We present a holographic quantum simulation algorithm to variationally prepare thermal states of $d$-dimensional interacting quantum many-body systems, using only enough hardware qubits to represent a $(d-1)$-dimensional cross section. This technique implements the thermal state by approximately unraveling the quantum matrix-product density operator (qMPDO) into a stochastic mixture of quantum matrix-product states (sto-qMPS). The parameters of the quantum circuits generating the qMPS and of the probability distribution generating the stochastic mixture are determined through a variational optimization procedure. We demonstrate a small-scale proof-of-principle demonstration of this technique on Quantinuum's trapped-ion quantum processor to simulate thermal properties of correlated spin chains over a wide temperature range using only a single pair of hardware qubits. Then, through classical simulations, we explore the representational power of two versions of sto-qMPS ansatzes for larger and deeper circuits and establish empirical relationships between the circuit resources and the accuracy of the variational free energy.

The work is devoted to the theoretical study of the process of recombination of nonequilibrium charge carriers with the emission of plasmons in structures with two and three quantum wells, and also in an infinite number quantum-well structure under conditions of inverse band population. The plasmon spectra in these structures have been calculated. The dependence of the average probability of electron recombination with plasmon emission on the concentration of nonequilibrium carriers for three temperatures has been found. It has been shown that an increase in the number of quantum wells leads to a slight increase in the average recombination probability.

A noncentrosymmetric superconductor can have a superconducting diode effect, where the critical current in opposite directions is different when time-reversal symmetry is also broken. We theoretically propose that a ferroelectric superconductor with coexisting ferroelectricity and superconductivity can support a ferroelectric reversible superconducting diode effect. Through first-principles calculation, we predict that monolayer ${\mathrm{CuNb}}_2{\mathrm{Se}}_4$ (i.e., bilayer ${\mathrm{NbSe}}_2$ intercalated with Cu) is such a ferroelectric superconductor, where ferroelectricity controls the layer polarization as well as the sign of spin-orbit-coupling-induced spin splittings. Because the nonreciprocal effect of the critical current is proportional to the spin splittings, the superconducting diode effect is reversible upon electric switch of ferroelectricity. While we use ${\mathrm{CuNb}}_2{\mathrm{Se}}_4$ as a model system, the predicted effect can appear in a class of two-dimensional superconducting bilayers with ferroelectricity induced by interlayer sliding. Our work opens the door to studying the interplay between superconductivity and ferroelectricity in two-dimensional materials.

We investigate hard-core bosons filled in a lattice chain in the presence of a weak linear potential. In the single-particle case, we find that the critical point of dynamical Stark localization is different from that of static Stark localization. This suggests an intermediate phase in which the eigenstates are Stark-localized, but the dynamic wave functions are extended after quenching. In the many-body case, by comparing the dynamical critical point with the static critical point, we find a many-body intermediate phase that is analogous to the single-particle intermediate phase. Furthermore, we also study the static transition for the ground state and the dynamical transition for domain-wall states. In the ground state, we find that the localization transition point is at $V\approx 2(U+W)$ for half-filling ($U$ is the nearest-neighbor interaction strength, $W$ is the half-bandwidth). For the typical domain-wall state $|111\cdots 000\rangle$, its dynamical transition points are at $V\approx 4(U+W)$ and $V\approx 4(U-W)$. By analyzing the distribution of the occupation, we also offer a phenomenological way to estimate the above transition points.

The spin Hall magnetoresistance (SMR) effect in single-crystalline Pt/CoO(001) bilayers has been systematically investigated. X-ray magnetic linear dichroism measurements prove that CoO antiferromagnetic (AFM) spins can be switched into the direction orthogonal to the applied field. We find the SMR signal is comprised of two components related to either the switching of CoO AFM Néel order or the applied strong field effect. Both SMR components show a “positive” angular dependence with $R_{\parallel }>R_{\perp }$, while $R_{\parallel }\left(R_{\perp }\right)$ is defined as the resistance with the applied in-plane field parallel (perpendicular) to the current. The observed positive SMR is mainly attributed to the uncompensated spins at the Pt/CoO interface, instead of the CoO AFM spins. Our study may attract a great deal of interest to understand the complicated SMR effect in AFM spintronics materials.

