The electronic structure of hexagonal boron nitride (h-BN) is explored using measurements of x-ray absorption and resonant inelastic x-ray scattering (RIXS) at the nitrogen K edge ($1\mathit{s}$) in tandem with calculations using many-body perturbation theory within the $\mathit{GW}$ and Bethe-Salpeter equation (BSE) approximations. Our calculations include the effects of lattice disorder from phonons activated thermally and from zero-point energy. They highlight the influence of disorder on near-edge x-ray spectra.

We show that the critical temperature of the ferromagnetic phase transition in a quasi-two-dimensional hole gas confined in a diluted magnetic semiconductor quantum well strongly depends on the hole chemical potential and hole density. Significant variations of the Curie temperature occur close to the Lifshitz topological transition points, where the hole Fermi surface acquires additional components of topological connectivity due to the filling of excited size-quantization subbands. The model calculations demonstrate that the Curie temperature can be doubled by a weak variation of the gate voltage for a CdMnTe/CdMgTe quantum-well-based device.

Strained two-dimensional crystals often offer novel physical properties that are usable to improve their electronic performance. Here we show by the theory of elasticity combined with the tight-binding approximation that local strains in silicene can open up new prospects for generating fully polarized spin and valley currents. The trajectory of electrons flowing through locally strained regions obeys the same behavior as light waves propagating in uniaxial anisotropic materials. The refraction angle of electrons at local strain boundaries exhibits a strong dependence on the valley degree of freedom, allowing for valley filtering based on the strain direction. The ability to control the spin polarization direction additionally requires a perpendicular electric field to be involved in combination with the local strain. Further similarities of the problem with optics of anisotropic materials are elucidated and possible applications in spin- and valleytronic nanodevices are discussed.

We consider a hydrodynamic description of transport for generic two-dimensional electron systems that lack Galilean invariance and do not fall into the category of Fermi liquids. We study magnetoresistance and show that it is governed only by the electronic viscosity provided that the wavelength of the underlying disorder potential is large compared to the microscopic equilibration length. We also derive the Coulomb drag transresistance for double-layer non-Fermi-liquid systems in the hydrodynamic regime. As an example, we consider frictional drag between two quantum Hall states with half-filled lowest Landau levels, each described by a Fermi surface of composite fermions coupled to a $U\left(1\right)$ gauge field. We contrast our results to prior calculations of drag of Chern-Simons composite particles and place our findings in the context of available experimental data.

While the atomistic Green's function (AGF) method has the potential to compute spectrally resolved phonon transport across interfaces, most prior formulations of the AGF method provide only the total phonon transmission function that includes contributions from all phonon branches or channels. In this work, we present a formulation of the conventional AGF technique in terms of phonon eigenspectra that provides a natural decomposition of the total transmission function into contributions from various phonon modes. The method involves the use of Dyson and Lippmann-Schwinger equations to determine surface Green's functions from the phonon eigenspectrum of the bulk, and establishes a direct connection between the transmission function and the bulk phonon spectra of the materials forming the interface. We elucidate our formulation of the AGF technique through its application to a microscopic picture of phonon mode conversion at Si-Ge interfaces with atomic intermixing. Intermixing of atoms near the interface is shown to increase the phase space available for phonon mode conversion and to enhance thermal interface conductance at moderate levels of atomic mixing. The eigenspectrum-based AGF method should be useful in determination of microscopic mechanisms of phonon scattering and identification of the specific modes that dominate thermal transport across an interface.

Using thermodynamic measurements, neutron diffraction, nuclear magnetic resonance, and muon spin relaxation, we establish putative quantum spin-liquid behavior in ${\mathrm{Ba}}_3{\mathrm{InIr}}_2{\mathrm{O}}_9$, where unpaired electrons are localized on mixed-valence ${\mathrm{Ir}}_2{\mathrm{O}}_9$ dimers with ${\mathrm{Ir}}^{4.5+}$ ions. Despite the antiferromagnetic Curie-Weiss temperature on the order of 10 K, neither long-range magnetic order nor spin freezing are observed down to at least 20 mK, such that spins are short-range correlated and dynamic over nearly three decades in temperature. Quadratic power-law behavior of both the spin-lattice relaxation rate and specific heat indicates the gapless nature of the ground state. We envisage that this exotic behavior may be related to an unprecedented combination of the triangular and buckled honeycomb geometries of nearest-neighbor exchange couplings in the mixed-valence setting.

We investigate spin Hall magnetoresistance (SMR) in heavy-metal (HM)/metallic-ferromagnet (FM) multilayers as a function of the FM thickness. We observe that the SMR is considerably modified when the FM layer is thinner than ∼1.5 nm, which is a critical thickness of a FM for the absorption or reflection of spin current generated from the spin Hall effect in a HM. Moreover, the FM thickness dependence of the SMR in HM/FM/HM trilayer structures relies on the relative signs of the spin Hall angle in HMs, indicating that there is non-negligible interaction between the accumulated spins at each HM/FM interface. We propose an analytical model of the SMR in a HM/FM/HM trilayer by incorporating the spin transmission through the HM/FM interfaces and the resultant interaction in the FM layer, which can explain the variation of the SMR with the relative signs of the spin Hall angle in HMs.

