We report low-energy inelastic neutron scattering data of the paramagnetic (PM) to hidden-order (HO) phase transition at $T_0=17.5\mathrm{K}$ in ${\mathrm{URu}}_2{\mathrm{Si}}_2$. While confirming previous results for the HO and PM phases, our data reveal a pronounced wave-vector dependence of low-energy excitations across the phase transition. To analyze the energy scans we employ a damped harmonic oscillator model containing a fit parameter $1/\mathrm{\Gamma }$ which is expected to diverge at a second-order phase transition. Counter to expectations the excitations at ${\vec{Q}}_1\approx (1.4,0,0)$ show an abrupt steplike suppression of $1/\mathrm{\Gamma }$ below $T_0$, whereas excitations at ${\vec{Q}}_0=(1,0,0)$, associated with large-moment antiferromagnetism (LMAF) under pressure, show an enhancement and a pronounced peak of $1/\mathrm{\Gamma } at T_0$. Therefore, at the critical HO temperature $T_0$, LMAF fluctuations become nearly critical as well. This is the behavior expected of a “supervector” order parameter with nearly degenerate components for the HO and LMAF leading to nearly isotropic fluctuations in the combined order-parameter space.

We develop a fully quantum-mechanical approach, based on stochastic trajectories, for an incoherently pumped system of interacting bosons in contact with a thermal reservoir. It enables a complete characterization of coherence in such a multimode system. We apply our model to microcavity exciton polaritons interacting with acoustic phonons and observe the formation of a macroscopic occupation in the lowest-energy mode accompanied by the simultaneous establishment of temporal and spatial coherence. We describe the transition from thermal to coherent statistics together with the expected emergence of an off-diagonal long-range order.

Transition-metal (TM) oxides play an increasingly important role in technology today, including applications such as catalysis, solar energy harvesting, and energy storage. In many of these applications, the details of their electronic structure near the Fermi level are critically important for their properties. We propose a first-principles–based computational methodology for the accurate prediction of oxygen charge transfer in TM oxides and lithium TM (Li-TM) oxides. To obtain accurate electronic structures, the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional is adopted, and the amount of exact Hartree-Fock exchange (mixing parameter) is adjusted to reproduce reference band gaps. We show that the HSE06 functional with optimal mixing parameter yields not only improved electronic densities of states, but also better energetics (Li-intercalation voltages) for $\mathrm{LiCo}{\mathrm{O}}_2$ and $\mathrm{LiNi}{\mathrm{O}}_2$ as compared to the generalized gradient approximation (GGA), Hubbard $U$ corrected GGA ($\mathrm{GGA}+U$), and standard HSE06. We find that the optimal mixing parameters for TM oxides are system specific and correlate with the covalency (ionicity) of the TM species. The strong covalent (ionic) nature of TM-O bonding leads to lower (higher) optimal mixing parameters. We find that optimized HSE06 functionals predict stronger hybridization of the $\mathrm{Co}3d$ and $\mathrm{O}2p$ orbitals as compared to GGA, resulting in a greater contribution from oxygen states to charge compensation upon delithiation in $\mathrm{LiCo}{\mathrm{O}}_2$. We also find that the band gaps of Li-TM oxides increase linearly with the mixing parameter, enabling the straightforward determination of optimal mixing parameters based on GGA ($\alpha =0.0$) and HSE06 $(\alpha =0.25)$ calculations. Our results also show that $G_0{W}_0@\mathrm{GGA}+U$ band gaps of TM oxides ($M\mathrm{O},M=\mathrm{Mn},\mathrm{Co},\mathrm{Ni}$) and $\mathrm{LiCo}{\mathrm{O}}_2$ agree well with experimental references, suggesting that $G_0{W}_0$ calculations can be used as a reference for the calibration of the mixing parameter in cases when no experimental band gap has been reported.

We demonstrate, using first-principles calculations, that the electronic structure of ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x(x=0.5)$ is topologically nontrivial and characterized by an odd ${\mathbb{Z}}_2$ invariant and Dirac cone type surface states, in sharp contrast to the end member FeSe $(x=0)$. This topological state is induced by the enhanced three-dimensionality and spin-orbit coupling due to Te substitution (compared to FeSe), and characterized by a band inversion at the $Z$ point of the Brillouin zone, which is confirmed by our ARPES measurements. The results suggest that the surface of ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ may support a nontrivial superconducting channel in proximity to the bulk.

