The mechanocaloric effect is the temperature change of a material upon application or removal of an external stress. Beyond its fundamental interest, this caloric response represents a promising and ecofriendly alternative to current cooling technologies. To obtain large mechanocaloric effects, we need materials whose elastic properties (e.g., strain, elastic compliance) are strongly temperature dependent. This is the case of ferroelectric perovskite oxides, where the development of the spontaneous electric polarization is accompanied by significant strains and lattice softening. Thus, in this work we study the mechanocaloric properties of model ferroelectric ${\mathrm{PbTiO}}_3$ by means of predictive atomistic (“second-principles”) simulations and a perturbative formalism introduced here. Our calculations reveal relatively large effects (up to $-4$ K for relatively small applied compressions of $-0.1$ GPa) and several striking features. In particular, we find that the mechanocaloric response is highly anisotropic in the ferroelectric phase, as it can be either conventional (temperature increases upon compression) or inverse (temperature decreases) depending on the direction of the applied stress. We discuss and explain these surprising results, which compare well with existing experimental information. Our analysis suggests that the coexistence of conventional and inverse mechanocaloric responses is probably common among ferroelectrics and materials displaying a negative thermal expansion.

The century-old Vegard‘s law has been remarkably accurate in describing the evolution of the lattice parameters of almost all solid solutions. Contractions or expansions of lattice parameters of such systems depend on the size of the guest atom being smaller or larger than the host atom it replaces to form the solid solution. This has given rise to the concept of “chemical pressure” in analogy to the physical pressure. We have investigated using EXAFS the evolution of the local structure in terms of atom-pair distances extending up to the third-nearest neighbors in the family of compounds, $\mathrm{Zn}{\mathrm{Se}}_x{\mathrm{S}}_{1-x}$ as an example of an anionic solid solution, in contrast to all previous studies focusing on cationic solid solutions. Our results establish several common features between these two types of solid solutions, while strongly suggesting that the concept of a chemical pressure is inaccurate and misleading. Most interestingly, we also find a qualitative difference between the cationic solid solutions, reported earlier, and the anionic solid solution.

Antiferromagnetic spintronics allows us to explore storing and processing information in magnetic crystals with vanishing magnetization. In this paper, we investigate magnetoresistance effects in antiferromagnetic CuMnAs upon switching into high-resistive states using electrical pulses. By employing magnetic field sweeps up to 14 T and magnetic field pulses up to $\sim 60$ T, we reveal hysteretic phenomena and changes in the magnetoresistance, as well as the resilience of the switching signal in CuMnAs to the high magnetic field. These properties of the switched state are discussed in the context of recent studies of antiferromagnetic textures in CuMnAs.

Recent experiments have observed correlated insulating and possible superconducting phases in twisted homobilayer transition metal dichalcogenides (TMDs). Besides the spin-valley locked moiré bands due to the intrinsic Ising spin-orbit coupling, homobilayer moiré TMDs also possess either logarithmic or power-law divergent Van Hove singularities (VHSs) near the Fermi surface, controllable by an external displacement field. The former and the latter are dubbed conventional and higher-order VHSs, respectively. Here, we perform a perturbative renormalization group (RG) analysis to unbiasedly study the dominant instabilities in homobilayer TMDs for both the conventional and higher-order VHS cases. We find that the spin-valley locking largely alters the RG flows and leads to instabilities unexpected in the corresponding extensively studied graphene-based moiré systems, such as spin- and valley-polarized ferromagnetism and topological superconductivity with mixed parity. In particular, for the case with two higher-order VHSs, we find a spin-valley-locking-driven metallic state with no symmetry breaking in the TMDs despite the diverging bare susceptibility. Our results show how the spin-valley locking significantly affects the RG analysis and demonstrate that moiré TMDs are suitable platforms to realize various interaction-induced spin-valley locked phases, highlighting physics fundamentally different from the well-studied graphene-based moiré systems.

We present a large-scale density matrix renormalization group (DMRG) study of the spin-1 SU(3) bilinear-biquadratic model on the square lattice, which was suggested to host a nematic spin liquid state in recent DMRG calculations. We report that this spin liquid appears to strongly compete with a three-sublattice magnetic order. To further study the competition between the two states, and the reason of the emergent nematic spin liquid, we included an additional next-nearest-neighbor SU(3) symmetric interactions along one of the two plaquette diagonal directions. This allows to tune the square-lattice model to the triangular-lattice model. By computing spin correlation functions and various order parameters, we find that the three-sublattice order may develop at infinitesimal additional new coupling, at least within the precision of our study. Compared with the previous findings that the nematic spin liquid is stable in extended parameter regions with additional couplings that respect the lattice symmetries of the square lattice, we argue that here the diagonal couplings, which frustrate the bipartite-lattice structure, rapidly suppress the two-sublattice fluctuations and the three-sublattice order thus wins. This numerical result is consistent with the conjecture that the nematic spin liquid emerges from the competition between two- and three-sublattice fluctuations.

