A large anisotropy in the electronic properties across a structural transition in several correlated systems has been identified as the key manifestation of electronic nematic order, breaking rotational symmetry. In this context, FeSe is attracting tremendous interest, since electronic nematicity develops over a wide range of temperatures, allowing accurate experimental investigation. Here we combine angle-resolved photoemission spectroscopy and theoretical calculations based on a realistic multiorbital model to unveil the microscopic mechanism responsible for the evolution of the electronic structure of FeSe across the nematic transition. We show that the self-energy corrections due to the exchange of spin fluctuations between hole and electron pockets are responsible for an orbital-dependent shrinking of the Fermi surface that affects mainly the $xz/yz$ parts of the Fermi surface. This result is consistent with our experimental observation of the Fermi surface in the high-temperature tetragonal phase, which includes the $xy$ electron sheet that was not clearly resolved before. In the low-temperature nematic phase, we experimentally confirm the appearance of a large ($\sim 50$ meV) $xz/yz$ splitting. It can be well reproduced in our model by assuming a moderate splitting between spin fluctuations along the $x$ and $y$ crystallographic directions. Our mechanism shows how the full entanglement between orbital and spin degrees of freedom can make a spin-driven nematic transition equivalent to an effective orbital order.

Indirect excitons in coupled quantum wells have long radiative lifetimes and form a cold quasi-two-dimensional population suitable for studying collective quantum effects. Here we report the observation of the exciton Mott transition from an insulating (excitons) to a conducting (ionized electron-hole pairs) phase, which occurs gradually as a function of carrier density and temperature. The transition is inferred from spectral and time-resolved photoluminescence measurements around a carrier density of $2\times {10}^{10}{\mathrm{cm}}^{-2}$ and temperatures of 12–16 K. An externally applied electric field is employed to tune the dynamics of the transition via the quantum-confined Stark effect. Our results provide evidence of a gradual nature of the exciton Mott transition.

We show that upon Tb substitution the interaction between the magnetic moments on the impurity ${\mathrm{Tb}}^{4+}$ ion and its surrounding ${\mathrm{Ir}}^{4+}$ ions is described by a “compass” model, i.e., an Ising-like interaction favoring the magnetic moments across each bond to align along the bond direction. Such an interaction nucleates quenched magnetic vortices near the impurities and drives a reentrant transition out of the antiferromagnetic ordered phase at low temperatures, hence quickly suppressing the Néel temperature, consistent with the experiment [J. C. Wang et al., Phys. Rev. B 92, 214411 (2015)]. As a by-product, we propose that the compass model can be realized in ordered double perovskites composed of spin-orbital-coupled $d^5$ ions and half-closed-shell $f^7$ ions.

Vanadium dioxide is of broad interest as a spin-$\frac{1}{2}$ electron system that realizes a metal-insulator transition near room temperature, due to a combination of strongly correlated and itinerant electron physics. Here, resonant inelastic x-ray scattering is used to measure the excitation spectrum of charge and spin degrees of freedom at the vanadium $L$ edge under different polarization and temperature conditions, revealing excitations that differ greatly from those seen in optical measurements. These spectra encode the evolution of short-range energetics across the metal-insulator transition, including the low-temperature appearance of a strong candidate for the singlet-triplet excitation of a vanadium dimer.

We present an effective Hamiltonian based real-space approach for studying the weak-coupling BCS to the strong-coupling Bose-Einstein condensate crossover in the two-dimensional attractive Hubbard model at finite temperatures. We introduce and justify an effective classical Hamiltonian to describe the thermal fluctuations of the relevant auxiliary fields. Our results for $T_c$ and phase diagrams compare very well with those obtained from more sophisticated and CPU-intensive numerical methods. We demonstrate that the method works in the presence of disorder and can be a powerful tool for a real-space description of the effect of disorder on superconductivity. From a combined analysis of the superconducting order parameter, the distribution of auxiliary fields, and the quasiparticle density of states, we identify the regions of metallic, insulating, superconducting, and pseudogapped behavior. Our finding of the importance of phase fluctuations for the pseudogap behavior is consistent with the conclusions drawn from recent experiments on NbN superconductors. The method can be generalized to study superconductors with nontrivial order-parameter symmetries by identifying the relevant auxiliary variables.

