We present a systematic investigation of the special twist method introduced by Rajagopal et al. [Phys. Rev. B 51, 10591 (1995)] for reducing finite-size effects in correlated calculations of periodic extended systems with Coulomb interactions and Fermi statistics. We propose a procedure for finding special twist values which, at variance with previous applications of this method, reproduce the energy of the mean-field infinite-size limit solution within an adjustable (arbitrarily small) numerical error. This choice of the special twist is shown to be the most accurate single-twist solution for curing one-body finite-size effects in correlated calculations. For these reasons we dubbed our procedure “exact special twist” (EST). EST only needs a fully converged independent-particles or mean-field calculation within the primitive cell and a simple fit to find the special twist along a specific direction in the Brillouin zone. We first assess the performances of EST in a simple correlated model such as the three-dimensional electron gas. Afterwards, we test its efficiency within ab initio quantum Monte Carlo simulations of metallic elements of increasing complexity. We show that EST displays an overall good performance in reducing finite-size errors comparable to the widely used twist average technique but at a much lower computational cost since it involves the evaluation of just one wave function. We also demonstrate that the EST method shows similar performances in the calculation of correlation functions, such as the ionic forces for structural relaxation and the pair radial distribution function in liquid hydrogen. Our conclusions point to the usefulness of EST for correlated supercell calculations; our method will be particularly relevant when the physical problem under consideration requires large periodic cells.

We investigate the effect of epitaxial strain on the Mott metal-insulator transition (MIT) in perovskite systems with $d^1$ and $d^2$ electron configurations of the transition metal (TM) cation. We first discuss the general trends expected from the changes in the crystal-field splitting and in the hopping parameters that are induced by epitaxial strain. We argue that the strain-induced crystal-field splitting generally favors the Mott-insulating state, whereas the strain-induced changes in the hopping parameters favor the metallic state under compressive strain and the insulating state under tensile strain. Thus the two effects can effectively cancel each other under compressive strain, while they usually cooperate under tensile strain, in this case favoring the insulating state. We then validate these general considerations by performing electronic structure calculations for several $d^1$ and $d^2$ perovskites, using a combination of density functional theory (DFT) and dynamical mean-field theory (DMFT). We isolate the individual effects of strain-induced changes in either hopping or crystal-field by performing DMFT calculations where we fix one type of parameter to the corresponding unstrained DFT values. These calculations confirm our general considerations for ${\mathrm{SrVO}}_3$ ($d^1$) and ${\mathrm{LaVO}}_3$ ($d^2$), whereas the case of ${\mathrm{LaTiO}}_3$ ($d^1$) is distinctly different, due to the strong effect of the octahedral tilt distortion in the underlying perovskite crystal structure. Our results demonstrate the possibility to tune the electronic properties of correlated TM oxides by using epitaxial strain, which allows to control the strength of electronic correlations and the vicinity to the Mott MIT.

We construct a bosonic analog of a two-dimensional topological Dirac semimetal (DSM). The low-energy description of the most basic 2D DSM model consists of two Dirac cones at positions $\pm {\mathbf{k}}_0$ in momentum space. The local stability of the Dirac cones is guaranteed by a composite symmetry $Z_2^{\mathcal{TI}}$, where $\mathcal{T}$ is time reversal and $\mathcal{I}$ is inversion. This model also exhibits interesting time-reversal and inversion symmetry breaking electromagnetic responses. In this work we construct a bosonic version by replacing each Dirac cone with a copy of the $O\left(4\right)$ nonlinear sigma model (NLSM) with topological theta term and theta angle $\theta =\pm \pi$. One copy of this NLSM also describes the gapless surface termination of the 3D bosonic topological insulator (BTI). We compute the time-reversal and inversion symmetry breaking electromagnetic responses for our model and show that they are twice the value one gets in the DSM case matching what one might expect from, for example, a bosonic Chern insulator. We also investigate the stability of the BSM model and find that the composite $Z_2^{\mathcal{TI}}$ symmetry again plays an important role. Along the way we clarify many aspects of the surface theory of the BTI including the electromagnetic response, the charges and statistics of vortex excitations, and the stability to symmetry-allowed perturbations. We briefly comment on the relation between the various descriptions of the $O\left(4\right)$ NLSM with $\theta =\pi$ used in this paper (a dual vortex description and a description in terms of four massless fermions) and the recently proposed dual description of the BTI surface in terms of $2+1$-dimensional quantum electrodynamics with two flavors of fermion $\left(N=2 {\text{QED}}_3\right)$. In a set of four Appendixes we review some of the tools used in the paper and also derive some of the more technical results.

