Traditional models for the growth of complex networks rely on a positive-feedback mechanism—nodes with the most existing connections get the most new connections. Theorists now present a new growth model involving a negative-feedback mechanism that may explain the prevalence of small, densely connected groups of nodes seen in real-world networks.

[Phys. Rev. X 3, 021016] Published Mon Jun 17, 2013

Many conventional quantum information-processing proposals fix data spatially and use temporally sequenced operations to carry out a computation. Researchers now propose a different approach, relying on connected modular elements—quantum transistors—that could enable clock-controlled quantum information processing similar to present-day classical integrated circuits.

[Phys. Rev. X 3, 021015] Published Fri Jun 14, 2013

The optical properties of graphene previously used to probe its electronic structure are its optical dispersion and absorption coefficients. Now, scientists report the third-harmonic generation of light in graphene and demonstrate the promise of this nonlinear optical response as a technically versatile microscopic imaging approach of the physical structure of graphene.

[Phys. Rev. X 3, 021014] Published Mon Jun 10, 2013

An innovative wiring of two familiar superconducting circuits, a microwave LC resonator containing a small mass on a spring and a microwave amplifier that measures the motion of the mass, feeds the output of the amplifier directly back to the first circuit, creating a quantum-devices-based network that can be continuously and dynamically tuned to optimize control and measurement capabilities.

[Phys. Rev. X 3, 021013] Published Wed Jun 05, 2013

So far two mechanisms, time-reversal symmetry and chirality, have been known to “protect” conducting edge states in a bulk insulator. Now Michael Levin of University of Maryland reveals “fractional statistics” of bulk particle-like excitations as the third (and final) mechanism of edge-state protection in two-dimensional insulators.

[Phys. Rev. X 3, 021009] Published Thu May 30, 2013

Whether polarization catastrophe or oxygen vacancies is responsible for the remarkable emergence of a two-dimensional electron gas at the interface of the insulating oxides, polar LaAlO₃ and nonpolar SrTiO₃, has been hotly debated. Using a series of experiments that compare the electrical properties of amorphous and crystalline LaAlO₃/SrTiO₃ heterostructures, researchers discover that the answer depends on the structure of the LaAlO₃ overlayer.

[Phys. Rev. X 3, 021010] Published Thu May 30, 2013

Establishing the strength of the electron-phonon interaction, which plays an important role in superconductivity, has proven difficult with current experimental and theoretical techniques. Theorists now propose a reliable first-principles method and also use it to explain the extraordinarily high superconducting temperatures seen in bismuthates and transition-metal chloronitrides.

[Phys. Rev. X 3, 021011] Published Thu May 30, 2013

The ampere, the basic measurement standard for electric current, is still defined based on the electromagnetic force between two parallel current-carrying wires. To achieve a quantum standard of current based on electron counting, an extremely accurate electron pump with a high current yield in the nanoampere range is required. Researchers now present a proof-of-principle demonstration of such a pump.

[Phys. Rev. X 3, 021012] Published Thu May 30, 2013

In sensing-feedback control of a quantum system, optimizing the timing of the sensing measurements turns out to be a key to getting around the fundamental difficulty that a measurement can randomly change the system’s state. Achieving optimal timing by combining technical advances with conceptual physical insight, researchers demonstrate, for the first time, high-fidelity control of a superconducting qubit along time-dependent trajectories.

[Phys. Rev. X 3, 021008] Published Wed May 29, 2013

While the tracks of the elusive Majorana fermion have been seen in semiconductor nanowires in contact with superconductors, this exotic particle is also predicted to appear in nanostructures made of three-dimensional topological insulators, such as HgTe, with induced superconducting surface current. Researchers show that such supercurrent can indeed be induced in HgTe Josephson junctions and offer insights into the underlying mechanism.

[Phys. Rev. X 3, 021007] Published Tue May 28, 2013

What cultural and social processes determine the size and growth of the vocabulary of a natural language? Does such a vocabulary grow forever? From large text databases, such as the Google Ngram, that have become available only recently, researchers tease out new and systematic insights into these fundamental questions and develop a mathematical model with predictive power that describes vocabulary growth as a simple stochastic process.

[Phys. Rev. X 3, 021006] Published Tue May 14, 2013

Slow oscillations in neuronal activity in the human brain are the defining feature of scalp-measured electroencephalography taken under general anesthesia. A theoretical investigation of a model for the human cortex reveals that slow spatiotemporal patterns emerge spontaneously as the result of a chemically modified balancing act between two instabilities in cortical dynamics—one to spatial organizations and the other to temporal bifurcation. Long-range interneuronal communication across the cortex is shown to be crucial to the pattern formation.

[Phys. Rev. X 3, 021005] Published Thu May 09, 2013

An analytical statistical-physical approach that is low in computational complexity, but high in accuracy is now available for theoretical studies of how behaviors, opinions, and infectious diseases spread among human populations.