We employ the fully anisotropic $\mathrm{DFT}+U+J$ approach with the PBEsol functional to investigate ground-state magnetic and electronic properties of bulk binary manganese oxides: MnO, ${\mathrm{Mn}}_3{\mathrm{O}}_4, \alpha -{\mathrm{Mn}}_2{\mathrm{O}}_3$, and $\beta -{\mathrm{MnO}}_2$, in order of increasing Mn valence. The computed crystal structures, noncollinear magnetic ground states, and corresponding electronic structures are in good agreement with the experimental data and hybrid functional calculations available in the literature. We take into account the nonlinear core-valence interaction in our Mn pseudopotential designed by ourselves, as it has been proven to be important for transition-metal systems. Although the Hubbard $U$ term is capable by itself of opening a band gap, the explicitly defined exchange parameter $J$ plays an important role in improving the detailed electronic and noncollinear magnetic structure profiles. Appropriate band gaps are obtained with $U$ values smaller than those used in previously reported calculations. Our results suggest that pseudopotential design together with $\mathrm{DFT}+U+J$ enables the acquisition of accurate properties of complex magnetic systems using a nonhybrid density functional.

An analytical formula is reported describing the time-resolved dynamical Franz-Keldysh effect (Tr-DFKE) produced by an elliptically polarized laser at subfemtosecond timescale. The Houston function is assumed as the time-dependent wave function of the parabolic two-band system. For elliptical polarization, the resulting formula exhibits subcycle changes in the optical properties; the modulation of the dielectric function is smaller than that for linear polarization. In contrast, the subcycle modulation of the dielectric function, a significant feature of the Tr-DFKE, disappears for a circularly polarized laser. This analytical formula shows good qualitative agreement with the first-principle calculation employing the time-dependent density functional theory for diamond.

We present an experimental study of the quantum spin ice candidate pyrochlore compound ${\mathrm{Pr}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ by means of magnetization measurements, specific heat, and neutron scattering up to 12 T and down to 60 mK. When the field is applied along the $\left[111\right]$ and $\left[1\overline{1}0\right]$ directions, $\mathbf{k}=0$ field-induced structures settle in. We find that the ordered moment rises slowly, even at very low temperature, in agreement with macroscopic magnetization. Interestingly, for $H\parallel \left[1\overline{1}0\right]$, the ordered moment appears on the so-called $\alpha$ chains only. The spin excitation spectrum is essentially inelastic and consists in a broad flat mode centered at about 0.4 meV with a magnetic structure factor which resembles the spin ice pattern. For $H\parallel \left[1\overline{1}0\right]$ (at least up to 2.5 T), we find that a well-defined mode forms from this broad response, whose energy increases with $H$, in the same way as the temperature of the specific-heat anomaly. We finally discuss these results in the light of mean field calculations and propose an interpretation where quadrupolar interactions play a major role, overcoming the magnetic exchange. In this picture, the spin ice pattern appears shifted up to finite energy because of those interactions. We then propose a range of acceptable parameters for ${\mathrm{Pr}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$  that allow to reproduce several experimental features observed under field. With these parameters, the actual ground state of this material would be an antiferroquadrupolar liquid with spin-ice-like excitations.

