We report comprehensive pair distribution function measurements of the hole-doped iron-based superconductor system ${\mathrm{Sr}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. Structural refinements performed as a function of temperature and length scale reveal orthorhombic distortions of the instantaneous local structure across a large region of the phase diagram possessing average tetragonal symmetry, indicative of fluctuating nematicity. These nematic fluctuations are present up to high doping levels ($x\gtrsim 0.48$, near optimal superconductivity) and high temperatures (above room temperature for $x=0$, decreasing to 150 K for $x=0.48$), with a typical length scale of 1–3 nm. This work highlights the ubiquity of nematic fluctuations in a representative iron-based superconductor and provides important details about the evolution of these fluctuations across the phase diagram.

Impurity scattering is found to lead to quasi-one-dimensional nanoscale modulation of the local density of states in iron pnictides and chalcogenides. This “quasiparticle interference” feature is remarkably similar across a wide variety of pnictide and chalcogenide phases, suggesting a common origin. We show that a unified understanding of the experiments can be obtained by simply invoking a fourfold symmetry breaking $d_{xz}\text{−}{d}_{yz}$ orbital splitting, of a magnitude already suggested by the experiments. This can explain the one-dimensional characteristics in the local density of states observed in the orthorhombic nematic, tetragonal paramagnetic, as well as the spin-density-wave and superconducting states in these materials.

We study the local density of the Bose-Hubbard model in the presence of on-site disorder near the Bose-glass transition using multifractal, typical medium, and percolation theories. At incommensurate filling our findings support the scenario of percolating superfluid clusters enhancing Anderson localization. Scaling analysis of the superfluid density at the incommensurate filling of $\rho =1.1$ and on-site interaction $U=80t$ predicts a superfluid-Bose-glass transition at disorder strength of ${\mathrm{\Delta }}_c\approx 30t$. At this filling the local-density distribution becomes more skew with increasing disorder strength. Multifractal analysis suggests a multifractal behavior resembling that of the Anderson localization. In the Bose-glass phase the mode of the local-density distribution approaches an integer value as expected from typical medium theory for the Anderson localization. Percolation analysis points to a phase transition of percolating noninteger filled sites around the same value of disorder. On the other hand, the behavior at commensurate filling is rather different. Close to the tip of the Mott lobe ($\rho =1,U=22t$) we find a Mott-insulator-Bose-glass transition at disorder strength of ${\mathrm{\Delta }}_c\approx 16t$. An analysis of the local-density distribution shows Gaussian-like behavior for a wide range of disorders above and below the transition.

We conduct experimental simulations of many-body quantum systems using a hybrid classical-quantum algorithm. In our setup, the wave function of the transverse field quantum Ising model is represented by a restricted Boltzmann machine. This neural network is then trained using variational Monte Carlo assisted by a $D$-wave quantum sampler to find the ground-state energy. Our results clearly demonstrate that already the first generation of quantum computers can be harnessed to tackle nontrivial problems concerning physics of many-body quantum systems.

For any symmorphic magnetic space group $G$, we prove that topological band insulators (with vanishing first Chern numbers) cannot have a ground state composed of a single, energetically isolated band. This no-go statement means that such topological insulators cannot be realized in tight-binding models with a single, filled, low-energy band. An implication is that the minimal dimension of the tight-binding Hamiltonian (at each wave vector) is four, if the topological insulator is stable, i.e., the filled bands remain topological upon addition of nontopological bands. Otherwise, if the topological insulator is unstable, the minimal dimension is three. In addition to our no-go statement, we present a surefire recipe to model Chern insulators and unstable topological insulators, by energetically splitting elementary band representations; this recipe, combined with recently constructed Bilbao tables on band representations, can be systematized for high-throughput identification of magnetic and time-reversal-invariant topological materials. All stated results follow from our theorem which applies to any single, isolated energy band of a $G$-symmetric Schrödinger-type or tight-binding Hamiltonian: for such bands, being topologically trivial (in the category of complex vector bundles) is equivalent to being a band representation of $G$.

