Research articles

Quantum many-body systems can show an elusive form of order known as topological order. Theoretical work now unifies several microscopic models whereby topological phases have been found, and predicts quantum phase transitions that are driven by quantum fluctuations of the topology.

Frequency combs have revolutionized frequency metrology. High-harmonic generation in atoms has led to fast sources of short-wavelength photons. Combining these two technologies enables the transfer of frequency combs to the vacuum-ultraviolet with potential applications in spectroscopy.

Formulating a consistent framework for relativistic thermodynamics has been a subject of controversy over the past century. A new approach for defining thermodynamic quantities makes predictions that are, in principle, testable, and which might lead to a natural extension of thermodynamics to general relativity.

As well as providing subatomic-scale real-space images of metals, the scanning tunnelling microscope also reveals momentum–space information. Now it is possible to use this technique to image a heavy-electron liquid and obtain information on orbital structures.

In a ‘striped’ superconductor, it may be possible to observe a superconducting state that, with increasing temperature, melts into a unique phase with charge-4e superconductivity, instead of the usual charge of 2e from paired electronic excitations.

The identification of the magnetic-fluctuation mode at a quantum phase transition of the archetypical heavy-fermion compound Ce_1−xLa_xRu₂Si₂ indicates that quantum criticality in this system is governed by collective antiferromagnetic behaviour, rather than by local magnetic moments as has been suggested.

The so-called hidden-order state in URu₂Si₂ is further obscured by conflicting experimental observations. A first-principles calculation shows that an order parameter with real and imaginary parts can explain many of these conflicts.

More efficient solar-energy conversion is possible if a single high-energy photon can be made to generate two electron–hole pairs in a cell, rather than a single pair plus heat. It is now shown that, contrary to expectation, this carrier multiplication is better in bulk semiconductor materials than in quantum dots.

One of the many unusual characteristics of graphene is that it shows ‘puddles’ of positive and negative charge throughout. A systematic scanning tunnelling microscope study shows that these puddles are not a consequence of ripples in graphene’s structure as originally thought, but are due to charged impurities below its surface.

Understanding the mechanical properties of DNA helps us to predict protein–DNA and DNA–DNA interactions. It is now shown that—with the aid of statistical physics—the melting temperature of DNA can be used to extract very detailed information about local flexibility.

When two identical photons hit a half-silvered mirror, quantum mechanics requires that both pass through or both be reflected in the same direction. Previously, this effect had only been demonstrated with photons from similar light sources. It has now been repeated with photons generated by two completely different physical processes.

Scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy are complementary probes, and yet the results of recent studies using these techniques on quasiparticle excitations in the copper oxide superconductors seem to be contradictory. In fact, there is no contradiction.

Using arguments from computational complexity theory, fundamental limitations are found for how efficient it is to calculate the ground-state energy of many-electron systems using density functional theory.

Bound entanglement is a particular class that is not distillable—that is, it cannot be converted into a pure maximally entangled state by means of local operations and classical communication. A four-qubit bound entangled state, or Smolin state, has now been created experimentally.

Magnetic switching is typically accomplished by using a driving field that stays on until the magnetization is rotated to its final position. An experiment demonstrates that, in antiferromagnets, inertial effects can be harnessed, such that only a short ‘kick’ is required to transfer sufficient momentum to the spin system for it to reorient.

When electrons are transported through a semiconductor quantum dot, they interact with nuclear spin in the host material. This interaction—often considered to be a nuisance—is now shown to provide a feedback mechanism that actively pulls the electron-spin Larmor frequency into resonance with that of an external microwave driving field.

Radiofrequency spectroscopy provides a microscopic probe of fermionic pairing in ultracold Fermi gases. Calculations now suggest that there is a one-to-one correspondence between the theory of these spectra and the theory of paraconductivity fluctuations in superconductors, that is, the effect of enhanced conductivity even before the system enters the superconducting state.

Pumping an atomic system with light at a wavelength that is longer than its resonance can lead to cooling. Conversely, it is now shown that pumping with shorter-wavelength light can lead to the stimulated emission of phonons—in analogy to the amplification of photons in lasers.

In semiconductor quantum dots, interactions between the confined electrons and the surrounding reservoir of nuclear spins limit the attainable electron-spin coherence. But the nuclear-spin reservoir can also take a constructive role, as it facilitates the locking of the optical quantum-dot resonance to the changing frequency of an external driving laser, as an experiment now demonstrates.

A two-dimensional lattice of vortices melts into an isotropic liquid with increasing temperature. A microscopic view of the melting transition reveals that this actually occurs in three steps, one of which is an unusual liquid-crystal-like vortex phase.