We have observed period-tripling subharmonic oscillations in a driven superconducting coplanar waveguide resonator operated in the quantum regime, $k_{B}T\ll \hslash \omega$. The resonator is terminated by a tunable inductance that provides a Kerr-type nonlinearity. We detected the output field quadratures at frequencies near the fundamental mode, $\omega /2\pi \sim 5\mathrm{GHz}$, when driving the resonator with a current at $3\omega$, with amplitude exceeding an instability threshold. We observed three stable radiative states with equal amplitudes, phase shifted by $2\pi /3$ rad, red detuned from the fundamental mode. The down-conversion from $3\omega$ to $\omega$ is strongly enhanced by near-resonant excitation of the second mode of the resonator and the cross-Kerr effect. Our experimental results are in quantitative agreement with a model for the driven dynamics of two coupled modes.

Exciton condensation in an electron-hole bilayer system of monolayer transition-metal dichalcogenides is analyzed at three different levels of theory to account for screening and quasiparticle renormalization. The large effective masses of the transition metal dichalcogenides place them in a strong coupling regime. In this regime, mean-field (MF) theory with either an unscreened or screened interlayer interaction predicts a room-temperature condensate. Interlayer and intralayer interactions renormalize the quasiparticle dispersion, and this effect is included in a $GW$ approximation. The renormalization reverses the trends predicted from the unscreened or screened MF theories. In the strong coupling regime, intralayer interactions have a large impact on the magnitude of the order parameter and its functional dependencies on effective mass and carrier density.

Periodic structures with a Bloch dispersion relation supporting a stationary inflection point (SIP) can display a unique scattering feature, the frozen mode regime (FMR). The FMR is much more robust than common cavity resonances; it is much less sensitive to boundary conditions, structural imperfections, and losses. Using perturbation theory, we analyze the FMR in a realistic case of a finite fragment of a periodic structure. We show that in close proximity to a SIP frequency, the character of the FMR is qualitatively different from the known case of a semi-infinite structure.

Motzkin spin chains are frustration-free models whose ground state is a combination of Motzkin paths. The weight of such path contributions can be controlled by a deformation parameter $t$. As a function of the latter, these models, besides the formation of domain wall structures, exhibit gapped Haldane topological orders with a constant decay of string order parameters for $t<1$. A behavior compatible with a Berezinskii-Kosterlitz-Thouless phase transition at $t=1$ is also presented. By means of numerical calculations we show that the topological properties of the Haldane phases depend on the spin value. This allows one to classify different kinds of hidden antiferromagnetic Haldane gapped regimes associated with nontrivial features such as symmetry-protected topological order. On one hand, our results allow one to clarify the physical properties of Motzkin frustration-free chains, and on the other hand, suggest them as an interesting and paradigmatic class of local spin Hamiltonians.

With decreasing temperature, ${\mathrm{Sr}}_2{\mathrm{VO}}_4$ undergoes two structural phase transitions, tetragonal-to-orthorhombic-to-tetragonal, without long-range magnetic order. Recent experiments suggest that only at very low temperature, ${\mathrm{Sr}}_2{\mathrm{VO}}_4$ might enter a yet-unknown phase with long-range magnetic order, but without orthorhombic distortion. By combining relativistic density functional theory with an extended spin-1/2 compass-Heisenberg model, we find an antiferromagnetic single-stripe ground state with highly competing exchange interactions, involving a non-negligible interlayer coupling, which places the system at the crossover between the XY-model and Heisenberg-model regimes. Most strikingly, we find a strong two-site “spin-compass” exchange anisotropy which is relieved by the orthorhombic distortion induced by the spin stripe order. Based on these results, we discuss the origin of the hidden-order phase and the possible formation of a spin liquid at low temperatures.

We present a solution to the problem of interpreting neural networks classifying phases of matter. We devise a procedure for reconstructing the decision function of an artificial neural network as a simple function of the input, provided the decision function is sufficiently symmetric. In this case one can easily deduce the quantity by which the neural network classifies the input. The method is applied to the Ising model and SU(2) lattice gauge theory. In both systems we deduce the explicit expressions of the order parameters from the decision functions of the neural networks. We assume no prior knowledge about the Hamiltonian or the order parameters except Monte Carlo–sampled configurations.

