The authors discuss the concept of the Brillouin zone, central to many fields of condensed matter physics. They challenge the universality of defining the Brillouin zone as a Wigner-Seitz cell of the reciprocal lattice. Experimental measurements of terahertz phonon-polariton dispersion in a photonic crystal fabricated in a thin slab of an anisotropic material (lithium niobate) demonstrate that the lowest band gap does not form at the boundary of the conventionally defined Brillouin zone, in contrast to a similar photonic crystal fabricated in a nearly isotropic lithium tantalate. The analysis motivated by this unexpected experimental result shows that in an anisotropic photonic crystal the Wigner-Seitz cell of the reciprocal lattice is no longer bounded by Bragg planes and, consequently, does not conform to the original definition of the Brillouin zone. The authors construct an alternative Brillouin zone bounded by the Bragg planes and show its utility in analyzing the dispersion bands of anisotropic photonic crystals.

[Phys. Rev. B 93, 054204] Published Tue Feb 09, 2016

]]>Noncentrosymmetric superconductors such as PbTaSe${}_{2}$ have been the focus of intensive study due to their unconventional properties and they have also recently been proposed as candidates for topological superconductivity and Majorana fermions. To test this, it is important to characterize the superconducting order parameter. The authors here report precise temperature-dependent measurements of the London penetration depth of PbTaSe${}_{2}$. The results indicate fully gapped $s$-wave BCS-type superconductivity, which is rather puzzling. This compound is known to have a topologically nontrivial band structure and a strong antisymmetric spin-orbit coupling, which, in 2D superconductors, is expected to produce an order parameter with mixed spin singlet and spin triplet components.

[Phys. Rev. B 93, 060506(R)] Published Fri Feb 05, 2016

]]>The half-filled Landau level can support a plethora of exotic strongly correlated states. The authors interconnect a recently proposed Dirac particle theory of the half-filled Landau level with the physics of spin liquids and the surface states of topological insulators. Additionally, they suggest that composite fermions can be viewed as dipolar bound states of charge/vortex composites and show that their transport properties violate the conventional Wiedemann-Franz law.

[Phys. Rev. B 93, 085110] Published Fri Feb 05, 2016

]]>The fate of magnetic ordering in frustrated systems with competing magnetic interactions is a long-standing question in the field of strong electronic correlation. Frustration can lead either to exotic phases that do not order down to zero temperature, the so-called spin liquids, or to spiral magnetism. In this paper, the authors consider a model for spins on the anisotropic triangular lattice, by using variational Monte Carlo, where they treat on the same ground spin liquids and magnetic states with spiral order. Moreover, they perform a systematic analysis of the spin-liquid states that can be constructed on the anisotropic triangular lattice by means of the fermionic projective symmetry group classification. The results allow for determining a complete phase diagram, as a function of the frustration ratio, encompassing different physical behaviors. They are relevant for the description of materials that are characterized by stacked triangular layers, such as charge-transfer salts and the compounds Cs${}_{2}$CuCl${}_{4}$ and Cs${}_{2}$CuBr${}_{4}$.

[Phys. Rev. B 93, 085111] Published Fri Feb 05, 2016

]]>The authors study the contribution of the chiral anomaly to various magnetotransport coefficients in Weyl semimetals. They do so using quasiclassical transport theory taking into account Berry curvature. In this framework, the authors compute the longitudinal conductivity, thermal conductivity, the spectrum of polaritons, and absorption coefficient for sound waves.

[Phys. Rev. B 93, 085107] Published Thu Feb 04, 2016

]]>It has recently been shown that disordered hyperuniform many-particle systems represent new distinguishable states of amorphous matter that are poised between a crystal and a liquid are are endowed with novel physical and thermodynamic properties. Such systems have shown to exist as ground states, i.e., at a temperature of absolute zero. Such “stealthy” and hyperuniform states are unique in that they are transparent to radiation for a range of wavelengths. In this paper, we ask whether Ising models of magnets, called spin chains in one dimension, can possess spin interactions that enable their ground states to be disordered, stealthy, and hyperuniform. Using inverse statistical-mechanical theoretical methods, we do demonstrate the existence of such states, which should be experimentally realizable.

[Phys. Rev. B 93, 064201] Published Wed Feb 03, 2016

]]>The authors report elastic and inelastic neutron scattering results on the quantum spin ice candidate Yb${}_{2}$Ti${}_{2}$O${}_{7}$. The experiments were performed on a well characterized stoichiometric powder, which displays a large and sharp heat capacity anomaly at 0.26 K. The authors show that, at low temperature, Yb${}_{2}$Ti${}_{2}$O${}_{7}$ exhibits long-range order with an ice-like ferromagnetic structure. However, the onset temperature is much higher than the temperature of the heat capacity anomaly. The spin excitations were found to be gapless on an energy scale < 0.09 meV and organized into a continuum of scattering with vestiges of highly overdamped ferromagnetic spin waves. The same spin dynamics is also observed for the single crystals, which indicates that those spin excitations are robust upon weak disorder.

[Phys. Rev. B 93, 064406] Published Wed Feb 03, 2016

]]>The authors show that static and Floquet topological insulators can be described (classified) by a unified-framework scattering theory, even when bulk Floquet bands fail to capture the topological nature of a phase. The paper introduces and characterizes for the first time invariants of weak topological Floquet insulators in two dimensions. This should help motivate research into Floquet weak topological insulators and Floquet topological crystalline insulators, fields which are largely unexplored.

[Phys. Rev. B 93, 075405] Published Tue Feb 02, 2016

]]>Inspired by the recently proposed Dirac composite fermion picture for half-filled Landau level the authors extend previously developed Hamiltonian formalism to incorporate considerations of particle-hole symmetry. They show that the magnetic translation algebra can be represented in the enlarged space of Dirac fermions, with particle-hole symmetry manifestly defined. A Hartree-Fock approximation then leads to a composite Fermi liquid of Dirac fermions.

[Phys. Rev. B 93, 085405] Published Tue Feb 02, 2016

]]>The multiorbital Hubbard model Hamiltonian is relevant to a host of strongly correlated materials, but there is no consistently accepted form for it. This manuscript derives the most general model Hamiltonian with $s$, $p$, and $d$-shell electrons and also identifies new terms describing pair hopping and quadrupole effects that were missed previously.

[Phys. Rev. B 93, 075101] Published Mon Feb 01, 2016

]]>Quantum kicked rotor (QKR) is a standard model of chaos that describes a particle moving on a ring under time-modulated kicking. This paper demonstrates that a large class of spin-$\frac{1}{2}$ quasiperiodic QKR models exhibits a dynamical analog of the integer quantum Hall effect, which is usually associated with two-dimensional electronic systems. Additionally, the authors reveal that the topological theta angle, a fundamental concept in quantum chromodynamics, can emerge in QKR models.