Inelastic tunneling of electrons can generate the emission of photons with energies intuitively limited by the applied bias voltage. However, experiments indicate that more complex processes involving the interaction of electrons with plasmon polaritons lead to photon emission with overbias energies. We recently proposed a model of this observation in Phys. Rev. Lett. 113, 066801 (2014), in analogy to the dynamical Coulomb blockade, originally developed for treating the electromagnetic environment in mesoscopic circuits. This model describes the correlated tunneling of two electrons interacting with a local plasmon-polariton mode, represented by a resonant circuit, and shows that the overbias emission is due to the non-Gaussian fluctuations. Here we extend our model to study the overbias emission at finite temperature. We find that the thermal smearing strongly masks the overbias emission. Hence, the detection of the correlated tunneling processes requires temperatures $k_{B}T$ much lower than the bias energy $eV$ and the plasmon energy $\hslash {\omega }_0$, a condition which is fortunately realized experimentally.

We experimentally study the spin relaxation mechanism in heavily doped $n$-type germanium (Ge) layers by electrically detecting pure spin current transport. The spin diffusion length (${\lambda }_{\mathrm{Ge}}$) in heavily doped $n$-type Ge layers at 125 K is less than $0.7\mu \mathrm{m}$, much shorter than that expected in the recent study by Dushenko et al. We find that the spin relaxation time ${\tau }_{\mathrm{s}}$ is independent of temperature in the range of 8 to 125 K, which can be interpreted by the recent theory by Song et al. This study clarifies that the spin-relaxation mechanism at low temperatures in degenerate Ge is dominated by extrinsic scattering with impurities.

Stückelberg interferometry is a phenomenon that has been well established for quantum-mechanical two-level systems. Here, we present classical two-mode interference of a nanomechanical two-mode system, realizing a classical analog of Stückelberg interferometry. Our experiment relies on the coherent energy exchange between two strongly coupled, high-quality factor nanomechanical resonator modes. Furthermore, we discuss an exact theoretical solution for the double-passage Stückelberg problem by expanding the established finite-time Landau-Zener single-passage solution. For the parameter regime explored in the experiment, we find that the Stückelberg return probability in the classical version of the problem formally coincides with the quantum case, which reveals the analogy of the return probabilities in the quantum-mechanical and the classical version of the problem. This result qualifies classical two-mode systems at large to simulate quantum-mechanical interferometry.

The mechanical properties of monolayer GaS and GaSe crystals are investigated in terms of their elastic constants: in-plane stiffness (C), Poisson ratio $\left(\nu \right)$, and ultimate strength $\left({\sigma }_U\right)$ by means of first-principles calculations. The calculated elastic constants are compared with those of graphene and monolayer ${\mathrm{MoS}}_2$. Our results indicate that monolayer GaS is a stiffer material than monolayer GaSe crystals due to the more ionic character of the Ga-S bonds than the Ga-Se bonds. Although their Poisson ratio values are very close to each other, 0.26 and 0.25 for GaS and GaSe, respectively, monolayer GaS is a stronger material than monolayer GaSe due to its slightly higher ${\sigma }_U$ value. However, GaS and GaSe crystals are found to be more ductile and flexible materials than graphene and ${\mathrm{MoS}}_2$. We have also analyzed the band-gap response of GaS and GaSe monolayers to biaxial tensile strain and predicted a semiconductor-metal crossover after $17\%$ and $14\%$ applied strain, respectively, for monolayer GaS and GaSe. In addition, we investigated how the mechanical properties are affected by charging. We found that the flexibility of single layer GaS and GaSe displays a sharp increase under $0.1e$/cell charging due to the repulsive interactions between extra charges located on chalcogen atoms. These charging-controllable mechanical properties of single layers of GaS and GaSe can be of potential use for electromechanical applications.

We consider a triple-quantum-dot (TQD) system composed by an interacting quantum dot connected to two effectively noninteracting dots, which in turn are both connected in parallel to metallic leads. As we show, this system can be mapped onto a single-impurity Anderson model with a nontrivial density of states. The TQD's transport properties are investigated under a continuous tuning of the noninteracting dots' energy levels, employing the numerical renormalization group technique. Interference between single and many-particle resonances splits the Kondo peak, fulfilling a generalized Friedel sum rule. In addition, a particular configuration in which one of the noninteracting dots is held out of resonance with the leads allows us to access a pseudogap regime where a Kosterlitz-Thouless-type quantum-phase transition (QPT) occurs, separating the Kondo and non-Kondo behavior. Within this same configuration, the TQD exhibits traces of the Fano-Kondo effect, which is in turn strongly affected by the QPT. Signatures of all these phenomena are neatly displayed by the calculated linear conductance.