The representation of ground states of fermionic quantum impurity problems as superpositions of Gaussian states has recently been given a rigorous mathematical foundation [S. Bravyi and D. Gosset, Commun. Math. Phys. 356, 451 (2017)]. It is natural to ask how many parameters are required for an efficient variational scheme based on this representation. An upper bound is $O\left(N^2\right)$, where $N$ is the system size, which corresponds to the number parameters needed to specify an arbitrary Gaussian state. We provide an alternative representation, with more favorable scaling, only requiring $O\left(N\right)$ parameters, that we illustrate for the interacting resonant-level model. We achieve the reduction by associating mean-field-like parent Hamiltonians with the individual terms in the superposition, using physical insight to retain only the most relevant channels in each parent Hamiltonian. We benchmark our variational ansatz against the numerical renormalization group, and compare our results to other variational schemes of a similar nature to ours. Apart from the ground-state energy, we also study the spectrum of the covariance matrix—a very stringent measure of accuracy. Our approach outperforms some existing variational schemes and remains quantitatively accurate in the numerically challenging near-critical regime.

We investigate the low-energy physics of non-Hermitian quantum spin models with $PT$ symmetry. To this end, we consider the one-dimensional Ising chain and the two-dimensional toric code in a non-Hermitian staggered field. For both systems, dual descriptions in terms of non-Hermitian staggered Ising interactions in a conventional transverse field exist. We perform high-order series expansions about the high- and low-field limit for both systems to determine the ground-state energy per site and the one-particle gap. The one-dimensional non-Hermitian Ising chain is known to be exactly solvable. Its ground-state phase diagram consists of second-order quantum phase transitions, which can be characterized by logarithmic singularities of the second derivative of the ground-state energy and, in the symmetry-broken phase, the gap closing of the low-field gap. In contrast, the gap closing from the high-field phase is not accessible perturbatively due to the complex energy and the occurrence of exceptional lines in the high-field gap expression. For the two-dimensional toric code in a non-Hermitian staggered field, we study the quantum robustness of the topologically ordered phase by the gap closing of the low-field gap. We find that the well-known second-order quantum phase transition of the toric code in a uniform field extends into a large portion of the non-Hermitian parameter space. However, the series expansions become unreliable for a dominant anti-Hermitian field. Interestingly, the analysis of the high-field gap reveals the potential presence of an intermediate region.

Recent theoretical calculations predict the presence of Dirac nodal lines with $\pi$ Berry phase and related topological surface states in elemental alkaline-earth metals. Here we provide experimental and theoretical evidence for the existence of similar nodal lines also in hexagonal close-packed Yb, an element of the lanthanide series, in the limit of zero spin-orbit coupling. These topological properties emerge after taking into account self-energy corrections, which permit one to correctly describe the experimental low-energy electronic structure of Yb. By angle-resolved photoemission spectroscopy we demonstrate the occurrence of topological surface states in Yb which are robust in the presence of the large spin-orbit coupling.

The precise control of spins in semiconductor spintronic devices requires electrical means to generate spin packets with a well-defined initial phase. We demonstrate a pulsed electrical scheme that triggers the spin ensemble phase in a similar way as circularly polarized optical pulses generate phase coherent spin packets. Here, we use fast current pulses to initialize phase coherent spin packets, which are injected across an Fe/GaAs Schottky barrier into $n$-GaAs. By means of time-resolved Faraday rotation, we demonstrate phase coherence by the observation of multiple Larmor precession cycles for current pulse widths down to 500 ps at 17 K. We show that the current pulses are broadened by the charging and discharging time of the Schottky barrier. At high frequencies, the observable spin coherence is limited only by the finite bandwidth of the current pulses, which is of the order of 2 GHz. These results therefore demonstrate that all-electrical injection and phase control of electron spin packets at microwave frequencies is possible in metallic-ferromagnet–semiconductor heterostructures.