[Phys. Rev. B 93, 075403] Published Mon Feb 01, 2016

]]>In quantum magnetism, Kitaev’s honeycomb model is well known for its gapped and gapless spin liquid ground states in which the elementary spin degrees of freedom fractionalize into Majorana fermions and a ${\mathbb{Z}}_{2}$ gauge field. In this paper, the authors discuss the gapless spin liquids arising in three-dimensional Kitaev models and show that the gapless Majorana fermions form metallic states which, depending on the underlying lattice structure, exhibit Fermi surfaces, nodal lines, or Weyl points.

[Phys. Rev. B 93, 085101] Published Mon Feb 01, 2016

]]>The authors have conducted an experimental study of hexagonal boron nitride using steady-state and time-resolved photoluminescence. The assignment of previously observed lines of unknown origin is made convincingly to indirect excitons assisted by acoustic and optical phonons and to defect-assisted transitions. This is further evidence for $h$-BN being an indirect band-gap semiconductor.

[Phys. Rev. B 93, 035207] Published Thu Jan 28, 2016

]]>The authors discuss magnetoplasma and cyclotron resonances under microwave excitation in a high quality, GaAs/AlGaAs two-dimensional electron system. They observe that the cyclotron resonance originates as a pure resonance that does not hybridize with dimensional magnetoplasma excitations. The magnetoplasma resonances form a fine structure of the cyclotron resonance.

[Phys. Rev. B 93, 041110(R)] Published Thu Jan 28, 2016

]]>Reports on sulfur hydride attaining metallicity under pressure and exhibiting superconductivity at temperatures as high as 200 K have spurred an intense search for another room-temperature superconductor among hydrogen-rich compounds. Recently, compressed phosphorus hydride (phosphine) was reported to metallize at pressures above 45 GPa, reaching a superconducting transition temperature (${T}_{c}$) of 100 K at 200 GPa. However, neither the exact composition nor the crystal structure of the superconducting phase have been conclusively determined. This work reports an extensive study of the phase diagram of PH${}_{n}$ ($n$=1–6) by means of \textit{ab initio} crystal structure predictions using the minima hopping method. The results do not support the existence of thermodynamically stable PH${}_{n}$ compounds, which exhibit a tendency for elemental decomposition at high pressure even when vibrational contributions to the free energies are taken into account. Although the lowest energy phases of PH${}_{1,2,3}$ display ${T}_{c}$’s comparable to experiments, it remains uncertain if the measured values of ${T}_{c}$ can be fully attributed to a phase-pure compound of PH${}_{n}$.

[Phys. Rev. B 93, 020508(R)] Published Tue Jan 26, 2016

]]>The authors report x-ray absorption spectroscopy of samples in large electrical fields. The observed effect is linear in electric field and only observable in polar materials. It can be thought of as the x-ray analogue of the linear Stark effect. It may open up a new area of research by studying electrical polarization with element specificity for multiconstituent functional materials

[Phys. Rev. B 93, 035136] Published Tue Jan 26, 2016

]]>Phonons in iron pnictide materials were predicted to show large effects from magnetic ordering that had not been observed, and, indeed, the overall phonon dispersion has been in poor agreement with \textit{ab initio} calculations. By carefully detwinning a sample of SrFe${}_{2}$As${}_{2}$, clear phonon splitting was observed below ${T}_{N}$, allowing the authors to suggest an improved model for the phonon response for both the antiferromagnetically ordered phase below ${T}_{N}$ and the paramagnetic phase above ${T}_{N}$.

[Phys. Rev. B 93, 020301(R)] Published Mon Jan 25, 2016

]]>The authors report a first-principles study of the structural and electronic properties of the transition-metal-based compounds TiO${}_{2}$, ZnS, and NiO. Total energies (and resulting quantities) are calculated within the random-phase approximation (RPA) total-energy functional, which is being discussed actively in the DFT community. The effect of Hubbard $U$ corrections are shown. These calculations should stimulate more study and use of the RPA functional.

[Phys. Rev. B 93, 035133] Published Mon Jan 25, 2016

]]>The $g$ tensor, which describes the interaction energy of the spin with an external magnetic field, plays an essential role in controlling and manipulating single spins in semiconductor quantum dots. Spin-correlated orbital currents can strongly affect the $g$ tensor. This paper investigates theoretically and experimentally how these currents depend on the shape of quantum dots and how they affect the anisotropy of the electron $g$ tensor. It is found that the spin-correlated orbital currents form a simple current loop perpendicular to the magnetic moment’s orientation, and are therefore directly sensitive to the shape of the nanostructure. This simple and intuitive picture is validated by a systematic experimental magnetoluminescence study of the size dependence of the separate electron and hole $g$ tensors of InAs/InP quantum dots.

[Phys. Rev. B 93, 035311] Published Mon Jan 25, 2016

]]>Phenomenon of many-body localization violates one of the basic rules of statistical mechanics: It states that certain ‘localized’ macroscopic systems cannot act as a bath for themselves and hence do not relax to equilibrium. The authors of this paper find that, whenever a system is not localized at all thermodynamic parameters (particle density, energy density), then local fluctuations into the non-localized phase, dubbed bubbles, can slowly destroy localization globally. This result provides a rather strong restriction on the existence of many-body localized phases and runs contrary to the idea that there could be genuine localization transitions as a function of temperature.

[Phys. Rev. B 93, 014203] Published Fri Jan 22, 2016

]]>A voltage biased Josephson junction coupled to two microwave cavities which are in turn coupled to two “hot” and “cold” thermal environments, constitutes the proposal for the first thermoelectric heat engine where the heat current is completely separated from the electronic degrees of freedom.

[Phys. Rev. B 93, 041418(R)] Published Fri Jan 22, 2016

]]>Diffusive transport of magnons in YIG has recently attracted a lot of research attention, particularly in the context of the spin Seebeck effect and the new field of magnon spintronics. Here, the authors perform a systematic study of the influence of an external magnetic field on the magnon spin diffusion length, making use of a nonlocal measurement scheme that allows for direct extraction of this length scale. The results demonstrate a new mechanism for the manipulation of diffusive magnonic spin currents.

[Phys. Rev. B 93, 020403(R)] Published Tue Jan 19, 2016

]]>This paper provides a warning to the acoustic metamaterials community to be careful not to mistake the sign or value of the effective parameters, i.e. the density and modulus, of acoustic metamaterials for want of an overall picture of the physics involved. The authors here provide a new picture of the functioning of acoustic metamaterials based on hidden forces and hidden sources of volume. The new ansatz is tested on some established acoustic metamaterials with elements based on membranes, Helmholtz resonators, springs, and masses. It should provide the basis for a clearer vision of acoustic metamaterials and a faster route to real-world applications.