Recent experiments [A. M. Schaeffer et al., Proc. Natl. Acad. Sci. USA 112, 60 (2015)] have shown that lithium presents an extremely anomalous isotope effect in the 15–25 GPa pressure range. In this article we have calculated the anharmonic phonon dispersion of ${}^7\mathrm{Li}$ and ${}^6\mathrm{Li}$ under pressure, their superconducting transition temperatures, and the associated isotope effect. We have found a huge anharmonic renormalization of a transverse acoustic soft mode along $\mathrm{\Gamma }K$ in the fcc phase, the expected structure at the pressure range of interest. In fact, the anharmonic correction dynamically stabilizes the fcc phase above 25 GPa. However, we have not found any anomalous scaling of the superconducting temperature with the isotopic mass. Additionally, we have also analyzed whether the two lithium isotopes adopting different structures could explain the observed anomalous behavior. According to our enthalpy calculations including zero-point motion and anharmonicity it would not be possible in a stable regime.

Under various conditions of the growth process, when the presumably unconventional superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ (SRO) contains microinclusions of Ru metal, the superconducting critical temperature increases significantly. An atomic resolution high-angle annular-dark-field scanning transmission electron microscopy study shows a sharp interface geometry which allows crystals of SRO and of Ru metal to grow side by side by forming a commensurate superlattice structure at the interface. In an attempt to shed light on why this happens, we investigated the atomic structure and electronic properties of the interface between the oxide and the metal microinclusions using density functional theory calculations. Our results support the observed structure, indicating that it is energetically favored over other types of Ru-metal/SRO interfaces. We find that $t_{2g}-e_g$ orbital mixing occurs at the interface with significantly enhanced magnetic moments. Based on our findings, we argue that an inclusion-mediated interlayer coupling reduces phase fluctuations of the superconducting order parameter, which could explain the observed enhancement of the superconducting critical temperature in SRO samples containing microinclusions.

The effects of superconducting fluctuation (SCF) on the thermodynamic properties of electron systems in the so-called BCS-BEC crossover regime are studied. As the attractive interaction between electrons becomes stronger upon approaching the BCS-BEC crossover regime, the importance of the mode coupling between SCF drastically increases. The enhanced mode coupling leads to lowering of both the zero-field superconducting critical temperature and the depairing field $B_{\text{c2}}\left(T\right)$. Consequently, SCF-induced contributions to the specific heat and the diamagnetic susceptibility can seemingly exceed the corresponding values in the Gaussian approximation. We discuss the relevance of the present results to the anomalous SCF-induced diamagnetic response observed in the iron selenide (FeSe).

We revisit the effects of the Hubbard repulsion on quantum spin Hall effects (QSHE) in two-dimensional quantum lattice models. We present both unbiased exact diagonalization and density-matrix renormalization group simulations with numerical evidence for a continuous quantum phase transition (CQPT) separating QSHE from the topologically trivial antiferromagnetic phase. Our numerical results suggest that the nature of CQPT exhibits distinct finite-size scaling behaviors, which may be consistent with either Ising or $XY$ universality classes for different time-reversal symmetric QSHE systems.

We report a comprehensive study of exciton confinement in self-assembled InAs quantum dots (QDs) in strain-engineered metamorphic $\mathrm{I}{\mathrm{n}}_x\mathrm{G}{\mathrm{a}}_{1\text{–}x}\mathrm{As}$ confining layers on GaAs using low-temperature magnetophotoluminescence. As the lattice mismatch (strain) between QDs and confining layers (CLs) increases from 4.8% to 5.7% the reduced mass of the exciton increases, but saturates at higher mismatches. At low QD-CL mismatch there is clear evidence of spillover of the exciton wave function due to small localization energies. This is suppressed as the In content $x$ in the CLs decreases (mismatch and localization energy increasing). The combined effects of low effective mass and wave-function spillover at high $x$ result in a diamagnetic shift coefficient that is an order of magnitude larger than for samples where In content in the barrier is low (mismatch is high and localization energy is large). Finally, an anomalously small measured Bohr radius in samples with the highest $x$ is attributed to a combination of thermalization due to low localization energy, and its enhancement with magnetic field, a mechanism which results in small dots in the ensemble dominating the measured Bohr radius.

Recently, two-dimensional (2D) transition-metal carbides and nitrides, namely, MXenes have attracted a lot of attention for electronic and energy storage applications. Due to a large spin-orbit coupling and the existence of a Dirac-like band at the Fermi energy, it has been theoretically proposed that some of the MXenes will be topological insulators (TIs). Up to now, all of the predicted TI MXenes belong to transition-metal carbides, whose transition-metal atom is W, Mo, or Cr. Here, on the basis of first-principles and ${\mathbb{Z}}_2$ index calculations, we demonstrate that some of the MXene nitrides can also be TIs. We find that ${\mathrm{Ti}}_3{\mathrm{N}}_2{\mathrm{F}}_2$ is a 2D TI, whereas ${\mathrm{Zr}}_3{\mathrm{N}}_2{\mathrm{F}}_2$ is a semimetal with nontrivial band topology and can be turned into a 2D TI when the lattice is stretched. We also find that the tensile strain can convert ${\mathrm{Hf}}_3{\mathrm{N}}_2{\mathrm{F}}_2$ semiconductor into a 2D TI. Since Ti is one of the most used transition-metal elements in the synthesized MXenes, we expect that our prediction can advance the future application of MXenes as TI devices.