We study the nonequilibrium effects of spin and/or electric currents on the helical indirect exchange interactions of local spins that embedded in general open electronic systems. Especially, besides the synthesized anisotropic Heisenberg interactions, we find that the synthetic helical indirect exchange interactions possess two parts: antisymmetric (Dzyaloshinskii-Moriya interaction) and symmetric (Kaplan-Shekhtman-Entin-Wohlman-Aharony interaction), which are all formulated in terms of Keldysh nonequilibrium Green's functions. The presence of either spin-orbit coupling or spin polarized currents alone is able to synthesize and control the antisymmetric Dzyaloshinskii-Moriya exchange interactions, as the same direction as spin splitting. However, the appearance of symmetric Kaplan-Shekhtman-Entin-Wohlman-Aharony interactions requires both, i.e., the spin-orbit coupling and spin polarized currents with different splitting directions. Our results show the detailed scheme of controlling the sign, magnitude, and direction of indirect Dzyaloshinskii-Moriya vectors and Kaplan-Shekhtman-Entin-Wohlman-Aharony interactions at nonequilibrium in open quantum devices.

Spin injection efficiency has been a key factor for designing spintronic devices based on the lateral nonlocal spin valve. Here, we suggest a type of nonlocal spin valve in which the spin is injected electrically via dual ferromagnetic electrodes similar to a vertical spin valve and the spin detection is done by another ferromagnetic electrode. Using spin drift-diffusion theory, we study the spin injection, transport, and detection analytically and numerically. The results show that the spin injection efficiency can be enhanced significantly compared to the usual single ferromagnetic electrode injection, and thus the detected spin signal is increased greatly. Besides, by controlling the magnetization orientations of the two injecting ferromagnetic electrodes as well as the magnetization direction of the detection electrode, one can switch on and off the spin injection and thus form three or four output spin states. This feature is used for designing all spin-based three-state logic devices.

Highly disordered superconductors have a rich phase diagram. At a moderate magnetic field ($B$) the samples go through the superconductor-insulator quantum phase transition. In the insulating phase, the resistance increases sharply with $B$ up to a magnetoresistance peak beyond which the resistance drops with $B$. In this paper we follow the temperature ($T$) evolution of this magnetoresistance peak. We show that as $T$ is reduced, the peak appears at lower $B$'s approaching the critical field of the superconductor-insulator transition. Due to experimental limitations we are unable to determine whether the $T=0$ limiting position of the peak matches that of the critical field or is at comparable but slightly higher $B$. We show that, although the peak appears at different $B$ values, its resistance follows an activated $T$ dependence over a large $T$ range with a prefactor that is very similar to the quantum of resistance for Cooper pairs.

${\mathrm{LaPtGe}}_3$ and ${\mathrm{CePtGe}}_3$ crystallize with a noncentrosymmetric body-centered tetragonal (space group $I4mm$)${\mathrm{BaNiSn}}_3$-type of structure. ${\mathrm{LaPtGe}}_3$ is a weakly coupled BCS-like $s$-wave type-I superconductor with $T_{\mathrm{c}}=0.55$ K, $B_{\mathrm{c}}\simeq 14$ mT, and Ginzburg-Landau parameter ${\kappa }_{\mathrm{GL}}=0.021<1/\sqrt{2}$. ${\mathrm{CePtGe}}_3$ is a nonsuperconducting ($T_{\mathrm{nsc}}=0.35$ K) metal with an effective magnetic moment ${\mu }_{\mathrm{eff}}=2.46{\mu }_{\mathrm{B}}$. The Ce moments show two antiferromagnetic ordering transitions at $T_{\mathrm{N}1}\approx 3.7$ K and $T_{\mathrm{N}2}\approx 2.7$ K and a ground-state multiplet $J=5/2$ splitting into three doublets (energy splittings from the ground state ${\mathrm{\Delta }}_1=46$ K and ${\mathrm{\Delta }}_2=137$ K). The moderately enhanced Sommerfeld coefficient ${\gamma }_0$ and the observation of almost the full critical magnetic entropy at $T_{\mathrm{N}1}$ suggest that magnetic Ruderman-Kittel-Kasuya-Yosida-type interactions are dominant in ${\mathrm{CePtGe}}_3$, leading to a magnetically ordered ground state.