Single crystals of ${\mathrm{PbZrO}}_3$ have been studied by dynamic mechanical analysis measurements in the low-frequency range $f=0.02$–50 Hz. The complex Young's modulus exhibits a quite rich behavior and depends strongly on the direction of the applied dynamic force. In pseudocubic ${\left[100\right]}_c$ direction, we found intrinsic elastic behavior as expected from the Landau theory; at the antiferroelectric transition $T_c\approx 510$ K, a downwards cusp anomaly in $Y^{\prime }$ accompanied by a peak in $Y^{\prime \prime }$ points to a quadratic/linear order parameter/strain coupling in the Landau free energy. Both anomalies are increasing with decreasing frequency showing that the measurements are performed in the limit $\omega {\tau }_{\mathrm{th}}>1$. Frequency scans around $T_c$ show energy dissipation, which could result from interphase boundary motion and/or heat diffusion. Above $T_c$, we observe a pronounced precursor softening, quite similar as it was found in other perovskites, which can be perfectly fitted including isotropic order parameter fluctuations. The low-frequency elastic response in ${\left[110\right]}_c$ direction is different. Below $T_c$, we find in addition to the intrinsic anomaly a strong contribution from ferroelastic domains, which leads to an additional softening in $Y^{\prime }$. With decreasing temperatures this superelastic softening gradually disappears, due to an increasing relaxation time ${\tau }_{\mathrm{DW}}$ for domain wall motion, indicating glassy behavior of domain freezing in ${\mathrm{PbZrO}}_3$. In contrast to the ${\left[100\right]}_c$ direction, for forces along ${\left[110\right]}_c$, we found a pronounced precursor hardening, starting at about 60 K above $T_c$. Since this anomaly is of dynamic nature, starting at the same temperature as the observed birefringence and piezoelectric anomalies [Ko et al. Phys. Rev. B 87, 184110 (2013)], we conclude that it originates from slow dynamic polar clusters, which freeze at $T^{*}\approx 550\text{K}>T_c$.

High resolution inelastic x-ray scattering experiments are widely used to study the electronic and chemical properties of materials under a range of conditions, from ambient temperature to the warm dense matter regime. We use the real-space multiple scattering (RSMS) Green's function formalism coupled with density functional theory molecular dynamics (DFT-MD) to study thermal effects on the Compton profile (CP) of disordered systems. The RSMS method is advantageous for calculations of highly disordered, aperiodic systems because it places no restriction on symmetry. As a test, we apply our approach to thermally disordered Be, Li, and Si in both liquid and solid phases. We find good agreement with experimental and other theoretical results, showing that the real-space multiple scattering approach coupled with DFT-MD is an efficient and reliable method for calculating the CP of disordered systems at finite temperatures.

The large epitaxial stress induced by the misfit between a triple atomic layer Fe film and an Ir(111) substrate is relieved by the formation of a dense dislocation line network. Spin-polarized scanning tunneling microscopy investigations show that the strain is locally varying within the Fe film and that this variation affects the magnetic state of the system. Two types of dislocation line regions can be distinguished and both exhibit spin spirals with strain-dependent periods (ranging from 3 to $10\mathrm{nm})$. Using a simple micromagnetic model, we attribute the changes of the period of the spin spirals to variations of the effective exchange coupling in the magnetic film. This assumption is supported by the observed dependence of the saturation magnetic field on the period of the zero-field spin spiral. Moreover, magnetic skyrmions appear in an external magnetic field only in one type of dislocation line area, which we impute to the different pinning properties of the dislocation lines.

Phase decomposition is a well-known process leading to the formation of two-phase mixtures. Here we show that a strain imposed on a ferroelastic crystal promotes the formation of mixed phases and domains, i.e., strain phase separation with local strains determined by a common tangent construction on the free energy versus strain curves. It is demonstrated that a domain structure can be understood using the concepts of domain/phase rule, lever rule, and coherent and incoherent strain phase separation, in a complete analogy to phase decomposition. The proposed strain phase separation model is validated using phase-field simulations and experimental observations of $\mathrm{PbTi}{\mathrm{O}}_3$ and $\mathrm{BiFe}{\mathrm{O}}_3$ thin films as examples. The proposed model provides a simple tool to guide and design domain structures of ferroelastic systems.