The Kondo insulator ${\mathrm{SmB}}_6$ has long been known to display anomalous transport behavior at low temperatures, $T <5$ K. In this temperatures range, a plateau is observed in the dc resistivity, contrary to the exponential divergence expected for a gapped system. Recent theoretical calculations suggest that ${\mathrm{SmB}}_6$ may be the first topological Kondo insulator (TKI) and propose that the residual conductivity is due to topological surface states which reside within the Kondo gap. Since the TKI prediction many experiments have claimed to observe high mobility surface states within a perfectly insulating hybridization gap. Here, we investigate the low energy optical conductivity within the hybridization gap of single crystals of ${\mathrm{SmB}}_6$ via time domain terahertz spectroscopy. Samples grown by both optical floating zone and aluminum flux methods are investigated to probe for differences originating from sample growth techniques. We find that both samples display significant three-dimensional bulk conduction originating within the Kondo gap. Although ${\mathrm{SmB}}_6$ may be a bulk dc insulator, it shows significant bulk ac conduction that is many orders of magnitude larger than any known impurity band conduction. The nature of these in-gap states and their coupling with the low energy spin excitons of ${\mathrm{SmB}}_6$ is discussed. Additionally, the well-defined conduction path geometry of our optical experiments allows us to show that any surface states, which lie below our detection threshold if present, must have a sheet resistance of $R/\text{square}\ge$ 1000 $\mathrm{\Omega }$.

We introduce and test a nonlocal energy-optimized model kernel (NEO) within the adiabatic connection fluctuation-dissipation (ACFD) density-functional theory for the jellium surface and uniform electron gas, as benchmarks for simple metallic systems. Our model kernel is short ranged for the uniform electron gas paradigm system and one-electron self-correlation free. One-electron self-interaction freedom is provided by an iso-orbital indicator. We show how several versions of the NEO kernel perform for the uniform electron gas and jellium surface energies, and in addition we explain the underlying physics of self-interaction-free exchange-only kernels for exponentially decaying surface densities.

We present a theory of the phononic thermal (Kapitza) resistance at the interface between graphene or another single-layer two-dimensional (2D) crystal (e.g., ${\mathrm{MoS}}_2)$ and a flat substrate, based on a modified version of the cross-plane heat transfer model by Persson, Volokitin, and Ueba [J. Phys.: Condens. Matter 23, 045009 (2011)]. We show how intrinsic flexural phonon damping is necessary for obtaining a finite Kapitza resistance and also generalize the theory to encased single-layer 2D crystals with a superstrate. We illustrate our model by computing the thermal boundary conductance (TBC) for bare and ${\mathrm{SiO}}_2$-encased single-layer graphene and ${\mathrm{MoS}}_2$ on a ${\mathrm{SiO}}_2$ substrate, using input parameters from first-principles calculation. The estimated room temperatures TBC for bare (encased) graphene and ${\mathrm{MoS}}_2$ on ${\mathrm{SiO}}_2$ are 34.6 (105) and 3.10 (5.07) ${\mathrm{MW}\mathrm{K}}^{-1}{\mathrm{m}}^{-2}$, respectively. The theory predicts the existence of a phonon frequency crossover point, below which the low-frequency flexural phonons in the bare 2D crystal do not dissipate energy efficiently to the substrate. We explain within the framework of our theory how the encasement of graphene with a top ${\mathrm{SiO}}_2$ layer introduces new low-frequency transmission channels, which significantly reduce the graphene-substrate Kapitza resistance. We emphasize that the distinction between bare and encased 2D crystals must be made in the analysis of cross-plane heat dissipation to the substrate.

A gate electric field is a powerful way to manipulate the physical properties of nanojunctions made of two-dimensional crystals. To simulate field effects on the electronic structure of trilayer graphene, we used density functional theory in combination with the effective screening medium method, which enables us to understand the field-dependent layer-layer interactions and the fundamental physics underlying band gap variations and the resulting band modifications. Two different graphene stacking orders, Bernal (or ABC) and rhombohedral (or ABA), were considered. In addition to confirming the experimentally observed band gap opening in ABC-stacked and the band overlap in ABA-stacked trilayer systems, our results reveal rich physics in these fascinating systems, where layer-layer couplings are present but some characteristics features of single-layer graphene are partially preserved. For ABC stacking, the electric-field-induced band gap size can be tuned by charge doping, while for ABA band the tunable quantity is the band overlap. Our calculations show that the electronic structures of the two stacking orders respond very differently to charge doping. We find that in the ABA stacking hole doping can reopen a band gap in the band-overlapping region, a phenomenon distinctly different from electron doping. The physical origins of the observed behaviors were fully analyzed, and we conclude that the dual-gate configuration greatly enhances the tunability of the trilayer systems.