Raman-assisted hopping can allow for the creation of density-dependent synthetic magnetism for cold neutral gases in optical lattices. We show that the density-dependent fields lead to a nontrivial interplay between density modulations and chirality. This interplay results in a rich physics for atoms in two-leg ladders, characterized by a density-driven Meissner-superfluid to vortex-superfluid transition, and a nontrivial dependence of the density imbalance between the legs. Density-dependent fields also lead to intriguing physics in square lattices. In particular, it leads to a density-driven transition between a nonchiral and a chiral superfluid, both characterized by nontrivial charge density-wave amplitude. We finally show how the physics due to the density-dependent fields may be easily probed in experiments by monitoring the expansion of doublons and holes in a Mott insulator, which presents a remarkable dependence on quantum fluctuations.

In the polar catastrophe scenario, polar discontinuity accounts for the driving force of the formation of a two-dimensional electron gas (2DEG) at the interface between polar and nonpolar insulators. In this paper, we substitute the usual, nonferroelectric, polar material with a ferroelectric thin film and use the ferroelectric polarization as the source for polar discontinuity. We use ab initio simulations to systematically investigate the stability, formation, and properties of the two-dimensional free-carrier gases formed in $\mathrm{PbTi}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures under realistic mechanical and electrical boundary conditions. Above a critical thickness, the ferroelectric layers can be stabilized in the out-of-plane monodomain configuration due to the electrostatic screening provided by the free carriers. Our simulations also predict that the system can be switched between three stable configurations (polarization up, down, or zero), allowing the nonvolatile manipulation of the free-charge density and sign at the interface. Furthermore, the link between ferroelectric polarization and free-charge density demonstrated by our analysis constitutes compelling support for the polar catastrophe model that is used to rationalize the formation of 2DEG at oxide interfaces.

Nonreciprocal devices that permit wave transmission in only one direction are indispensible in many fields of science including, e.g., electronics, optics, acoustics, and thermodynamics. Manipulating phonons using such nonreciprocal devices may have a range of applications such as phonon diodes, transistors, switches, etc. One way of achieving nonreciprocal phononic devices is to use materials with strong nonlinear response to phonons. However, it is not easy to obtain the required strong mechanical nonlinearity, especially for few-phonon situations. Here we present a general mechanism to amplify nonlinearity using parity-time ($\mathcal{PT}$)-symmetric structures, and show that an on-chip microscale phonon diode can be fabricated using a $\mathcal{PT}$-symmetric mechanical system, in which a lossy mechanical resonator with very weak mechanical nonlinearity is coupled to a mechanical resonator with mechanical gain but no mechanical nonlinearity. When this coupled system transits from the $\mathcal{PT}$-symmetric regime to the broken-$\mathcal{PT}$-symmetric regime, the mechanical nonlinearity is transferred from the lossy resonator to the one with gain, and the effective nonlinearity of the system is significantly enhanced. This enhanced mechanical nonlinearity is almost lossless because of the gain-loss balance induced by the $\mathcal{PT}$-symmetric structure. Such an enhanced lossless mechanical nonlinearity is then used to control the direction of phonon propagation, and can greatly decrease (by over three orders of magnitude) the threshold of the input-field intensity necessary to observe the unidirectional phonon transport. We propose an experimentally realizable lossless low-threshold phonon diode of this type. Our study opens up perspectives for constructing on-chip few-phonon devices and hybrid phonon-photon components.

We suggest a more nuanced view of the merit and utility of generalized gradient approximations (GGAs) for the noninteracting kinetic energy (KE) than the critique of Xia and Carter (XC) [Phys. Rev. B 91, 045124 (2015)]. Specifically, the multiple valuedness of the Pauli term enhancement factor (denoted $G\left[n\right]$ by XC) with respect to the inhomogeneity variable $s$ can be excluded by enforcement of a bound on the Kohn-Sham KE to achieve universality of the functional along with enforcement of proper large-$s$ behavior. This is physically sensible in that the excluded $G$ values occur for $s$ values that correspond to low densities. The behavior is exacerbated by peculiarities of pseudodensities. The VT84F KE GGA, constructed with these constraints, does not have the numerical instability in our older PBE2 functional analyzed by XC.

