Phonon lifetimes are a key component of quasiparticle theories of transport; yet first-principles lifetimes are rarely directly compared with inelastic neutron scattering (INS) results. Existing comparisons show discrepancies even at temperatures where perturbation theory is expected to be reliable. In this paper, we demonstrate that the reciprocal space voxel ($q$ voxel), which is the finite region in reciprocal space required in INS data analysis, must be explicitly accounted for within theory in order to draw a meaningful comparison. We demonstrate accurate predictions of peak widths of the scattering function when accounting for the $q$ voxel in ${\mathrm{CaF}}_2$ and ${\mathrm{ThO}}_2$. Passing this test implies high fidelity of the phonon interactions and the approximations used to compute the Green's function, serving as a critical benchmark of theory and indicating that other material properties should be accurately predicted, which we demonstrate for thermal conductivity.

We present a study combining neutron diffraction and ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy on a powder sample of ${\mathrm{CaBaMn}}_2{\mathrm{Fe}}_2{\mathrm{O}}_7$ belonging to the large family of swedenborgite compounds. The undistorted hexagonal crystal structure (space group $P{6}_{3}mc$) is preserved down to low temperatures, and all employed techniques reveal a transition into a magnetically long-range ordered phase at $T_{\mathrm{N}}$ = 205 K. The magnetic Bragg peak intensities from the powder diffraction patterns together with a symmetry analysis of the employed models unambiguously reveal the classical $\sqrt{3}\times \sqrt{3}$ magnetic structure on a hexagonal lattice with propagation vector $\mathbf{q}=\left(\frac{1}{3}\frac{1}{3}0\right)$. The nuclear Bragg peak intensities allowed the statistical distribution of Fe and Mn ions on both trigonal and kagome sites of the complex swedenborgite structure to be analyzed which was considered to explain the complex shape of the Mössbauer spectra.

A superconducting (SC) mixed state occurs in type-II superconductors where the upper critical field $H_{c2}$ is higher than the thermodynamic critical field $H_c$. When an applied field is in between these fields, the free energy depends weakly on the order parameter which therefore can be small (SC state) or zero (normal state) at different parts of the sample. In this paper we demonstrate how a normal state along a line traversing a superconductor can be turned on and off externally in zero field. The concept is based on a long, current-carrying excitation coil, piercing a ring-shaped superconductor. The ring experiences zero field, but the vector potential produced by the coil generates a circular current that destroys superconductivity along a radial line starting at preexisting nucleation points in the sample. Unlike the destruction of superconductivity with magnetic field, the vector potential method is reversible and reproducible; full superconductivity is recovered upon removing the current from the coil, and different cooldowns yield the same normal lines. We suggest potential applications of this magnetic-field-free mixed state.

According to topological quantum chemistry (TQC), a class of electronic materials has been called obstructed atomic insulators (OAIs), in which a portion of valence electrons necessarily have their centers located on some empty Wyckoff positions (WPs) without atom occupation in the lattice. The obstruction of centering these electrons coinciding with their host atoms is nontrivial and results in metallic boundary states when the boundary is properly cut. Here, on the basis of first-principles calculations in combination with TQC analysis, we propose two-dimensional ${MA}_2{Z}_4$ $(M=\mathrm{Cr}, \mathrm{Mo}, and \mathrm{W}; A=\mathrm{Si} \mathrm{and} \mathrm{Ge}; Z=\mathrm{N}, \mathrm{P}, \mathrm{and} \mathrm{As})$ monolayers are all OAIs. A typical case is the recently synthesized ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ (${\alpha }_1\text{−}{\mathrm{MoSi}}_2{\mathrm{N}}_4$). Although both ${\alpha }_1$- and ${\alpha }_2\text{−}{\mathrm{MoSi}}_2{\mathrm{N}}_4$ monolayers are topological trivial insulators, it has valence electrons centering empty WPs and the in-gap corner states at the three vertices of triangle-shaped nanodisk samples respecting $C_3$ rotation symmetry, which can be explained by the filling anomaly of electrons. In addition, the ${\alpha }_2\text{−}{\mathrm{MoSi}}_2{\mathrm{N}}_4$ monolayer exhibits unique OAI-induced metallic edge states along the $\left(1\overline{1}\right)$ edge. The readily synthesized ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ is quite stable and has a large bulk bandgap of 1.94 eV, which makes the identification of these edge and corner states most possible for experimental clarification.