A newly developed hypergeometric resummation technique [H. Mera et al., Phys. Rev. Lett. 115, 143001 (2015)] provides an easy-to-use recipe to obtain conserving approximations within the self-consistent nonequilibrium many-body perturbation theory. We demonstrate the usefulness of this technique by calculating the phonon-limited electronic current in a model of a single-molecule junction within the self-consistent Born approximation for the electron-phonon interacting system, where the perturbation expansion for the nonequilibrium Green's function in powers of the free bosonic propagator typically consists of a series of noncrossing sunset diagrams. Hypergeometric resummation preserves conservation laws and it is shown to provide substantial convergence acceleration relative to more standard approaches to self-consistency. This result strongly suggests that the convergence of the self-consistent sunset series is limited by a branch-cut singularity, which is accurately described by Gauss hypergeometric functions. Our results showcase an alternative approach to conservation laws and self-consistency where expectation values obtained from conserving divergent perturbation expansions are summed to their self-consistent value by analytic continuation functions able to mimic the convergence-limiting singularity structure.

We study the effects that ripples induce on the electrical and magnetic properties of graphene. The variation of the interatomic distance created by the ripples translates in a modulation of the hopping parameter between carbon atoms. A tight-binding Hamiltonian including a Hubbard interaction term is solved self-consistently for ripples with different amplitudes and periods. We find that, for values of the Hubbard interaction $U$ above a critical value $U_C$, the system displays a superposition of local ferromagnetic and antiferromagnetic ordered states. Nonetheless, the global ferromagnetic order parameter is zero. The $U_C$ depends only on the product of the period and hopping amplitude modulation. When the Hubbard interaction is close to the critical value of the antiferromagnetic transition in pristine graphene, the antiferromagnetic order parameter becomes much larger than the ferromagnetic one, being the ground state similar to that of flat graphene.

Electron transport in thin metallic wires and films is strongly influenced by the quality of their surface. Weak localization and magnetoconductivity are also sensitive to the electron scattering at the edges of the sample. We study weak localization effects in a two-dimensional electron gas patterned in the form of a narrow quasi-one-dimensional channel in transverse magnetic field. The most general boundary conditions interpolating between the limits of mirror and diffuse edge scattering are assumed. We calculate magnetoconductivity for an arbitrary width of the sample including the cases of diffusive and ballistic lateral transport as well as the crossover between them. We find that in a broad range of parameters, the electron mobility is limited by the boundary roughness, while the magnetotransport is only weakly influenced by the quality of the edges. In addition, we calculate magnetoconductivity for a metallic cylinder in the transverse field and a quasi-two-dimensional metallic film in the parallel field.