We find that the multivalued character of the $G$ factor as a function of the reduced gradient ($s$) still exists after accounting for pseudopotential artifacts and the kinetic energy global upper bound. We also find that the VT84F functional indeed exhibits stable convergence and more reasonable results for self-consistent bulk properties compared to other generalized gradient approximation (GGA) kinetic energy density functionals (KEDFs) that we tested earlier. However, VT84F generally yields overestimated equilibrium volumes, which may result from its inability (as with all GGAs) to reproduce the $G-s$ multivalued character. The analogous failure to predict the multivalued character of $G$ as a function of the reduced density $\left(d\right)$ is also likely to be responsible for the inaccuracy of our vWGTF functionals reported earlier. Our multivaluedness analysis therefore does not impugn any particular GGA KEDF. Instead, it merely confirms the importance of pointwise analysis for improving KEDFs by emphasizing the need to resolve the multivaluedness of $G$ with respect to various density variables.

Exciton harvesting is demonstrated in an ensemble of quantum emitters coupled to localized surface plasmons. When the interaction between emitters and the dipole mode of a metallic nanosphere reaches the strong-coupling regime, the exciton conductance is greatly increased. The spatial map of the conductance matches the plasmon field intensity profile, which indicates that transport properties can be tuned by adequately tailoring the field of the plasmonic resonance. Under strong coupling, we find that pure dephasing can have detrimental or beneficial effects on the conductance, depending on the effective number of participating emitters. Finally, we show that the exciton transport in the strong-coupling regime occurs on an ultrafast time scale given by the inverse Rabi splitting ($\sim 10$ fs), which is orders of magnitude faster than transport through direct hopping between the emitters.

Valleys in monolayer transition-metal dichalcogenides seamlessly connect two basic carriers of quantum information, namely, the electron spin and photon helicity. Lifting the valley degeneracy is an attractive route to achieve further optoelectronic manipulations. However, the magnetic field only creates a very small valley splitting. We propose a strategy to create giant valley splitting by the proximity-induced Zeeman effect. Our first principles calculations of monolayer ${\mathrm{MoTe}}_2$ on a EuO substrate show that valley splitting over 300 meV can be generated. Interband transition energies become valley dependent, leading to selective spin-photon coupling by optical frequency tuning. The valley splitting is also continuously tunable by rotating the substrate magnetization. The giant and tunable valley splitting adds a different dimension to the exploration of unique optoelectronic devices based on magneto-optical coupling and magnetoelectric coupling.

We present a theory describing spiral magnets with Dzyaloshinskii-Moriya interaction (DMI) subject to bond disorder at small concentration $c$ of defects. It is assumed that both DMI and exchange coupling are changed on imperfect bonds. Qualitatively the same physical picture is obtained in two models which are considered in detail: B20 cubic helimagnets and layered magnets in which DMI leads to a long-period spiral ordering perpendicular to layers. We find that the distortion of the spiral magnetic ordering around a single imperfect bond is long range: values of additional turns of spins decay with the distance $r$ to the defect as $1/r^2$ being governed by the Poisson's equation for electric dipole. At finite concentration of randomly distributed imperfect bonds, we calculate correction to the spiral vector. We show that this correction can change the sign of spin chirality even at $c\ll 1$ if defects are strong enough. It is demonstrated that impurities lead to a diffuse elastic neutron scattering which has power-law singularities at magnetic Bragg peaks positions. Then, each Bragg peak acquires power-law decaying tails. Corrections are calculated to the magnon energy and to its damping caused by scattering on impurities.

A three-dimensional electron topological insulator (ETI) is a phase of matter protected by particle-number conservation and time-reversal symmetry. It was previously believed that the surface of an ETI must be gapless unless one of these symmetries is broken. A well-known symmetry-preserving, gapless surface termination of an ETI supports an odd number of Dirac cones. In this paper, we deduce a symmetry-respecting, gapped surface termination of an ETI, which carries an intrinsic two-dimensional (2d) topological order, $\text{Moore-Read}\times \mathrm{U}{\left(1\right)}_{-2}$. The Moore-Read sector supports non-Abelian charge $1/4$ anyons, while the Abelian, $\mathrm{U}{\left(1\right)}_{-2}$, (antisemion) sector is electrically neutral. Time-reversal symmetry is implemented in this surface phase in a highly nontrivial way. Moreover, it is impossible to realize this phase strictly in 2d, simultaneously preserving its implementation of both the particle-number and time-reversal symmetries. A one-dimensional (1d) edge on the ETI surface between the topologically ordered phase and the topologically trivial time-reversal-broken phase with a Hall conductivity ${\sigma }_{xy}=1/2$ carries a right-moving neutral Majorana mode, a right-moving bosonic charge mode, and a left-moving bosonic neutral mode. The topologically ordered phase is separated from the surface superconductor by a direct second-order phase transition in the $X{Y}^{*}$ universality class, which is driven by the condensation of a charge $1/2$ boson, when approached from the topologically ordered side, and proliferation of a flux $4\pi (2hc/e)$ vortex, when approached from the superconducting side. In addition, we prove that time-reversal invariant (interacting) electron insulators with no intrinsic topological order and electromagnetic response characterized by a $\theta$ angle, $\theta =\pi$, do not exist if the electrons transform as Kramers singlets under time reversal.