Inverted-gap InAs/GaSb quantum wells have long been predicted to show quantum spin Hall insulator (QSHI) behavior. The experimental characterization of the QSHI phase in these systems has relied on the presence of quantized edge transport near charge neutrality. However, experimental data showing the presence of edge conductance in the trivial regime suggest that additional experimental signatures are needed to characterize the QSHI phase. Here we show that electric field-induced gap oscillations can be used as an indicator of the presence of helical edge states in the system. By studying a realistic low-energy model InAs/GaSb quantum wells derived from $\mathbf{k}·\mathbf{p}$ band theory, we show that such oscillations are bound to appear in narrow samples as the system is driven to the topological phase by the electric field. Our results can serve as a guide for the search of additional experimental signatures of the presence of topologically protected helical edge states in InAs/GaSb systems.

In this work, we foresee the structure of a new class of borophenes with smaller two-dimensional (2D) densities of atoms than those explored so far for 2D boron crystals. Boron atoms in the porous borophenes tend to be five-coordinated in contrast with commonly investigated structures with hexagonal holes for which the number of nearest neighbors of each atom varies from three to six. High metallic character is the usual feature of borophenes, however, we have also identified a semimetallic borophene that turns into a semiconductor upon unit-cell expansion. A $10\%$ increase in lattice constant of this structure gives rise to an energy gap of $0.3\text{eV}$. This extends to semiconductor industry the possible application of 2D boron crystals.

We construct a valley dynamics model, involving both bright and dark excitonic states, to determine magneto-optical properties of excitons and singlet and triplet trions (charged excitons), in monolayer ${\mathrm{WSe}}_2$ subjected to magnetic fields. Our computed excitonic peak energy and valley Zeeman splitting, which are essential for the magnetic-field-associated valley dynamics, in great agreement with experimental data by Lyons et al. [Nat. Commun. 10, 2330 (2019)]. In addition, we find that the valley polarizations of excitons and trions respectively exhibit the “X”- and “V”-shape dependence on magnetic field, consistent with experimental measurements by Aivazian et al. [Nat. Phys. 11, 148 (2015)]. Remarkably, beyond available experimental measurements, our theory predicts an X-V shape conversion and even a new paradigm more than the X and V shapes, which depend on the trion fine structure arising from exchange interactions and involving a magnetic swap of ground states between singlet and triplet trions. Our results are helpful for elucidating recent experimental data about the valley-degeneracy-lifting-mediated valley dynamics of different trion species and should stimulate experiments probing relevant new magneto-optical features.

Recent numerical and experimental works have revealed a disorder-free many-body localization (MBL) in an interacting system subjected to a linear potential, known as the Stark MBL. The conventional MBL, induced by disorder, has been widely studied by using quantum simulations based on superconducting circuits. Here, we consider the Stark MBL in two types of superconducting circuits, i.e., the one-dimensional array of superconducting qubits, and the circuit where nonlocal interactions between qubits are mediated by a resonator bus. We calculate the entanglement entropy and participate entropy of the highly excited eigenstates, and study the nonequilibrium properties of the Stark MBL. In particular, we observe an anomalous relaxation of the imbalance, dominated by the power-law decay $t^{-\xi }$. The exponent $\xi$ satisfies $\xi \propto |\gamma -{\gamma }_c{|}^{\nu }$ when $\gamma <{\gamma }_c$, and vanishes for $\gamma \ge {\gamma }_c$, which can be employed to estimate the ${\gamma }_c$. Our work indicates that superconducting circuits are a promising platform for investigating the critical properties of the Stark MBL transition.

The real part of optical conductivity $\text{Re}\sigma \left(\omega \right)$ of the Mott insulators has a large amount of information on how spin and charge degrees of freedom interact with each other. By using the time-dependent density-matrix renormalization group, we study $\text{Re}\sigma \left(\omega \right)$ of the two-dimensional Hubbard model on a square lattice at half filling. We find an excitonic peak at the Mott-gap edge of $\text{Re}\sigma \left(\omega \right)$ not only for the two-dimensional square lattice but also for two- and four-leg ladders. For the square lattice, however, we do not clearly find a gap between an excitonic peak and continuum band, which indicates that a bound state is not well defined. The emergence of an excitonic peak in $\text{Re}\sigma \left(\omega \right)$ implies the formation of a spin polaron. Examining the dependence of $\text{Re}\sigma \left(\omega \right)$ on the on-site Coulomb interaction and next-nearest-neighbor hoppings, we confirm that an excitonic peak is generated from a magnetic effect. Electron scattering due to an electron-phonon interaction is expected to easily suppress an excitonic peak since spectral width of an excitonic peak is very narrow. Introducing a large broadening in $\text{Re}\sigma \left(\omega \right)$ by modeling the electron-phonon coupling present in ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$, we obtain $\text{Re}\sigma \left(\omega \right)$ comparable with experiments.