We develop a theory for a continuous bandwidth-tuned transition at fixed fractional electron filling from a metal with a generic Fermi surface to a “Wigner-Mott” insulator that spontaneously breaks crystalline space-group symmetries. Across the quantum critical point, (i) the entire electronic Fermi surface disappears abruptly upon approaching from the metallic side, and (ii) the insulating charge gap and various order parameters associated with the spontaneously broken space-group symmetries vanish continuously upon approaching from the insulating side. Additionally, the insulating side hosts a Fermi surface of neutral spinons. We present a framework for describing such continuous metal-insulator transitions (MITs) and analyze the example of a bandwidth-tuned transition at a filling, $\nu =1/6$, for spinful electrons on the triangular lattice. By extending the theory to a certain large-$N$ limit, we provide a concrete example of such a continuous MIT and discuss numerous experimental signatures near the critical point. We place our results in the context of recent experiments in moiré transition metal dichalcogenide materials.

The magnetotransport experiments based on the weak antilocalization (WAL) effect have confirmed the common belief that the Rashba spin-orbit coupling (SOC) of two-dimensional (2D) holes in semiconductor quantum wells (QWs) is a $k$-cubic term as the lowest order with negligible linear terms. However, an emerging finite linear Rashba SOC was recently found in 2D holes by semiempirical pseudopotential method (SEPM) due to the direct dipolar coupling of an external electric field to the valence subbands in the presence of heavy-hole–light-hole (HH-LH) mixing. Here, we resolve this discrepancy by illustrating that the hole densities in the experiments are so high that the emerging linear term becomes undetectable since its strength declines substantially as increasing the wavevector $k$. Taking the example of a strained $\mathrm{Ge}/{\mathrm{Si}}_{0.5}{\mathrm{Ge}}_{0.5}$ QW utilized in the experiment [Phys. Rev. Lett. 113, 086601 (2014)], we demonstrate that the hole density must be reduced by a factor of 5 to below $2.1\times {10}^{11}{\text{cm}}^{-2}$ in order to probe the $k$-linear term. We also evaluate the possibility to achieve WAL at low hole densities in order to measure SOC. These findings shed new light on the experimental measurement of Rashba SOC.

Flat bands in twisted moiré superlattices support a variety of topological and strongly correlated phenomena along with easily tunable electrical and optical properties. Here, we demonstrate the existence of long lived, flat intraband and interband plasmons in twisted double bilayer graphene. We show that the interband plasmons originate from the presence of a Van Hove singularity in the joint density of states and a finite Berry connection between the pair of bands involved. We find that the gapped interband plasmon mode has a universal dispersion, and the plasmon gap is specified by the location of the Van Hove singularity in the joint density of states. Metallic moiré systems support an additional intraband plasmon mode which becomes flat in the large momentum limit because of the influence of higher interband transitions. We demonstrate that the undamped and flat plasmon modes in moiré systems are highly tunable and can be controlled by varying the vertical electric field and electron doping, and they persist over a wide range of twist angles.

In recent years, programmable Rydberg-atom arrays have been widely used to simulate new quantum phases and phase transitions, generating great interest among theorists and experimentalists. Based on the large-scale density matrix renormalization group method, the ground-state phase diagram of one-dimensional Rydberg chains is investigated with fidelity susceptibility as an efficient diagnostic method. We find that the competition between Rydberg blockade and external detuning produces unconventional phases and phase transitions. As the Rydberg blockade radius increases, the phase transition between disordered and density-wave ordered phases changes from the standard Potts universality class to an unconventional chiral one. As the radius increases further (above Potts point but still close to the tip of the lobe), a very narrow intermediate floating-phase region begins to appear. A concise physical picture is also provided to illustrate the numerical results. Compared with previous studies, this work brings more evidence for commensurate-incommensurate quantum phase transitions in programmable quantum simulators from the perspective of quantum information, showing that fidelity susceptibility can be used to study such phase transitions.

Plasmons are elementary quantum excitations of conducting materials with Fermi surfaces. In two dimensions they may carry a static dipole moment that is transverse to their momentum which is quantum geometric in nature, the quantum geometric dipole (QGD). We show that this property is also realized for such materials confined in nanowire geometries. Focusing on the gapless, intrasubband plasmon excitations, we compute the transverse dipole moment ${\mathcal{D}}_x$ of the modes for a variety of situations. We find that single chiral fermions generically host nonvanishing ${\mathcal{D}}_x$, even when there is no intrinsic gap in the two-dimensional spectrum, for which the corresponding two-dimensional QGD vanishes. In the limit of very wide wires, the transverse dipole moment of the highest velocity plasmon mode matches onto the two-dimensional QGD. Plasmons of multivalley systems that are time-reversal symmetric have a vanishing transverse dipole moment but can be made to carry nonvanishing values by breaking the valley symmetry, for example, via a magnetic field. The presence of a nonvanishing transverse dipole moment for nanowire plasmons in principle offers the possibility of continuously controlling their energies and velocities by the application of a static transverse electric field.

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.