We report the magnetic structure and the magnetocaloric effect (MCE) of the ternary compound $\mathrm{Ho}{\mathrm{Al}}_2{\mathrm{Ge}}_2$ with a trigonal ${\mathrm{CaAl}}_2{\mathrm{Si}}_2$-type crystal structure. A neutron powder diffraction experiment reveals that $\mathrm{Ho}{\mathrm{Al}}_2{\mathrm{Ge}}_2$ exhibits an incommensurate spin density wave (SDW) with a propagation vector $\mathit{k}=(0.23,0,0.06)$. The special arrangement of magnetic moments in $\mathrm{Ho}{\mathrm{Al}}_2{\mathrm{Ge}}_2$ induces interesting physical phenomena and large magnetocaloric effects. The rise in resistivity at low temperatures indicates the effect of the SDW state in the electronic transport. The maximum magnetic-entropy change is $-16.1\mathrm{J}/\mathrm{kg}\mathrm{K}$ under a magnetic field change of 0–70 kOe for an isotropic $\mathrm{Ho}{\mathrm{Al}}_2{\mathrm{Ge}}_2$ powder and it increases to $-17.9\mathrm{J}/\mathrm{kg}\mathrm{K}$ for a single crystal when the magnetic field ($H$) is applied parallel to the $ab$ plane. A large rotating magnetic-entropy change of $-5.1\mathrm{J}/\mathrm{kg}\mathrm{K}$ for $H=20\mathrm{kOe}$ in a $\mathrm{Ho}{\mathrm{Al}}_2{\mathrm{Ge}}_2$ single crystal is obtained, which is closely associated to the magnetic anisotropy of the SDW order and its response to the external magnetic field. We discuss the large MCE in terms of the field-induced metamagnetic transition from the incommensurate SDW order to the ferromagnetic order. Our study establishes the triangular lattice $R{\mathrm{Al}}_2{\mathrm{Ge}}_2$ ($R$ = rare-earth elements) as a unique family of compounds to explore the existence of the incommensurate spin density waves and the correlated physical properties.

The polaron dynamics in organic ladders is calculated in the presence of electron-lattice and spin-orbit couplings (SOCs), employing an extended Su-Schrieffer-Heeger (SSH) model and a nonadiabatic dynamics method. The time-dependent total charge and spin in the chains is averaged over initial polaron states with spins up and down. This average reveals a strong oscillating spin Hall effect (SHE). We show the necessity of all terms in the SSH Hamiltonian for the SHE, including intrachain and interchain hopping as well as intrachain and interchain SOC. The large and rapidly oscillating behavior of the SHE is verified to originate from the existence of polarons by comparison with the case of rigid chains without polarons. We attribute the enhancement of the SHE to skew scattering off transient deformations of the chains. Spectral analysis exhibits three dominant parts in the Fourier-transformed spin Hall signal. The high-frequency part is associated with the pure spin-flip dynamics due to SOC, while the low-frequency parts, which are also observed in the charge response, are related to the intrinsic electron transfer between the chains and the appearance of polarons, respectively. In this paper, we reveal the distinct properties of the dynamical SHE in organics dominated by polaron transport.

The ground state properties of the two-dimensional $J_1-J_2$-model are very challenging to analyze via classical numerical methods due to the high level of frustration. This makes the model a promising candidate for which quantum computers could be helpful to possibly explore regimes that classical computers cannot reach. The $J_1-J_2$-model is a quantum spin model composed of Heisenberg interactions along the rectangular lattice edges and along diagonal edges between next-nearest-neighbor spins. We propose an ansatz for the variational quantum eigensolver to approximate the ground state of an antiferromagnetic $J_1-J_2$ Hamiltonian for different lattice sizes and different ratios of $J_1$ and $J_2$. Moreover, we demonstrate that this ansatz can work without the need for gates along the diagonal next-nearest-neighbor interactions. This simplification is of great importance for solid-state-based hardware with qubits on a rectangular grid, where it eliminates the need for swap gates. In addition, we provide an extrapolation for the number of gates and parameters needed for larger lattice sizes, showing that these are expected to grow linearly in the qubit number up to lattice sizes which eventually can no longer be treated with classical computers.