[Phys. Rev. B 93, 024302] Published Thu Jan 14, 2016

]]>Massless Dirac fermions with a pseudospin of $\frac{1}{2}$ are tied to many intriguing properties of graphene. Previous studies have discussed the possibility of constructing a higher pseudospin system in artificial lattices of ultracold atoms, which can also support equally interesting physical properties. The authors of this paper demonstrate that photon transport in certain photonic crystals corresponds to a pseudospin-1 system. This is a significant development because, contrary to ultracold systems, photonic crystals offer a better opportunity to study novel pseudospin-1 physics in experimentally realizable materials and at room temperature.

[Phys. Rev. B 93, 035422] Published Thu Jan 14, 2016

]]>Spin-orbitronics, which exploits the coupling between the spin and the orbital momentum of electrons, relies on the possibility to electrically create and detect pure spin currents without need of ferromagnetic elements. An efficient way to achieve this spin-to-charge conversion (and vice versa) is expected by exploiting the Rashba-Edelstein effect. This phenomenon is related to the well-known spin Hall effect, but in the former, the spin-to-charge current conversion occurs at the interface of materials with a strong spin splitting of the surface states, instead of the bulk. This paper reports an observation of the inverse Rashba-Edelstein effect, i.e., conversion of a spin current into a charge current, at a bismuth/copper interface, by using spin absorption with lateral spin valves. The induced charge current changes sign with temperature, a phenomenon that the authors can explain theoretically owing to the complex spin structure and dispersion of the surface states at the Fermi energy.

[Phys. Rev. B 93, 014420] Published Wed Jan 13, 2016

]]>In quantum magnetism, peculiar features of quantum spin system sometimes forbid the existence of gapped “featureless” paramagnets which are fully symmetric and unfractionalized. The celebrated Lieb-Schultz-Mattis theorem is an example of such a constraint, but it is not known what the most general restriction might be. For given lattice structure and symmetries, it is an interesting question whether a featureless paramagnetic phase exists. In this paper, the authors focus on the existence of such featureless paramagnets on the square and honeycomb lattices and, on the basis of analytical and numerical arguments, propose the corresponding wave functions for several such models. The existence of these nontrivial states not only offers a proof of principle, but also suggests an exotic field theory description when these states are brought to the vicinity of a second-order phase transition.

[Phys. Rev. B 93, 035114] Published Wed Jan 13, 2016

]]>Using a perturbation expansion to the fourth order in a junction critical current, the authors study the generation of nonclassical microwave radiation by inelastic Cooper pair tunneling in a small voltage-biased Josephson junction connected to a superconducting transmission line. The second order in the tunneling current only allows consideration of independent events. Pushing the calculation to the fourth order allows capture of the temporal correlations in the charge transport and their connection to the ones in the emitted electromagnetic radiation.

[Phys. Rev. B 93, 014506] Published Mon Jan 11, 2016

]]>Hydrogen sulfides have recently received a great deal of interest due to the record high-${T}_{c}$ of up to 203 K observed on strong compression of H${}_{2}$S . In this paper, a joint theoretical and experimental study is presented to characterize the dissociation products of compressed H${}_{2}$S , which is essential to understand its complex superconducting states. Based on the results here, the authors found H${}_{2}$S partially decomposes into S + H${}_{3}$S + H${}_{4}$S${}_{3}$ above 27 GPa, and H${}_{4}$S${}_{3}$ emerges as the major component at around 66 GPa. Interestingly, x-ray diffraction (XRD) experiments observed a small fraction of H${}_{3}$S and residual H${}_{2}$S at least up to 140 GPa, which are believed to be responsible for the two superconducting states observed in experiments. This paper provides the first XRD evidence on the existence of H${}_{3}$S at high pressure from the decomposition of H${}_{2}$S.

[Phys. Rev. B 93, 020103(R)] Published Mon Jan 11, 2016

]]>The prevailing theme of “form follows function” requires structural coherence over sufficient distances to allow subtle magnetic coupling to be revealed in high magnetic field studies at low temperatures. Single-crystal studies of the $S$=2 quasi-one dimensional chain material MnCl${}_{3}$(2,2’-bipridine) allows the long-range order to be identified, while also revealing the presence of an energy gap at the spin-flop transition field, whose nature remains intriguing.

[Phys. Rev. B 93, 014407] Published Fri Jan 08, 2016

]]>In modern quantum many-body physics, the Kadanoff-Baym equations have become a crucial component in the treatment of strongly and weakly correlated systems far from equilibrium. From the one-particle Green function $G$($t$,${t}^{\prime}$), they allow for the calculation of time-dependent expectation values of all one-particle observables and the total energy. In this work, for isolated Coulomb systems, the numerical behavior of the Kadanoff-Baym equations is investigated. The electron density dynamics are damped to an unphysical homogeneous density distribution, across both the linear and nonlinear response regimes. Unphysical features are shown to exist for $\mathrm{\Phi}$-derivable self-energy approximations, such as Hartree-Fock, second-Born, or $G\phantom{\rule{0}{0ex}}W$, in Hubbard and Coulomb systems, irrespective of interaction strength. With this degree of universality, these findings are pertinent to all two-time formalisms, and suggest the need for a different approach to the dynamics of quantum systems.

[Phys. Rev. B 93, 041103(R)] Published Fri Jan 08, 2016

]]>Doping a semiconductor induces a change in its lattice parameter. This change is caused not only by the different sizes of the impurity and host atoms, but also by the strain developed when the occupation of electronic energy bands is modified. While these ideas are conceptually simple, experimentalists and theorists alike have struggled for decades to separate the size and electronic effects. Following up on the most significant earlier developments, which focused on doped Si, the authors present data for $n$-type Ge doped with novel precursors and suggest that trends as a function of the donor species in both Ge and Si hold the key to resolving the doping dependence of the lattice parameter into its two fundamental physical components.

[Phys. Rev. B 93, 041201(R)] Published Fri Jan 08, 2016

]]>Through a combination of modeling and density functional theory calculations, the authors solve the structure of the only known quasicrystalline oxide, Ba-Ti-O, and introduce a new crystal-chemistry motif that explains all previously unsolved ultrathin Ba-Ti-O structures.

[Phys. Rev. B 93, 020101(R)] Published Thu Jan 07, 2016

]]>This paper reports on a photophysics study of gate-doped single-wall carbon nanotubes suspended over trenches. The authors experimentally demonstrate trion emissions from electrostatically doped nanotubes. They observe that the trion binding energies can be manipulated by varying the nanotube diameter.