We report the change of electronic structure associated with the hidden-order (HO) transition in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ through Compton scattering experiment on the (001) plane of the single crystal. The two-dimensional electron occupation number densities (EONDs) obtained at 14 and 20 K, which are the projection of Fermi volume onto the $k_x-k_y$ plane of the first Brillouin zone, clearly reflect a whole image of electronic structure in the HO and paramagnetic (PM) phases, respectively. The change in electronic structure is well described by theoretical EONDs derived from the result of a band calculation, where $\mathrm{U}5f$ electrons are treated as itinerant ones. We also evaluate the electron (hole) numbers in the HO and PM phases and, therefore, its change on the HO transition. The HO transition is deeply involved with a significant change in $5f$ electrons from partially localized to itinerant states through hybridization with conduction electrons, resulting in the reconstruction of Fermi-surface structure.

Many techniques have been developed in the past in order to compute positron lifetimes in materials from first principles. However, there is still a lack of a fast and accurate self-consistent scheme that could handle accurately the forces acting on the ions induced by the presence of the positron. We will show in this paper that we have reached this goal by developing the two-component density functional theory within the projector augmented-wave (PAW) method in the open-source code abinit. This tool offers the accuracy of the all-electron methods with the computational efficiency of the plane-wave ones. We can thus deal with supercells that contain few hundreds to thousands of atoms to study point defects as well as more extended defects clusters. Moreover, using the PAW basis set allows us to use techniques able to, for instance, treat strongly correlated systems or spin-orbit coupling, which are necessary to study heavy elements, such as the actinides or their compounds.

Inelastic scattering off magnetic impurities in a spin-chiral two-dimensional electron gas, e.g., the Rashba system, is shown to generate topological changes in the spin texture of the electron waves emanating from the scattering center. While elastic scattering gives rise to a purely in-plane spin texture for an in-plane magnetic scattering potential, out-of-plane components emerge upon activation of inelastic scattering processes. This property leads to a possibility to make controlled transitions between trivial and nontrivial topologies of the spin texture.

A resonance Raman study of graphene samples with different ${}^{13}\mathrm{C}$ isotopic concentrations and using different laser excitation energies is presented. The main Raman peaks ($D, G, G^{*}$, and $2D$) of graphene were measured and the dependence of their frequencies on the isotope atomic mass follows a simple harmonic oscillator relation. The $G^{*}$ and $2D$ double-resonance peak positions were measured as a function of the laser energy, and we observed that the slopes of the laser energy dependence are the same independently of isotope concentration. This result shows that isotopic substitution does not alter the electron and phonon dispersions near the $K$ point of the graphene Brillouin zone. From the linewidth of $G$ and $2D$ Raman peaks, we have also obtained a dependence of the phonon lifetime on the ${}^{13}\mathrm{C}$ isotope concentration.

We experimentally investigate the width of extended states in disorder-broadened Landau levels (LLs) in top-gated epitaxial graphene on silicon carbide using two different methods: gated transport spectroscopy and activation gap measurements on integer quantum Hall states. The transport spectroscopy reveals that the widths of the extended states in the zero-energy ($N=0$) and first excited ($N=1$) LLs are of similar magnitude over the ranges of magnetic field (4–16 T) and temperature studied (1.6–150 K). Under certain assumptions we find that the extended-state width follows a power-law temperature dependence with the exponent $\eta \sim 0.3$ in the $N=0$ ($N=1$) LL, with almost no (very weak) magnetic-field dependence. Activation gap measurements at the filling factors of $\nu =2$ and 6 give results consistent with transport spectroscopy for the $N=1$ LL, but indicate a larger broadening for the $N=0$ LL than deduced from the spectroscopy.