The diagrammatic formalism and transport equation are conventionally considered as separate but complementary basic techniques to tackle the impurity scattering effect. To compare with previous studies from the gauge-invariant kinetic equation approach [F. Yang and M. W. Wu, Phys. Rev. B 98, 094507 (2018); F. Yang and M. W. Wu, Phys. Rev. B 102, 144508 (2020)], we analytically perform a diagrammatic formulation of the impurity scattering in superconductors, with both transport and collective Higgs modes studied, to fill the long missing calculation of the Kubo current-current correlation in superconductors with impurity scattering and resolve the controversy (whether the impurity scattering can lead to the damping of Higgs mode) between the gauge-invariant kinetic equation and Eilenberger equation. For transport behavior, through a special unitary transformation that is equivalent to the Wilson-line technique for the diamagnetic response, we derive the Meissner-supercurrent vertex. Then, by formulating the supercurrent-supercurrent correlation with Born and vertex corrections from impurity scattering, we recover the previously revealed microscopic momentum-relaxation rate of superfluid by gauge-invariant kinetic equation. This rate is finite only when the superconducting velocity is larger than a threshold, at which the normal fluid emerges and causes friction with the superfluid current, similar to Landau's superfluid theory of liquid helium. This derivation also provides a physical understanding of the relaxation-time approximation in the previous diagrammatic formulation in the literature, which leads to the friction resistance of the Meissner supercurrent. For the collective Higgs mode, we calculate the amplitude-amplitude correlation with Born and vertex corrections from impurity scattering. The vertex correction, which only emerges at the nonequilibrium case, leads to a Higgs-mode damping, whereas the Born correction that is equivalent to equilibrium self-energy makes no contribution due to the Anderson theorem. This induced damping agrees with the analysis through the Heisenberg equation of motion and is also exactly the same as the one obtained from the gauge-invariant kinetic equation.

By using density functional theory calculations including a Coulomb repulsion term, we explore the formation of oxygen vacancies and their impact on the electronic and magnetic properties of strained bulk ${\mathrm{LaNiO}}_3$ and (${\mathrm{LaNiO}}_3{)}_1$/(${\mathrm{LaAlO}}_3{)}_1$(001) superlattices. For bulk ${\mathrm{LaNiO}}_3$, we find that epitaxial strain induces a substantial anisotropy in the oxygen vacancy formation energy. In particular, tensile strain promotes the selective reduction of apical oxygen, which may explain why the recently observed superconductivity in infinite-layer nickelates is limited to strained films. For (${\mathrm{LaNiO}}_3{)}_1$/(${\mathrm{LaAlO}}_3{)}_1$(001) superlattices, the simulations reveal that the ${\mathrm{NiO}}_2$ layer is most prone to vacancy formation, whereas the ${\mathrm{AlO}}_2$ layer exhibits generally the highest formation energies. The reduction is consistently endothermic, and a largely repulsive vacancy-vacancy interaction is identified as a function of the vacancy concentration. The released electrons are accommodated exclusively in the ${\mathrm{NiO}}_2$ layer, reducing the vacancy formation energy in the ${\mathrm{AlO}}_2$ layer by $\sim 70\%$ with respect to bulk ${\mathrm{LaAlO}}_3$. By varying the vacancy concentration from 0 to $8.3\%$ in the ${\mathrm{NiO}}_2$ layer at tensile strain, we observe an unexpected transition from a localized site-disproportionated ($0.5\%$) to a delocalized ($2.1\%$) charge accommodation, a reentrant site disproportionation leading to a metal-to-insulator transition despite a half-filled majority-spin Ni $e_g$ manifold ($4.2\%$), and finally a magnetic phase transition ($8.3\%$). While a band gap of up to 0.5 eV opens at $4.2\%$ for compressive strain, it is smaller for tensile strain or the system is metallic, which is in sharp contrast to the defect-free superlattice. The strong interplay of electronic reconstructions and structural modifications induced by oxygen vacancies in this system highlights the key role of an explicit supercell treatment beyond rigid-band methods and exemplifies the complex response to defects in artificial transition metal oxides.

We present an algorithm for reliably and systematically proving the existence of spectral gaps in Hamiltonians with quasicrystalline order, based on numerical calculations on finite domains. We apply this algorithm to prove that the Hofstadter model on the Ammann-Beenker tiling of the plane has spectral gaps at certain energies, and we are able to prove the existence of a spectral gap where previous numerical results were inconclusive. Our algorithm is applicable to more general systems with finite local complexity and eventually finds all gaps, circumventing an earlier no-go theorem regarding the computability of spectral gaps for general Hamiltonians.