[Phys. Rev. B 93, 041402(R)] Published Tue Jan 05, 2016

]]>Previously, closed optical transitions were only possible in crystals where the rare-earth ion site had perfect axial symmetry. The possibility of creating closed hyperfine transitions for all site symmetries by applying an appropriate oriented magnetic field is explored. The new technique should allow studies that are currently impossible for rare-earth-ion crystals both in the single ion and ensemble regimes.

[Phys. Rev. B 93, 014401] Published Mon Jan 04, 2016

]]>Dirac cones are at the heart of all the quasirelativistic phenomena oberved or predicted to occur in graphene. For a long time, it was believed that their existence is tightly bound to the honeycomb geometry of the graphene lattice. However, recent studies show that Dirac cones can be found in a variety of two-dimensional materials, with square or more complicated lattices. Here, the authors present a complete classification of these different systems, and show how Dirac cones emerge as a consequence of the symmetries of the underlying crystal. These results also provide guidelines to engineer novel materials that go beyond graphene.

[Phys. Rev. B 93, 035401] Published Mon Jan 04, 2016

]]>Based on experiments and simulations, the authors use waveguides of nonuniform cross-sectional width to tailor the spatial distribution of the energy density of transmission channels and to realize conversion between evanescent and propagating channels. For particular geometries, perfect reflection channels are created, and their large penetration depth into the medium as well as the complete return of probe light to the input end should greatly benefit sensing and imaging applications.

[Phys. Rev. B 92, 214206] Published Thu Dec 31, 2015

]]>Enhancing light transmission through scattering media is of much interest in many fields. Here, numerical simulations are performed to study the impact of gain on the value of reflection of the minimum reflection channel in a random medium. Surprisingly, in some random configurations, the reflection falls when gain is added even through the intensity inside the medium presumably increases once gain is introduced. This unexpected behavior is explained based on a decomposition of quasinormal modes.

[Phys. Rev. B 92, 224202] Published Tue Dec 29, 2015

]]>The first observation of two pressure-induced quantum phase transitions in a YbB${}_{6}$ single crystal is reported. This material is currently of much interest as there is conflicting experimental support for it belonging to a new class of topological insulators. Based on the results here the authors conclude that while at ambient-pressure YbB${}_{6}$ is topologically trivial, a newly found high-pressure gapped phase could be topologically nontrivial, although a partially gapped semimetal state has also been suggested. This interesting phase deserves further investigation.

[Phys. Rev. B 92, 241118(R)] Published Tue Dec 29, 2015

]]>BiFeO${}_{3}$ is one of the few known materials in which ferroelectricity and magnetism coexist at room temperature. Apart from possible practical applications, this multiferroic material has been attracting significant interest because of the interactions between its complex magnetic structure and different structural modes. The authors of this state-of-the-art first-principles study explore the electrical phase diagram of bulk BiFeO${}_{3}$ and reveal important aspects of the competition between ferroelectric and antiferroelectric phases.

[Phys. Rev. B 92, 235148] Published Mon Dec 28, 2015

]]>Room-temperature electronic properties of semiconductors, especially in the case of higher charge densities, are commonly discussed in terms of single-particle excitations – free electrons and holes. Many-particle effects, such as the formation of excitons (Coulomb-bound electron-hole pairs), are usually seen as low-temperature and low-density phenomena. In this paper, using ultrafast terahertz and photoluminescence measurements, the authors find that under certain conditions typical for wide-band-gap semiconductors, the radiative excitons can be efficiently formed at high charge density and at room temperature. This effect is believed to contribute to the extraordinarily high quantum efficiency of group III nitride light emitters.

[Phys. Rev. B 92, 241305(R)] Published Mon Dec 28, 2015

]]>Exactly solvable models often provide valuable insights in theoretical studies of topological phases. Here the authors introduce exactly solvable models of interacting Majorana fermions each with extensive topological ground-state degeneracy and a hierarchy of pointlike, topological excitations that are only free to move within submanifolds of the full lattice. These very different models make up a new kind of topological quantum order.

[Phys. Rev. B 92, 235136] Published Mon Dec 21, 2015

]]>For the first time, it is experimentally shown that a cloud of cesium (Cs) atoms can be used as a spectrally selective and tunable delay line for single photons emitted by semiconductor quantum dots. This delay line – significant slowing down of photons separated in frequency by only a few gigahertz – may represent the missing ingredient for the demonstration of a time-reordering scheme for entangled photon generation, and it could play a key role in future quantum networks and quantum communication.

[Phys. Rev. B 92, 235306] Published Mon Dec 21, 2015

]]>Understanding the conditions determining the appearance of stable chiral spin topological excitations is a central problem in condensed matter physics and related fields. This work provides the first unambiguous demonstration of discretization due to the macroscopic phase coherence and metastability for chiral topological excitations, and evidence of the mechanisms underlying its appearance. This was achieved through the use of an advanced Lorentz electron microscope with the highest possible resolution imaging of spin textures. The results provide new insights into dynamics in the phase transition of the chiral system, and will raise new questions concerning the mechanisms governing loss of chiral ordering.

[Phys. Rev. B 92, 220412(R)] Published Thu Dec 17, 2015

]]>van der Waals heterostructures, created by stacking two-dimensional materials, represent a novel and largely unexplored class of materials with very interesting optoelectronic properties. Excitons, strongly bound electron-hole pairs, play a crucial role in determining these properties, especially in 2D materials where the electron-hole binding is strong. However, a complete understanding of excitonic effects in 2D layered materials, i.e., when the electronic system transitions from a 2D geometry to a 3D one, is still missing. Here, the authors present a first-principles based multiscale method that attempts to fill this gap. With the help of their framework, one can predict the optoelectronics properties of van der Waals heterostructures and make a closer connection between the available theoretical models and experimental measurements in these materials.

[Phys. Rev. B 92, 245123] Published Thu Dec 17, 2015

]]>Building on recent advances by several groups to solve the fermion-sign problem in continuous-time quantum Monte Carlo calculations, the authors present two new algorithms and simulate the magnetic properties and phase transitions of the mass-imbalanced Hubbard model for square and honeycomb lattices. These theoretical calculations provide unbiased predictions that are relevant for experiments on ultracold atoms.