[Phys. Rev. X 3, 021004] Published Mon Apr 29, 2013

A study of the two-dimensional version of a topological insulator, the quantum spin Hall (QSH) system, finds that the current flow in the one-dimensional edge channels of the QSH state is affected by small puddles of electrons that lift the protection against backscattering.

[Phys. Rev. X 3, 021003] Published Mon Apr 22, 2013

[Phys. Rev. X 3, 029901] Published Wed Apr 17, 2013

A study of the thermoelectric properties of the doped insulator, strontium titanate, shows that it superconducts with the lowest charge density ever observed.

[Phys. Rev. X 3, 021002] Published Mon Apr 15, 2013

The fundamental principle for electrophoresis, the motion of a charged particle in solution driven by an applied electric field, is well understood. But experimental measurements of the electrophoretic retardation force, one of those responsible for electrophoresis, have been scarce and equivocal. Now a Belgian group develops a creative new approach to control the source of the retardation—the counterion cloud surrounding the charged particle—by gradual depletion and makes unambiguous measurement of the retardation force.

[Phys. Rev. X 3, 021001] Published Thu Apr 11, 2013

The LaAlO₃/SrTiO₃ interface is already known to have interesting properties such as superconductivity and magnetism. Now nanoscale charge-transport networks created at the interface show extraordinary evidences of violation of Ohm’s law.

[Phys. Rev. X 3, 011021] Published Tue Mar 26, 2013

New experimental findings about the electronic structure of NaFe_0.9825Co_0.0175As explain the fundamentally intriguing and important puzzle of why magnetic order and superconductivity can coexist in such iron-based superconductors and reveal an intimate tie between the coexistence and the electron pairing underlying the superconductivity.

[Phys. Rev. X 3, 011020] Published Mon Mar 18, 2013

Spectroscopic studies of the relaxation dynamics of excited single “acceptor” impurities in silicon, such as boron or aluminum, show that these impurities both have the potential to work as “qubits” and can also enrich trapped-atom experiments in solids.

[Phys. Rev. X 3, 011019] Published Thu Mar 14, 2013

State-of-the-art x-ray resonant photoemission spectroscopy combined with ab initio calculations maps out, for the first time, the actual shapes of the vibrational wave functions of highly excited nitrogen molecules.

[Phys. Rev. X 3, 011017] Published Fri Mar 08, 2013

Many interesting and important systems in natural science, such as that of a large protein molecule in water, show regions of different molecular activities and therefore of different interest to the investigator. A new method of molecular dynamics simulations now allows the simulator to zoom in and out of the region(s) of interest “on the fly” with computational ease and high efficiency and brings simulations of such systems into the realm of computational capability.

[Phys. Rev. X 3, 011018] Published Fri Mar 08, 2013

Symmetry-protected topological states in systems where electronic interactions can be safely ignored have been theoretically predicted and experimentally confirmed. Theorists now investigate 3D systems of interacting bosons and find a new plethora of topological surface states with symmetry properties that are impossible to realize in a purely 2D electronic system.

[Phys. Rev. X 3, 011016] Published Thu Feb 28, 2013

Spin fluxons created by inserting magnetic fluxes into 2D correlated topological insulators provide a simple and effective way to identify these remarkable states of matter.

[Phys. Rev. X 3, 011015] Published Tue Feb 26, 2013

A combined theoretical and experimental study of a frustrated magnet (Ho₂Ti₂O₇) indicates the presence of a low-temperature spin-liquid state in which the correlations in the spin fluctuations are topological in nature.

[Phys. Rev. X 3, 011014] Published Thu Feb 21, 2013

Combining a technique that compresses information during measurement with standard detector arrays allows high-dimensional quantum entanglement to be efficiently characterized.

[Phys. Rev. X 3, 011013] Published Wed Feb 20, 2013

Surface spectroscopy shows that a material long known as a Kondo insulator also exhibits the metallic surface states of a topological insulator.

[Phys. Rev. X 3, 011011] Published Thu Feb 14, 2013

The Kochen-Specker theorem, which excludes noncontextual hidden-variable explanations for the counterintuitive puzzles of quantum mechanics, has been realized for the first time in two different single-photon experiments.

[Phys. Rev. X 3, 011012] Published Thu Feb 14, 2013

Voltage-gated ion channels regulate the flows of sodium or potassium ions across nerve cell membranes. A new study of a model potassium channel reveals that behind the channel’s regulation of the ion flow lies a viscoelastic molecular structural behavior similar to that of Silly Putty.

[Phys. Rev. X 3, 011010] Published Mon Feb 11, 2013

A molecular rheology experiment on a biological enzyme leads to a proposal that a fundamental representation of the functional cycle of the enzyme is provided by its molecular strain-stress response.

[Phys. Rev. X 3, 011009] Published Fri Feb 01, 2013