Depositing monolayer graphene on a transition metal dichalcogenide (TMD) semiconductor substrate has been shown to change the dynamics of the electronic states in graphene, inducing spin-orbit coupling (SOC) and staggered potential effects. Theoretical studies on commensurate supercells have demonstrated the appearance of interesting phases, as different materials and relative gate voltages are applied. Here we address the effects of the real incommensurability between lattices by implementing a continuum model approach that does not require small-period supercells. The approach allows us to study the role of possible relative twists of the layers, and verify that the SOC transfer is robust to twists, in agreement with observations. We characterize the nature of the different phases by studying an effective Hamiltonian that fully describes the graphene-TMD heterostructure. We find the system supports topologically nontrivial phases over a wide range of parameter ranges, which require the dominance of the intrinsic SOC over the staggered and Rashba potentials. This tantalizing result suggests the possible experimental realization of a tunable quantum spin Hall phase under suitable conditions. We estimate that most TMDs used to date likely result in weak intrinsic SOC that would not drive the heterostructure into topologically nontrivial phases. Additional means to induce a larger intrinsic SOC, such as strain fields or heavy metal intercalation, may be required.

We explore the anisotropy in low frequency conductivity as a function of hole doping in multiorbital models of pnictides by analyzing the Drude weight in the $x$ and $y$ directions of a ($\pi$, 0) spin-density wave state. A reduction in the conductivity anisotropy with increased hole doping, and subsequent sign reversal, both observed in experiments, is found to be a common trend in these models. This behavior arises due to the interplay of low energy orbitally resolved density of states and the geometrical features of the Fermi surface. An understanding of anisotropy in the electron doped regime, however, would require additional ingredients.

The many-body ground state wave function of a fractional Chern insulator (FCI) can be constructed by mapping a fractional quantum Hall state to a FCI state through a substitution of the Bloch state of the flat Chern band for the magnetic Bloch state of the Landau level. There is a gauge freedom in choosing the single particle Bloch basis of the flat Chern band. Instead of considering only one form of interaction in FCI when choosing the gauge as done in previous works, we determine the optimal gauges for FCIs with different forms of interaction, including a short-range interaction, the Coulomb interaction, and an interpolation between them, by applying the variational principle proposed by Zhang et al. [Phys. Rev. B 93, 165129 (2016)]. We find that the optimal gauge strongly depends on the form of interaction. It contradicts with the common belief that the wave function of FCI is not sensitive to the interaction. In comparing the optimal gauge with the previous gauges proposed by Qi [Phys. Rev. Lett. 107, 126803 (2011)] and Wu et al. [Phys. Rev. B 86, 085129 (2012)], we find Wu et al.'s gauge is close to the optimal one when the interaction is a certain mixture of the Coulomb interaction and the short-range interaction, while Qi's gauge is qualitatively different from the optimal gauge in all the cases.

The effects of epitaxial strain on the structural, electronic, and magnetic properties in perovskite $R{\mathrm{MnO}}_3$ ($R=\mathrm{La}–\mathrm{Lu}$) films grown on (001) and (110) ${\mathrm{SrTiO}}_3$ substrates have been studied by density functional theory calculations. We demonstrate that the epitaxial strain causes the magnetic phase transitions from antiferromagnetic to ferromagnetic in most of the $R{\mathrm{MnO}}_3$ films, except those with smaller $R$ ions. These transitions can be attributed to the suppression of $Q_3$-type Jahn-Teller distortion caused by epitaxial strain, which significantly enhances the ferromagnetism in the nearest-neighbor exchange interactions. The strain gradient resulting from the strain relaxation provides the possibility to realize the coexistence of ferroelectricity and ferromagnetism, and a magnetoelectric coupling interface can be formed at the phase boundary between ferromagnetic and ferroelectric antiferromagnetic phases. Our results not only explain the observed ferromagnetism and the coexistence of multiple magnetic orders but also shed light on how to achieve the coexistence and coupling between ferroelectricity and ferromagnetism in $R{\mathrm{MnO}}_3$ films.

The layered transition metal dichalcogenides which show an inherent tendency towards charge density wave order are traditionally considered to realize rather two-dimensional electronic systems. However, recent theoretical and experimental results suggest that the stacking of the charge density wave and related orbital order in the direction perpendicular to the layers plays a key role for the in-plane electronic structure. Here we present a state-of-the-art density functional theory (DFT) study which models crucial features of the partially disordered orbital order stacking in the prototypical layered transition metal dichalcogenide 1T-${\mathrm{TaS}}_2$. We show that a DFT model with realistic assumptions about the orbital order perpendicular to the layers yields band structures which agree remarkably well with experiments and also correctly predicts the formation of an excitation gap at the Fermi energy. Our results not only imply that the widely accepted paradigm of local Mott physics as the driving mechanism behind the gap formation in 1T-${\mathrm{TaS}}_2$ needs to be reconsidered but they also highlight the crucial role of interlayer interactions in layered transition metal dichalcogenides in general.