We report a high-pressure study of monoclinic monazite-type $\mathrm{SrCr}{\mathrm{O}}_4$ up to 26 GPa. Therein we combined x-ray diffraction, Raman, and optical-absorption measurements with ab initio calculations, to find a pressure-induced structural phase transition of $\mathrm{SrCr}{\mathrm{O}}_4$ near 8–9 GPa. Evidence of a second phase transition was observed at 10–13 GPa. The crystal structures of the high-pressure phases were assigned to the tetragonal scheelite-type and monoclinic $\mathrm{AgMn}{\mathrm{O}}_4$-type structures. Both transitions produce drastic changes in the electronic band gap and phonon spectrum of $\mathrm{SrCr}{\mathrm{O}}_4$. We determined the pressure evolution of the band gap for the low- and high-pressure phases as well as the frequencies and pressure dependencies of the Raman-active modes. In all three phases most Raman modes harden under compression, however the presence of low-frequency modes which gradually soften is also detected. In monazite-type $\mathrm{SrCr}{\mathrm{O}}_4$, the band gap blueshifts under compression, but the transition to the scheelite phase causes an abrupt decrease of the band gap in $\mathrm{SrCr}{\mathrm{O}}_4$. Calculations showed good agreement with experiments and were used to better understand the experimental results. From x-ray-diffraction studies and calculations we determined the pressure dependence of the unit-cell parameters of the different phases and their ambient-temperature equations of state. The results are compared with the high-pressure behavior of other monazites, in particular $\mathrm{PbCr}{\mathrm{O}}_4$. A comparison of the high-pressure behavior of the electronic properties of $\mathrm{SrCr}{\mathrm{O}}_4$ ($\mathrm{SrW}{\mathrm{O}}_4$) and $\mathrm{PbCr}{\mathrm{O}}_4$ (${\mathrm{PbWO}}_4$) will also be made. Finally, the possible occurrence of a third structural phase transition is discussed.

The orbital magnetic susceptibility of an electron gas in a periodic potential depends not only on the zero field energy spectrum but also on the geometric structure of cell-periodic Bloch states which encodes interband effects. In addition to the Berry curvature, we explicitly relate the orbital susceptibility of two-band models to a quantum metric tensor defining a distance in Hilbert space. Within a simple tight-binding model allowing for a tunable Bloch geometry, we show that interband effects are essential even in the absence of Berry curvature. We also show that for a flat band model, the quantum metric gives rise to a very strong orbital paramagnetism.

We study the magnetization dynamics of thin-film magnetic elements with in-plane magnetization subject to a spin current flowing perpendicular to the film plane. We derive a reduced partial differential equation for the in-plane magnetization angle in a weakly damped regime. We then apply this model to study the experimentally relevant problem of switching of an elliptical element when the spin polarization has a component perpendicular to the film plane, restricting the reduced model to a macrospin approximation. The macrospin ordinary differential equation is treated analytically as a weakly damped Hamiltonian system, and an orbit-averaging method is used to understand transitions in solution behaviors in terms of a discrete dynamical system. The predictions of our reduced model are compared to those of the full Landau-Lifshitz-Gilbert-Slonczewski equation for a macrospin.

The pyrovanadate $\alpha -{\mathrm{Cu}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ belongs to the orthorhombic ($Fdd2$) class of crystals with noncentrosymmetric crystal structure. Recently, the compound has been identified to be a magnetic multiferroic with a substantial electric polarization below the magnetic transition temperature $T_C$ = 35 K. Here we report the results of our inelastic neutron-scattering (INS) studies on a polycrystalline sample of $\alpha -{\mathrm{Cu}}_2{\mathrm{V}}_2{\mathrm{O}}_7$. Our INS data clearly show the existence of dispersive spin wave excitations below $T_C$ with a zone-boundary energy of 11 meV at 5 K. We have analyzed the data using linear spin wave theory, which shows good agreement between the experiment and calculation. The analysis is consistent with the third-nearest-neighbor exchange interaction playing a dominant role in the magnetism of the material.

We study the hitherto unaddressed phenomenon of the quantum Hall effect with a magnetic and electric field oscillating in time with resonant frequencies. This phenomenon highlights an example of a heterodyne device with the magnetic field acting as a driving force, and it is analyzed in detail in its classical and quantum versions using Floquet theory. A bulk current flowing perpendicularly to the applied electric field is found, with a frequency shifted by integer multiples of the driving frequency. When the ratio of the cyclotron and driving frequency takes special values, the electron's classical trajectory forms a loop and the effective mass diverges, while in the quantum case we find an analog of the Landau quantization. A possible realization using metamaterial plasmonics is discussed.