In the conceptual framework of phase ordering after temperature quenches below transition, we consider the underdamped Bales-Gooding-type “momentum conserving” dynamics of a 2D martensitic structural transition from a square-to-rectangle unit cell. The one-component or $N_{\mathrm{OP}}=1$ order parameter is one of the physical strains, and the Landau free energy has a triple well, describing a first-order transition. We numerically study the evolution of the strain-strain correlation, and find that it exhibits dynamical scaling, with a coarsening length $L\left(t\right)\sim t^{\alpha }$. We find at intermediate and long times that the coarsening exponent sequentially takes on respective values close to $\alpha =2/3$ and $1/2$. For deep quenches, the coarsening can be arrested at long times, with $\alpha \simeq 0$. These exponents are also found in 3D. To understand such behavior, we insert a dynamical-scaling ansatz into the correlation function dynamics to give, at a dominant scaled separation, a nonlinear kinetics of the curvature $g\left(t\right)\equiv 1/L\left(t\right)$. The curvature solutions have time windows of power-law decays $g\sim 1/t^{\alpha }$, with exponent values $\alpha$ matching simulations, and manifestly independent of spatial dimension. Applying this curvature-kinetics method to mass-conserving Cahn-Hilliard dynamics for a double-well Landau potential in a scalar $N_{\mathrm{OP}}=1$ order parameter yields exponents $\alpha =1/4$ and $1/3$ for intermediate and long times. For vector order parameters with $N_{\mathrm{OP}}\ge 2$, the exponents are $\alpha =1/4$ only, consistent with previous work. The curvature kinetics method could be useful in extracting coarsening exponents for other phase-ordering dynamics.

We report on the electron spin resonance (ESR) study of ${\mathrm{LiCuVO}}_4$ in spiral and spin-modulated phases ($0<{\mu }_{0}H<10$ T). The measurements have been made on high quality samples that allowed getting rid of absorption lines arising from defects. In the field range of spiral phase ($0<{\mu }_{0}H<7.5$ T) all observed branches of the spectrum can be ascribed to uniform oscillations of planar spin structure. The ESR spectrum observed in spin-modulated phase (${\mu }_{0}H>7.5$ T) has two branches. One of them can be explained by uniform oscillations of spin structure, whereas the second branch, surprisingly demonstrates only a weak dependence on field orientation, which indicates the exchange nature of this mode. Allegedly this mode can be associated with soliton excitations.

We investigate the quantum phases of the frustrated spin-$\frac{1}{2}{J}_1-J_2-J_3$ Heisenberg model on the square lattice with ferromagnetic $J_1$ and antiferromagnetic $J_2$ and $J_3$ interactions. Using the pseudofermion functional renormalization group technique, we find an intermediate paramagnetic phase located between classically ordered ferromagnetic, stripy antiferromagnetic, and incommensurate spiral phases. We observe that quantum fluctuations lead to significant shifts of the spiral pitch angles compared to the classical limit. By computing the response of the system with respect to various spin rotation and lattice symmetry-breaking perturbations, we identify a complex interplay between different nematic spin states in the paramagnetic phase. While retaining time-reversal invariance, these phases either break spin-rotation symmetry, lattice-rotation symmetry, or a combination of both. We therefore propose the $J_1-J_2-J_3$ Heisenberg model on the square lattice as a paradigmatic example where different intimately connected types of nematic orders emerge in the same model.

We present a de Haas–van Alphen study of the Fermi surface of the low-temperature antiferromagnet ${\mathrm{CeZn}}_{11}$ and its nonmagnetic analog ${\mathrm{LaZn}}_{11}$, measured by torque magnetometry up to fields of $33\mathrm{T}$ and at temperatures down to $320\mathrm{mK}$. Both systems possess similar de Haas–van Alphen frequencies, with three clear sets of features—ranging from $50\mathrm{T}$ to $4\mathrm{kT}$—corresponding to three bands of a complex Fermi surface, with an expected fourth band also seen weakly in ${\mathrm{CeZn}}_{11}$. The effective masses of the charge carriers are very light ($<1m_e$) in ${\mathrm{LaZn}}_{11}$ but a factor of 2–4 larger in ${\mathrm{CeZn}}_{11}$, indicative of stronger electronic correlations. We perform detailed density functional theory (DFT) calculations for ${\mathrm{CeZn}}_{11}$ and find that only DFT+$U$ calculations with $U=1.5\mathrm{eV}$, which localize the $4f$ states, provide a good match to the measured de Haas–van Alphen frequencies, once the presence of magnetic breakdown orbits is also considered. Our study suggests that the Fermi surface of ${\mathrm{CeZn}}_{11}$ is very close to that of ${\mathrm{LaZn}}_{11}$ being dominated by Zn $3d$, as the Ce $4f$ states are localized and have little influence on its electronic structure, however, they are responsible for its magnetic order and contribute to enhance electronic correlations.