First-principles calculations indicate that, before falling into dielectric breakdown, charge transport induced by a strong-intensity few-cycle optical waveform in the subfemtosecond time domain can be precisely controlled depending on band distortion engineered by strain along the [0001] direction in wurtzite-AlN. It is further discovered from a model of electron-hole interaction that the subfemtosecond charge driving with band engineering can be substantially strengthened by excitonic correlation and dynamics. With these findings, we reveal band engineering to be a route to the ultrafast charge control of semiconductors and indeed suggest an unexplored prototype of solid-state petahertz (${10}^{15}\mathrm{Hz}$) device.

We report on high-field (up to 30 T) magnetotransport experiments in topological crystalline insulator (111) SnTe epitaxial films. The longitudinal magnetoresistance $R_{\mathrm{xx}}$ exhibits pronounced Shubnikov–de Haas (SdH) oscillation at 4.2 K that persists up to 80 K. The second derivative $(-d^2{R}_{\mathrm{xx}}/d{B}^2)$ versus $1/B$ curve shows a clear beating pattern and the fast Fourier-transform analysis reveals that the SdH oscillations are composed of two close frequencies. As SnTe has elongated bulk Fermi ellipsoids, the $1/cos\theta$ dependence obtained in the angular evolution of both SdH frequencies is not sufficient to assure conduction via surface states. The Lifshitz-Kosevich fitting of the $R_{\mathrm{xx}}$ oscillatory component confirms the two frequencies and enables us to extract the Berry phase of the charge carriers. The most likely scenario obtained from our analysis is that the beating pattern of these quantum oscillations originates from the Rashba splitting of the bulk longitudinal ellipsoid in SnTe.

Effects of electronic correlation and spin-orbit coupling (SOC) on electronic structure of iron selenides (${\mathrm{Li}}_{1-x}{\mathrm{Fe}}_x$)OHFeSe have been investigated with the combination of density functional theory (DFT) and dynamical mean-field theory. It is found that the electronic correlation substantially changes the Fermi surface topology for $x=0.2$, resulting in a tiny electron pocket around the zone center $\mathrm{\Gamma }$ and two large electron pockets around the zone corner $M$, respectively. Moreover, the SOC also considerably affects the low-energy electronic structure near the Fermi level, especially inducing a gap in the Dirac-like dispersion around $\mathrm{\Gamma }$ for $x=0.2$. Our calculations show a correlation-driven Lifshitz transition from FeSe to the heavily electron-doped compound, leading to a transition of the nesting wave vector from ($\pi$, 0) to ($\pi$, $0.5\pi \pm \delta$) accompanied by an orbital-weight redistribution between the $d_{xy}$ and $d_{xz}/d_{yz}$ orbitals. These correlation-driven electronic structures enrich the understanding of different DFT and experimental results, suggesting a quite distinct superconducting state of (${\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}$)OHFeSe in comparison with FeSe.

We provide an analytical and numerical description of relaxation oscillations in the nonresonantly pumped polariton condensate. Presented considerations are based on coupled rate equations that are derived from the open dissipative Gross-Pitaevskii model. The evolution of the condensate density can be explained qualitatively by studying the topology of the trajectory in phase space. We use a fixed points analysis for the classification of the different regimes of condensate dynamics, including fast stabilization, slow oscillations, and ultrashort pulse emission. We obtain an analytical condition for the occurrence of relaxation oscillations. Continuous and pulsed condensate excitations are considered and we demonstrate that, in the latter case, the existence of the second reservoir is necessary for the emergence of oscillations. We show that relaxation oscillations should be expected to occur in systems with relatively short polariton lifetimes.

We study a generalized Anderson model that mixes two localized configurations—one formed by two degenerate doublets and the other by a triplet with single-ion anisotropy $D{S}_z^2$—by means of two degenerate conduction channels. The model has been derived for a single Ni impurity embedded into an O-doped Au chain. Using the numerical renormalization group, we find a topological quantum phase transition, at a finite value $D_c,$ between two regular Fermi liquid phases of high (low) conductance and topological number $2I_L/\pi =0$ (+1) for $D<D_c$ ($D>D_c$), where $I_L$ is the well-known Luttinger integral. At finite temperature the two phases are separated by a non-Fermi liquid phase with fractional impurity entropy $\frac{1}{2}\ln 2$ and other properties which are similar to those of the two-channel Kondo model.