Microcavity exciton-polaritons are two-dimensional bosonic quasiparticles composed by excitons and photons. Using a model Hamiltonian with parameters generated from ab initio density-functional theory and Bethe-Salpeter equation calculations, we investigate the exciton and the exciton-polariton properties of hexagnonal boron nitride (hBN) based microcavity. We show that hBN-based microcavities, including monolayer and all-dielectric ones, are promising in optoelectronic applications. Room temperature exciton-polariton Bose-Einstein condensation can be achieved because of the large oscillator strength and binding energy of the exciton, and the strong interaction between the exciton-polaritons and the longitudinal optical phonons. Based on this BEC state, an exciton-polariton mediated superconducting device can also be fabricated at a few tens of kelvin using the microcavity structure proposed by Laussy et al. Phys. Rev. Lett. 104, 106402 (2010).

Systems of interacting fermions can give rise to ground states whose correlations become effectively free-fermion-like in the thermodynamic limit, as shown by Baxter for a class of integrable models that include the one-dimensional XYZ spin-$\frac{1}{2}$ chain. Here, we quantitatively analyze this behavior by establishing the relation between system size and correlation length required for the fermionic gaussianity to emerge. Importantly, we demonstrate that this behavior can be observed through the applicability of Wick's theorem, and thus, it is experimentally accessible. To establish the relevance of our results to possible experimental realizations of XYZ-related models, we demonstrate that the emergent gaussianity is insensitive to weak variations in the range of interactions, coupling inhomogeneities, and local random potentials.

Protected by the interplay of on-site Coulomb interactions and spin-orbit coupling, ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ at high pressure is a rare example of a Mott insulator with a paramagnetic ground state. Here, using optical Raman scattering, we measure both the phonon and magnon evolution in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ under pressure and identify three different magnetically-ordered phases, culminating in a spin-disordered state beyond 18 GPa. A strong first-order structural phase transition drives the magnetic evolution at $\sim 10$ GPa with reduced structural anisotropy in the ${\mathrm{IrO}}_6$ cages, leading to increasingly isotropic exchange interactions between the Heisenberg spins and a spin-flip transition to $c$-axis-aligned antiferromagnetic order. In the disordered phase of Heisenberg $J_{\mathrm{eff}}=1/2$ pseudospins, the spin excitations are quasielastic and continuous to 10 meV, potentially hosting a gapless quantum spin liquid in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$.

We use magnetotransport measurements to probe quantum Hall ground states in graphene/${\mathrm{WSe}}_2$ heterobilayers. Compared to pristine graphene, inter-Landau level (LL) gaps at half-filled quartets away from filling factor $\nu =0$ show significantly weaker dependence on the magnetic field $B$, while odd $\nu$ fillings show a stronger dependence. We interpret this behavior using a model incorporating Ising and Rashba spin-orbit coupling (SOC) along with Coulomb interactions within the self-consistent Hartree-Fock framework. A model fit yields Ising SOC in range $\sim 1–2 \mathrm{meV}$, Rashba $\sim 10 \mathrm{meV}$, and the in-plane dielectric constant $\sim 12$, in agreement to previously found values. In the zeroth LL quartet, the $\nu =0$ gap as a function of magnetic field exhibits a plateau near 5 T, compared to $\sim 20–25 \mathrm{T}$ for pristine graphene. This behavior is in agreement with a model in which the SOC causes a phase transition from a canted antiferromagnetic state to a ferromagnetic state to occur at a much lower field. Our studies demonstrate how the interplay of SOC and electronic interactions affect graphene's electronic structure.

Based on the Boltzmann transport equation, we demonstrate that the thermal conductance per unit width of a sufficiently thin polar nanofilm supporting the propagation of surface-phonon polaritons along its surfaces is independent of the material properties and is given by $12z\left(3\right)k_B^3{T}^2/c{h}^2$, where $k_B$ and $h$ are the respective Boltzmann and Planck constants, while $c$ is the light speed in vacuum, $T$ is the temperature, and $z\left(3\right)$ is the Riemann zeta function. The huge propagation length of these energy carriers establishes that this quantization holds not only for temperatures much smaller than 1 K, as is the case of electrons and phonons, but also for those comparable to room temperature, which can significantly facilitate its observation and implementation in the thermal management of nanoscale electronics and photonics.