Oxide materials exhibit several structural, magnetic, and electronic properties. Their stability under ambient conditions, easy synthesis, and high transition temperatures provide such systems with an ideal ground for realizing topological properties and real-life technological applications. However, experimental evidence of topological states in oxide materials is rare. In this paper, we have synthesized single crystals of oxide double perovskite ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6$ and revealed its topological nature by investigating its structural, magnetic, and electronic properties. We observed that the system crystallized in the cubic space group $Fm\overline{3}m$, which is a half-metallic ferromagnet. Transport measurements show an anomalous Hall effect (AHE), and it is evident that the Hall contribution originates from the Berry curvature. Assuming a shift of the Fermi energy toward the conduction band, the contribution of the AHE is enhanced owing to the presence of a gapped nodal line. This paper can be used to explore and realize the topological properties of bulk oxide systems.

We study the flat-band ferromagnetic phase of a spinfull and time-reversal symmetric Haldane-Hubbard model on a honeycomb lattice within a bosonization formalism for flat-band ${\mathrm{Z}}_2$ topological insulators. Such a study extends our previous one [Phys. Rev. B 104, 155129 (2021)] concerning the flat-band ferromagnetic phase of a correlated Chern insulator described by a Haldane-Hubbard model. We consider the topological Hubbard model at $1/4$ filling of its corresponding noninteracting limit and in the nearly flat band limit of its lower free-electronic bands. We define boson operators associated with two distinct spin-flip excitations, one that changes (mixed-lattice excitations) and a second one that preserves (same-lattice excitations) the index related to the two triangular sublattices. Within the bosonization scheme, the fermion model is mapped into an effective interacting boson model, whose quadratic term is considered at the harmonic approximation in order to determine the spin-wave spectrum. For both mixed- and same-lattice excitations, we find that the spin-wave spectrum is gapped and has two branches, with an energy gap between the lower and the upper bands at the $K$ and $K^{\prime }$ points of the first Brillouin zone. We find that the same-lattice excitations are indeed the lowest-energy (elementary) excitations that characterize the flat-band ferromagnetic phase, a feature that contrasts with the behavior of a previously studied correlated topological insulator on a square lattice, whose flat-band ferromagnetic phase is characterized by mixed-lattice excitations. We also find some evidences that the spin-wave bands for the same-lattice excitations might be topologically nontrivial even in the completely flat band limit.

Intrinsic magnetic topological insulators (MTIs), such as an ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ family, have proved to be an essential platform for the study of various quantum effects and can be used for a range of applications from information storage and dissipationless spin and charge transfer to quantum computers. The latter requires the creation of Majorana fermions, which are expected to emerge when a superconductor is contacted with an MTI surface. Therefore the study of the features arising at the interface between the MTI and the superconductor is of great interest. For this purpose in this paper, the gradual growth of the elemental superconductor (Pb) film on the surface of $\mathrm{Mn}{\left({\mathrm{Bi}}_{0.7}{\mathrm{Sb}}_{0.3}\right)}_2{\mathrm{Te}}_4$, which has a Dirac point near the Fermi level, was experimentally investigated by ARPES and XPS methods. We observed that the shape of the Dirac cone state remains almost unchanged up to coverage of $\sim 1.5$ Pb monolayers, thus remaining topological properties of the material. It was shown that Pb atoms deposition results in surface modification. They detached upper Te atoms and form TePb alloy on the surface. Moreover, Pb deposition leads to disorder of Te, Bi(Sb) layers closest to the surface. As a result, the localization of the Dirac cone state shifts towards the bulk. All these features should be taken into account in the implementation of a topological superconducting state based on the Pb/Mn(Bi,${\mathrm{Sb})}_2{\mathrm{Te}}_4$ systems.

We theoretically study a strongly disordered superconducting layer heated by near-field microwave radiation from a nanometric metallic tip. The microwaves heat up the quasiparticles, which cool by phonon emission and conduction away from the heated area. Due to a bistability with two stable states of the electron temperature under the tip, the heating can be tuned to induce a submicrometer-sized normal region bounded by a sharp domain wall between high- and low-temperature states. We propose this as a local probe to access different physics from existing methods, for example, to map out inhomogeneous superfluid flow in the layer. The bistability-induced domain wall can significantly improve its spatial resolution.