[Phys. Rev. B 92, 235129] Published Wed Dec 16, 2015

]]>A recent experimental identification of mono-pnictides (TaAs, TaP, NbP, and others) as Weyl semimetals has triggered an enormous effort to understand the unconventional physical phenomena in these materials. The chiral magnetic effect (CME), i.e., the occurrence of electrical current along the direction of an applied magnetic field, has been a particularly controversial subject, with a number of claims and counter-claims published in the literature. In this work Jing Ma and D. A. Pesin from the University of Utah provide a systematic theory of the CME and the related natural optical activity in these materials. They show that the natural optical activity of metals can be distinguished from well known phenomena in insulators by its singular frequency dependence in the terahertz range.

[Phys. Rev. B 92, 235205] Published Wed Dec 16, 2015

]]>The current through a driven two-level system, here realized in a double quantum dot, is determined by the phases acquired between avoided crossings. As a function of the detuning and the driving amplitude, it exhibits a characteristic interference pattern both in real space and in Fourier space. The authors demonstrate theoretically and experimentally the case of bichromatic driving creating fundamental relations between commensurability and symmetry properties.

[Phys. Rev. B 92, 245422] Published Tue Dec 15, 2015

]]>In an experimental and theoretical study, the authors find that the ac stress by a diamond mechanical resonator leads to a dressed spin basis whereby an NV center spin qubit is protected against magnetic fluctuations but remains sensitive to other environmental perturbations such as strain and temperature. By analyzing how the qubit protection scales with mechanical resonator amplitude, they demonstrate this technique’s value for precision thermometry, strain sensing, and quantum information.

[Phys. Rev. B 92, 224419] Published Mon Dec 14, 2015

]]>The ability to control multipolar light-matter interactions in metamaterials and other photonic systems has traditionally relied on engineering the physical properties of the resonators. In this paper, the authors follow the reverse approach. By tailoring the optical beam that illuminates a spherical nanoparticle, they demonstrate selective and enhanced coupling to the optical modes excited on the nanoparticle.

[Phys. Rev. B 92, 241110(R)] Published Mon Dec 14, 2015

]]>Increasing the external quantum efficiency is currently an important goal in organic light-emitting diodes (OLED) research. Utilizing an altered optical design that involves a low refractive index hole transport layer in a monochrome green top-emitting OLED, the authors have managed to shift the surface plasmon polariton dispersion relation, which in turn decreases the power dissipated into lost evanescent excitations, thus enhancing the external quantum efficiency.

[Phys. Rev. B 92, 245306] Published Fri Dec 11, 2015

]]>It is usually assumed that the magnetoelectric polarizability, which belongs to a broad family of topological magnetoelectric effects, is exactly quantized based on general topological arguments. By evaluating the Hall conductivity of surface states described by a massive Dirac model, the authors show that there is in fact a nonuniversal interaction correction to the quantized magnetoelectric coefficient of thin films formed from topological insulators.

[Phys. Rev. B 92, 245118] Published Thu Dec 10, 2015

]]>Multiferroic films have gained a large amount of attention due to the possible applications in spintronics. In this report, the authors provide a systematic study of the spin-Hall magnetoresistance and spin Seebeck effect in CoCr${}_{2}$O${}_{4}$ (CCO) thin films with platinum contacts. Larger responses of the spin-Hall magnetoresistance and spin Seebeck effect at the interface, as opposed to bulk measurements, indicate a greater interaction between interfacial magnetic moments. This interface coupling manifests magnetic- phase-dependent spin transport.

[Phys. Rev. B 92, 224410] Published Tue Dec 08, 2015

]]>A collaboration of researchers from France, Germany, and Spain present a comprehensive study of the structure and motion of dislocations in a technologically important ferroelectric KNbO${}_{3}$ perovskite.

[Phys. Rev. B 92, 214101] Published Mon Dec 07, 2015

]]>A comprehensive theory for the vortex unbinding transition in disordered superconducting films is developed.

[Phys. Rev. B 92, 214503] Published Mon Dec 07, 2015

]]>Using various macroscopic and microscopic techniques, including magnetometry and neutron and x-ray diffraction measurements, the authors investigate the magnetic properties of the quasi-two-dimensional antiferromagnet NiPS${}_{3}$. Contrary to previous measurements, the authors find that the material orders antiferromagnetically with a propagation vector of $\mathbf{k}$=[010]. This clarification of the magnetic structure is hoped to lead to a better understanding of the overall dynamics in these types of systems.

[Phys. Rev. B 92, 224408] Published Mon Dec 07, 2015

]]>Understanding the impact of the unique electronic structure of rare-earth quasicrystals on their magnetic properties has been a long-standing problem in the study of these fascinating materials. The issue is addressed here by studying the RKKY interaction of Ising spins that is mediated by electrons on a tight-binding quasiperiodic tiling. After deriving the form of the RKKY interaction, the authors examine the statistical properties of the resulting model using extensive Monte Carlo simulations. They show that, despite the lack of periodicity and the frustrated nature of the interaction between spins, the system displays long-range magnetic quasiperiodic order at low temperatures.

[Phys. Rev. B 92, 224409] Published Mon Dec 07, 2015

]]>The problem of electron-phonon interactions in strongly correlated systems is addressed by a new method which establishes a rigorous link between \textit{ab initio} density functional perturbation theory and low-energy model Hamiltonians. This allows the construction of model Hamiltonians for strongly correlated systems in a predictable and quantitative way, with respect to material oriented applications, especially where both electron-electron and electron-phonon couplings have the same magnitude. This could lead either to a competition or a cooperation between the two interactions, and stabilize interesting phases such as superconductivity in fullerenes.

[Phys. Rev. B 92, 245108] Published Mon Dec 07, 2015

]]>The electronic correlation strength – which is a defining property of correlated materials such as LaNiO${}_{3}$ – is studied in this manuscript experimentally, using angle-resolved photoemission spectroscopy, and theoretically, via density functional and dynamical mean field theory calculations. By carefully comparing the experiment and theory, the authors aim to quantify how accurately the DFT+DMFT approach can describe the correlated physics in LaNiO${}_{3}$. This is important because a precise knowledge of the predictive capabilities of the theory will aid in engineering new emergent properties in the nickelates.

[Phys. Rev. B 92, 245109] Published Mon Dec 07, 2015

]]>Indirect absorption plays an important role in the transparency of transparent conducting oxides. The authors perform a detailed first-principles study of phonon- and charged-impurity-assisted free-carrier absorption, using SnO${}_{2}$ as an example system. They were able to make a quantitative prediction of the absorption cross section due to free carriers, to determine the range of validity of the Fröhlich model, which is commonly used to describe the electron-phonon interaction, and to explain where and how the model breaks down. This study also accounts for the weak temperature dependence of the free-carrier absorption commonly seen by experiment, and compares the phonon-related absorption mechanism with the absorption caused by interactions with defects to find that defects dominate only for concentrations about 10${}^{20}$ cm${}^{-3}$.