The paramagnetic metallic phase of the Anderson-Hubbard model (AHM) is investigated using a nonperturbative local moment approach within the framework of dynamical mean-field theory with a typical medium. Our focus is on the breakdown of the metallic phase near the metal-insulators transition as seen in the single-particle spectra, scattering rates, and the associated distribution of Kondo scales. We demonstrate the emergence of a universal, underlying low-energy scale, $T_K^{\mathrm{peak}}$. This lies close to the peak of the distribution of Kondo scales obtained within the metallic phase of the paramagnetic AHM. Spectral dynamics for energies $\omega \lesssim T_K^{\mathrm{peak}}$ display Fermi liquid universality crossing over to an incoherent universal dynamics for $\omega \gg T_K^{\mathrm{peak}}$ in the scaling regime. Such universal dynamics indicate that within a local theory the low to moderately low-energy physics is governed by an effective, disorder renormalized Kondo screening.

We examine the dynamics of a single hole in the gapless phase of the Kitaev honeycomb model, focusing on the slow-hole regime where the bare hopping amplitude $t$ is much less than the Kitaev exchange energy $J$. In this regime, the hole does not generate gapped flux excitations and is dressed only by the gapless fermion excitations. Investigating the single-hole spectral function, we find that the hole propagates coherently with a quasiparticle weight that is finite but approaches zero as $t/J\to 0$. This conclusion follows from two approximate treatments, which capture the same physics in complementary ways. Both treatments use the stationary limit as an exactly solvable starting point to study the spectral function approximately (i) by employing a variational approach in terms of a trial state that interpolates between the limits of a stationary hole and an infinitely fast hole and (ii) by considering a special point in the gapless phase that corresponds to a simplified one-dimensional problem.

Interfaces between normal and topological insulators are bound to host metallic states that are protected by time-reversal symmetry and are therefore robust against disorder and interface reconstruction. Two-dimensional topological insulators (quantum spin Hall insulators) offer a unique opportunity to change the local topology by adsorption of atoms or molecules and thus comprise an ideal platform for designing topological heterostructures. Here we apply first-principles calculations to show that the quantum spin Hall insulator $1{\mathrm{T}}^{\prime }-{\mathrm{MoS}}_2$ exhibits a phase transition to a trivial insulator upon adsorption of various atoms. It is then demonstrated that one-dimensional metallic states indeed arise at the boundary of regions with and without adsorbed oxygen and that these boundary states generically constitute simple linear connections between valence and conduction bands in reciprocal space. This is in sharp contrast to topological edge states, which typically exhibit strong dispersion that are sensitive to a particular edge termination. The heterostructure is also suggestive of a simple design of one-dimensional metallic networks in sheets of $1{\mathrm{T}}^{\prime }-{\mathrm{MoS}}_2$.

We propose an experimental setup using ultracold atoms to implement a bilayer honeycomb lattice with Bernal stacking. In the presence of a potential bias between the layers and at low densities, fermions placed in this lattice form an annular Fermi sea. The presence of two Fermi surfaces leads to interesting patterns in Friedel oscillations and RKKY interactions in the presence of impurities. Furthermore, a repulsive fermion-fermion interaction leads to a Stoner instability towards an incommensurate spin density wave order with a wave vector equal to the thickness of the Fermi sea. The instability occurs at a critical interaction strength which goes down with the density of the fermions. We find that the instability survives interaction renormalization due to vertex corrections and discuss how this can be seen in experiments. We also track the renormalization group flows of the different couplings between the fermionic degrees of freedom, and find that there are no perturbative instabilities, and that Stoner instability is the strongest instability which occurs at a critical threshold value of the interaction. The critical interaction goes to zero as the chemical potential is tuned towards the band bottom.