The Rashba spin-orbit (SO) coupling can flexibly be controlled via an external gate, while the Dresselhaus term, which mainly depends on quantum confinement, is in general immune to electrical control. Here we theoretically report optical manipulation of the SO coupling by resorting to an intense high-frequency laser field, which “dresses” the confining potential for electrons as a result of optical stark effect and enables a flexible and simultaneous control of the Rashba and Dresselhaus couplings. Focusing on ordinary GaInAs/AlInAs quantum wells with two occupied subbands subject to both laser and gate fields, we perform a self-consistent Poisson-Schrödinger calculation in the Hartree approximation to determine electro-optical control of the intrasubband (intersubband) Rashba ${\alpha }_{\nu }$ ($\eta$) and Dresselhaus ${\beta }_{\nu }$ ($\mathrm{\Gamma }$) SO terms with $\nu =1,2$. Under the impact of laser field, we find that the Rashba terms of the two subbands ${\alpha }_1$ and ${\alpha }_2$ may remain locked to equal strength in a broad gate range, providing a means for unified manipulation of the two-subband Rashba couplings. Further, as the laser field varies, we observe that ${\alpha }_1$ and ${\alpha }_2$ may have either the same or opposite signs, or even ${\alpha }_2$ vanishes while ${\alpha }_1$ is finite, greatly fascinating for selective SO control of distinct subbands. For the Dresselhaus coupling, we disclose two distinct scenarios depending on the interplay of the well width and the laser field strength, and reveal that ${\beta }_2$ may decrease rapidly when the laser field strengthens, even though ${\beta }_1$ remains essentially constant. Regarding the intersubband Rashba ($\eta$) and Dresselhaus ($\mathrm{\Gamma }$) terms, which mainly depend on the overlap and parity of the wave functions of the two subbands, they have relatively weak dependence on the laser field. Moreover, the combined effect of intra- and intersubband SO terms may lead to crossings and avoided crossings of the energy dispersion of multiband spin branches and may even trigger the spin polarization of an originally spin degenerate (unpolarized) band, tunable by the laser field. Our results should stimulate experiments probing the laser field mediated multiband SO control and further enables its spintronic applications.

Materials carrying topological surface states (TSS) provide a fascinating platform for the hydrogen evolution reaction (HER). Based on systematic first-principles calculations for $A_{3}B$ ($A$ = Ni, Pd, Pt; $B$ = Si, Ge, Sn), we propose that topological electric polarization characterized by the Zak phase can be crucial to designing efficient catalysts for the HER. For $A_{3}B$, we show that the Zak phase takes a nontrivial value of $\pi$ in the whole (111) projected Brillouin zone, which causes quantized electric polarization charges at the surface. There, depending on the adsorption sites, the hydrogen (H) atom hybridizes with the TSS rather than with the bulk states. When the hybridization has an intermediate character between the covalent and ionic bonds, the H states are localized in the energy spectrum, and the change in the Gibbs free energy ($\mathrm{\Delta }G$) due to the H adsorption becomes small. Namely, the interaction between the H states and the substrate becomes considerably weak, which is a highly favorable situation for the HER. Notably, we show that $\mathrm{\Delta }G$ for ${\mathrm{Pt}}_3\mathrm{Sn}$ and ${\mathrm{Pd}}_3\mathrm{Sn}$ are just $-0.066$ and $-0.092$ eV, respectively, which are almost half of the value of Pt.

The nature of a metal-insulator and magnetic transitions is still a subject under intense debate in condensed-matter physics. Among the many possible mechanisms, the interplay between electronic correlations and spin-orbit couplings is an issue of a great deal of interest, in particular, when dealing with quasi-two-dimensional compounds. In view of this, here we use a Hartree-Fock approach to investigate how the Rashba spin-orbit coupling $V_{\text{SO}}$ affects the magnetic ordering provided by a Hubbard interaction $U$ on a square lattice. At half-filling, we have found a sequence of transitions for increasing $V_{\text{SO}}$: from a Mott insulator to a metallic antiferromagnet and then to a paramagnetic Rashba metal. Also, our results indicate that the Rashba coupling favors magnetic striped phases in the doped regime. By analyzing spectral properties, we associate the rearrangement of the magnetic ordering with the emerging chirality created by the spin-orbit coupling. Our findings provide insights towards clarifying the competition between these tendencies.