[Phys. Rev. B 92, 235201] Published Wed Dec 02, 2015

]]>The presence of a distinct magnetic insulating state in Sr${}_{4}$RhO${}_{6}$ is revealed. Using various experimental and theoretical methods, the authors find previously unseen long-range magnetic order. While the underlying spin-orbit coupling is reduced in the rhodate compound as compared to an iridate analog, its influence on the physical properties is still strong and relevant.

[Phys. Rev. B 92, 180413(R)] Published Mon Nov 30, 2015

]]>Single-crystal neutron diffraction and various bulk probes are used to investigate the magnetic structure and properties of the rare-earth half-Heusler antiferromagnet NdBiPt. While the family of compounds $R$BiPt ($R$=Ce–Lu) have been suggested as candidates for a new family of antiferromagnetic topological insulators, the magnetic structure for the Nd compound, as determined in this report, seems to exclude it from this distinction.

[Phys. Rev. B 92, 184432] Published Mon Nov 30, 2015

]]>Identifying the symmetry of the superconducting order parameter in multiband systems such as the iron pnictides is a tricky task. A new model-free approach based on the temperature dependence of the momentum-integrated quasiparticle interference data is proposed to resolve this puzzle.

[Phys. Rev. B 92, 184513] Published Mon Nov 30, 2015

]]>Superconductivity is believed to be a key ingredient to the realization of Majorana modes. The authors consider domain walls between different chiral phases of a quantum Hall state and show that, contrary to expectations, neither superconductivity nor even its analogs are necessary for Majorana zero modes in this setup. It is further suggested that Majorana zero modes might be observable in a system composed entirely of GaAs or graphene.

[Phys. Rev. B 92, 195152] Published Mon Nov 30, 2015

]]>A microscopic theory for ultrafast spin dynamics across a wide range of energies in magnetic heterostructures has been developed.

[Phys. Rev. B 92, 180412(R)] Published Mon Nov 23, 2015

]]>The physics of underdoped cuprates has been an unresolved longstanding problem. New measurements at ultrahigh magnetic fields on underdoped YBCO are used to show that superconducting diamagnetism survives at fields far above the irreversibility line at low temperatures. Two scenarios can account for these observations - superconductivity competes with charge order and is considerably diminished or a fluctuating superconductivity survives above a certain magnetic field.

[Phys. Rev. B 92, 180509(R)] Published Mon Nov 23, 2015

]]>Through an examination of the dynamic nuclear spin polarization and resonant spin-flip Raman scattering, the authors directly measure the optical-induced Overhauser shift in singly charged (In,Ga)As/GaAs quantum dots. Using a two-color laser excitation scheme, they observe the time evolution of the Overhauser shift, which demonstrates the nuclear spin depolarization over a few seconds. Furthermore, it is shown that this depolarization is strongly dependent on temperature.

[Phys. Rev. B 92, 195421] Published Fri Nov 20, 2015

]]>The perovskite semiconductor CsSnI${}_{3}$ has recently attracted much attention due to its potential applications in solar energy generation. In this work, the authors went beyond the standard (quasi)harmonic approximation, and explained the stability of CsSnI${}_{3}$ at experimental conditions through anharmonic phonon-phonon interactions between the Cs ions and their iodine cages. They also studied the renormalization of the electronic energies due to vibrations and found an unusual opening of the band gap at high temperatures, which demonstrates the importance of anharmonic temperature effects to the realistic modeling of the X(Sn,Pb)Y${}_{3}$ perovskites.

[Phys. Rev. B 92, 201205(R)] Published Fri Nov 20, 2015

]]>There has been some controversy regarding the dominant correlations in weakly doped Hubbard ladders. Analytical approaches find them to be superconducting but numerical approaches have thus far pointed otherwise. Using cutting-edge DMRG techniques on extremely large system sizes this controversy is now resolved to show that superconducting correlations indeed dominate.

[Phys. Rev. B 92, 195139] Published Thu Nov 19, 2015

]]>The two-dimensional superfluid-to-insulator quantum critical point is expected to be accompanied by a collective Goldstone Higgs mode that is massive. However, experimental confirmation within ultracold atoms in an optical lattice is not straightforward. Using Monte Carlo methods and a comparison with recent experiments, the authors of this paper have found that the signature of this massive mode can be observed in the spectral function of the amplitude response under the right conditions.

[Phys. Rev. B 92, 174521] Published Wed Nov 18, 2015

]]>Several established techniques could theoretically be combined to produce a large nuclear polarization in randomly oriented nanodiamonds. This addresses what is currently a critical barrier to the widespread application of nanodiamonds in in-vivo medical diagnostics and MRI.

[Phys. Rev. B 92, 184420] Published Wed Nov 18, 2015

]]>Using angle-resolved photoemission spectroscopy (ARPES), a collaboration of authors from Tokyo and Hiroshima, Japan study the evolution of the nodal d-wave quasiparticles in the trilayer Bi2223 cuprate superconductor across the phase transition into the normal state.

[Phys. Rev. B 92, 195135] Published Wed Nov 18, 2015

]]>The puzzle of the observed nonsaturating and linear magnetoresistance in 3D Dirac materials is addressed by a new mechanism of magnetoresistance in metals that derives from the dynamics of ”squeezed” electrons.

[Phys. Rev. B 92, 180204(R)] Published Mon Nov 16, 2015

]]>Heterostructures of two-dimensional materials have shown unusual properties and rich physical phenomena. This paper reports on micrometer-scale angle-resolved photoemission spectroscopy of van der Waals heterostructures of graphene and MoS${}_{2}$ monolayers. The authors directly measured the electronic structure of monolayer stacking and its tunability due to the twist-angle between the layers. They show that the electronic states of graphene and MoS${}_{2}$ are not hybridized, and the band gap of MoS${}_{2}$ can be engineered by changing the orientation of the two layers.

[Phys. Rev. B 92, 201409(R)] Published Mon Nov 16, 2015

]]>The authors develop an algorithm to identify approximately conserved quantities in models perturbed away from integrability. The authors show that these quantities are able to determine the long time behavior of local correlation functions via Mazur’s inequality. They apply the model to a XXZ Heisenberg perturbed by a next-nearest-neighbor interaction.

[Phys. Rev. B 92, 195121] Published Wed Nov 11, 2015

]]>High-quality $G\phantom{\rule{0}{0ex}}W$ calculations of the quasiparticle band structure in thin films of topological insulator Bi${}_{2}$Se${}_{3}$ show that the topological nature of the surface states is $n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t$ the spin-polarized quantum spin Hall state, but the less interesting trivial topology. This is in strong contrast to the expected topology of the surface states of $b\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}k$ Bi${}_{2}$Se${}_{3}$ and the predictions of density functional theory and parametrized calculations for Bi${}_{2}$Se${}_{3}$ thin films, which produce fundamentally less accurate predictions of the band gap than the $G\phantom{\rule{0}{0ex}}W$ approach.

[Phys. Rev. B 92, 201404(R)] Published Mon Nov 09, 2015

]]>The authors explore the possibility of long range coupling between two RX qubits, qubits formed out of sets of triple quantum dots. The coupling is electromagnetic in nature and is facilitated by a superconducting strip line. The authors demonstrate that within this setup, they can implement the quantum iSWAP gate with high fidelity.

[Phys. Rev. B 92, 205412] Published Mon Nov 09, 2015

]]>Martensites, which are the product of diffusionless phase transitions, can form phases with small periodic modulations of crystalline atomic lattice positions. Experiments have shown both that the modulation function is smooth and that when more than one modulation is present they can be incommensurate. These observations have appeared to be inconsistent with nanotwinning models. Here the authors show theoretically that in the case of the ferromagnetic shape memory alloy NiMnGa, the presence of two competing mechanisms favoring different commensurate periodic structures, a result of the interaction between different minima in the energy landscape, can lead to effective smooth incommensurate modulations consistent with nanotwinning models.

[Phys. Rev. B 92, 180101(R)] Published Fri Nov 06, 2015

]]>Cellular dynamical mean-field theory combined with continuous Monte Carlo techniques is used to account for the most important features of the doped quasi-two-dimensional organic Mott insulators.

[Phys. Rev. B 92, 195112] Published Fri Nov 06, 2015

]]>The transport properties of three-dimensional Weyl and Dirac semimetals are studied under a transverse magnetic field. The analysis is based on the continuum Dirac/Weyl model in the presence of potential disorder. The results are successfully compared to recent transport experiments in CdAs.

[Phys. Rev. B 92, 205113] Published Fri Nov 06, 2015

]]>The authors present the first example of a quasi-2D experimental realization of a transverse field Ising model, which is one of the canonical examples of a quantum phase transition driven by quantum mechanical fluctuations induced by a magnetic field applied perpendicular to the preferred direction of the magnetic moments. Experimental realizations of this model are rare, and include the prominently discussed quasi-1D Ising chain system CoNb${}_{2}$O${}_{6}$, and the 3D dipolar-coupled Ising ferromagnet LiHoF${}_{4}$, but the experimental realization of a quasi-2D system has been lacking until now.

[Phys. Rev. B 92, 180404(R)] Published Thu Nov 05, 2015

]]>The modeling of the electronic structure is the key to understanding layered transition-metal dichalcogenides (TMDCs) heterostructures. The authors present a full-range tight-binding hamiltonian for TMDCs by Wannier transformation of density functional theory results, which preserves both the orbital character and phase information. The tight-binding hamiltonians is expected to form the basis for further theoretical investigations of many-body physics and simulations for potential applications under external electric or magnetic fields in finite-size nanostructures, in either monolayer or heterostructure forms.

[Phys. Rev. B 92, 205108] Published Thu Nov 05, 2015

]]>The ability to spectrally resolve phonon energies and dispersion relations is central to the understanding and controlling of structural properties and thermal transport of crystalline solids. However, many of the existing probing techniques are limited by the requirement of large sample sizes. In this work, the authors have demonstrated a new method for determining phonon dispersion relations of solids based upon synchrotron reflection x-ray thermal diffuse scattering measurements. The application of this method was demonstrated in the model system of MgO thin films. Such techniques may become valuable for determining vibrational properties of heteroepitaxial thin films since x-ray penetration depths can be selectively tuned through the use of grazing incidence.

[Phys. Rev. B 92, 174301] Published Tue Nov 03, 2015

]]>It is commonly believed that fractional Chern insulators can only occur in Bloch bands with non-zero Chern numbers. The authors of this paper outline a number of ways in which specifically crafted interactions can be used to create a fractional topological insulator from a noninteracting system with zero Berry curvature.

[Phys. Rev. B 92, 195104] Published Tue Nov 03, 2015

]]>Previous electrical transport experiments on thin films of the topological insulator Bi${}_{2}$Se${}_{3}$ using ferromagnetic contacts have claimed to see spin-momentum locking based on changes in the measured voltage at the contacts. A careful new transport experiment using a unique device configuration reveals that such a voltage arises from fringe magnetic fields. The findings emphasize the necessity of performing detailed experiments so as to exclude effects such as local Hall fields on the measured signal.

[Phys. Rev. B 92, 201102(R)] Published Tue Nov 03, 2015

]]>There are many instances where superconductivity seems to emerge from the condensation of a non-Fermi liquid metal in the vicinity of a quantum critical point. The authors of this paper address this problem by studying a Fermi surface coupled to a critical bosonic order parameter within a large-N approach. The ensuing competition between the tendency for pairing and the destruction of Landau quasiparticles is found to result in a novel non-Fermi liquid fixed point with a finite scale invariant BCS coupling.

[Phys. Rev. B 92, 205104] Published Tue Nov 03, 2015

]]>How does the presence of a finite length scale in a negatively curved spatial geometry, namely the spatial curvature, modify statistical physics properties of many-particle systems such as critical exponents or the texture of ordered states? Here the authors show that phenomena such as unusual magnetization textures and a strong curvature fixed point can emerge, and that this is a promising theoretical approach for investigating uniform frustration and nontrivial critical behavior.

[Phys. Rev. B 92, 134423] Published Fri Oct 30, 2015

]]>Neutron scattering data show that the magnetic response function in the quantum critical region for two different materials, an iron-based superconductor and a heavy fermion compound, compare well to a universal theory.

[Phys. Rev. B 92, 155150] Published Fri Oct 30, 2015

]]>Due to the absence of a strong electron-acoustic phonon interaction, a system made up of graphene sheets encapsulated between thin hexagonal boron nitride slabs is ideal for investigating the hydrodynamic behavior of an electron liquid. Using a fully analytical theoretical approach, the authors demonstrate that nonlocal dc transport measurements can be used to extract the hydrodynamic shear viscosity of 2D electrons in graphene far from the neutrality point. They also suggest that the currently available scanning probe techniques should be able to spatially resolve viscosity-dominated electron flow regions.

[Phys. Rev. B 92, 165433] Published Fri Oct 30, 2015

]]>A thorough test of the eigenstate thermalization hypothesis (ETH) for the single-impurity Anderson model, performed with the aid of the numerical renormalization group (NRG) method, finds good support for ETH. The mechanism responsible for effective thermalization within the NRG can be identified as Anderson orthogonality: the more charge that needs to flow to or from infinity after applying a local excitation within the Wilson chain, the more the system looks thermal afterwards at an increased temperature. For the same reason, however, thermalization fails if charge rearrangement after the excitation remains mostly local. In these cases, the different statistical ensembles lead to different results, and their behavior needs to be understood as a microscopic quantum quench only. This analysis provides new and important insight into the process of thermalization and dynamics of systems exhibiting strong, nontrivial correlations.

[Phys. Rev. B 92, 155435] Published Thu Oct 29, 2015

]]>The authors have investigated the magnetic and structural properties of Fe3PO4O3 using various methods including thermodynamic probes, synchrotron X-ray diffraction, and neutron powder diffraction. In this context, the authors demonstrate the formation of an incommensurate helical antiferromagnetic structure with highly anisotropic domains, which may be produced through magnetoelastic strain that blocks long-range magnetic order. The overall helical nature of the magnetic domains may be of interest to those studying antiferromagnetic Skyrmions and other topological spin textures.

[Phys. Rev. B 92, 134419] Published Mon Oct 26, 2015

]]>Exploiting the analogy between Pfaffian and anti-Pfaffian states the authors construct a particle-hole conjugate of the composite Fermi liquid state. They suggest that a transition between composite Fermi liquid and anti-composite Fermi liquid states may occur at half-filling and discuss the relevance of such transition to recent magnetoresistance oscillation experiments.

[Phys. Rev. B 92, 165125] Published Fri Oct 23, 2015

]]>At low temperatures, the study of iron arsenide superconductors is hindered by the appearance of twin domains that strongly affect superconducting and magnetic properties of the system. To detwin the crystal, a uniaxial pressure is usually applied, which may itself affect the magnetic and electronic properties of the system. In this manuscript, a collaboration of authors from USA, China, France, and Germany develop a technique to obtain de-twinned strain-free samples and use transport measurements and neutron scattering to study electronic nematic phase in electron-doped BaFe${}_{2-x}$Ni${}_{x}$As${}_{2}$. Their results indicate that the in-plane resistivity anisotropy found earlier in uniaxially strained paramagnetic tetragonal phase of iron pnictides is also present in the stress-free samples, but is due to magnetoelastic coupling.

[Phys. Rev. B 92, 134521] Published Thu Oct 22, 2015

]]>The Renyi entropies for an interacting quantum dot are computed under a multi-contour Keldysh formalism. With this in hand, the self-information generating function is determined and the probability distribution of self-information is computed, including the effects of Coulomb interactions in the Hartree approximation. The author argues through these computations that fluctuations in the self-information and the Renyi entropies are tied together intimately and that these fluctuations thus offer a way to give the Renyi entropies physical meaning.

[Phys. Rev. B 92, 165312] Published Thu Oct 22, 2015

]]>The chemical vapor deposition of diamond is known to introduce complexes of silicon, vacancy, and hydrogen. Here the authors theoretically examine several such complexes, some of which have already been observed, others which could potentially form. Using hybrid density functional theory for the treatment of highly correlated orbitals, many measurable quantities are calculated. The SiV${}_{2}$H(-) negatively charged defect is found to be a promising candidate for a long lived solid state quantum memory.

[Phys. Rev. B 92, 165203] Published Tue Oct 20, 2015

]]>This paper reports NMR spin-lattice and spin-spin relaxation rate data for iron arsenide superconductors involving multiple crystals with different dopants, doping levels, and magnetic fields. The results present evidence for glassy nematic fluctuations throughout an extensive region of phase space in these materials. The authors propose that the inhomogeneous relaxation dynamics that dominates the NMR response arises because the dopants introduce quenched random fields which couple to the nematic order. This disorder-induced frustration plays a significant role in suppressing antiferromagnetism and in the emergence of superconductivity.

[Phys. Rev. B 92, 165116] Published Fri Oct 16, 2015

]]>The authors revisited the Mollwo–Ivey relation of $F$ centers in alkali halide crystals, a prototypical color center, based on post-density-functional-theory and post-Hartree-Fock methods. In contrast to earlier interpretations, which stress the importance of the Madelung potential, they find ion-size effects to be the predominant mechanism. These determine the shape of the defect-electron wave function and are responsible for the fractional Mollwo–Ivey exponent of 1.8.

[Phys. Rev. B 92, 144107] Published Wed Oct 14, 2015

]]>The authors apply non-linear spin wave theory to the problem of anisotropic spin-S Heisenberg models on the Kagome lattice. They argue that the non-linear terms of the analysis lead to two unusual effects: 1) the presence of a quantum phase transition as a function of the anisotropy parameter between $q=0$ and $\sqrt{3}\times \sqrt{3}$ order (whereas thermal fluctuations favor the $\sqrt{3}\times \sqrt{3}$ order regardless of the strength of the anisotropy) and 2) strong quantum effects in the spectral features that are not suppressed with $1/S$ as is typical.

[Phys. Rev. B 92, 144415] Published Tue Oct 13, 2015

]]>Magnetic force measurements are used to measure the penetration depth and pinning forces for vortices in the superconductor NdFeAsO${}_{1-x}$F${}_{x}$.

[Phys. Rev. B 92, 134509] Published Mon Oct 12, 2015

]]>The use of terahertz time-resolved studies in conducting and superconducting systems is gaining considerable interest, especially in the light of recent studies demonstrating induction of superconductivity in various materials with optical impulsive excitation. The authors carry out a theoretical analysis of the response functions typically measured by terahertz time-domain spectroscopy (THz-TDS) in metallic and superconducting samples. Within the Drude model, the authors found that THz-TDS does not simply measure the instantaneous conductivity change, in agreement with previous reports. They identify limits wherein the time-resolved THz-TDS spectrum does and does not approach the instantaneous conductivity change. They further show that the difference between the THz-TDS response and the instantaneous conductivity is largest when the photo-induced state changes from a normal metal to a superconductor.

[Phys. Rev. B 92, 134507] Published Fri Oct 09, 2015

]]>A fundamental question for understanding the decoherence of quantum spin qubits interacting with a bath of spins in the environment is the nature of the noise affecting the qubit - can it be modeled as classical noise or are quantum effects important? Here the authors experimentally demonstrate that in the favorable case of bismuth electron spin donors in silicon near values of a magnetic field known as clock transitions, the nuclear spin noise can be approximated as classical Gaussian noise. This finding is expected to influence efforts to optimize the control of qubits in silicon based quantum computation.

[Phys. Rev. B 92, 161403(R)] Published Thu Oct 08, 2015

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