We present a study of the principal deuterium Hugoniot for pressures up to 150 GPa, using machine learning potentials (MLPs) trained with quantum Monte Carlo (QMC) energies, forces, and pressures. In particular, we adopted a recently proposed workflow based on the combination of Gaussian kernel regression and $\mathrm{\Delta }$-learning. By fully taking advantage of this method, we explicitly considered finite-temperature electrons in the dynamics, whose effects are highly relevant for temperatures above 10 kK. The Hugoniot curve obtained by our MLPs shows a good agreement with the most recent experiments, particularly in the region below 60 GPa. At larger pressures, our Hugoniot curve is slightly more compressible than the one yielded by experiments, whose uncertainties generally increase, however, with pressure. Our work demonstrates that QMC can be successfully combined with $\mathrm{\Delta }$-learning to deploy reliable MLPs for complex extended systems across different thermodynamic conditions, by keeping the QMC precision at the computational cost of a mean-field calculation.

We study theoretically the magic-angle twisted bilayer graphene with proximity-induced Ising and Rashba spin-orbit couplings on the top layer. Topological flat bands (with three distinct phases) are generically realized by the spin-orbit couplings. Using a mean field analysis, we find that (partial) valley polarization prevails for a wide range of doping, suppressing the usual superconductivity with a pairing between time-reversal partners. Remarkably, we uncover that observable unconventional intervalley interband phonon-mediated superconductivity (with the highest $T_c\approx 1.2\mathrm{K}$) can coexist with strong valley imbalance due to the approximate Fermi-surface nesting between two flat bands not related by time-reversal symmetry, and the dominant pairing is an intersublattice Ising pairing, corresponding to a mixture of $p$ and $d$ waves. In contrast, the intrasublattice Ising phonon-mediated superconductivity with $s$- and $f$-wave mixing emerges in the absence of valley imbalance. Our work reveals an unprecedented route of realizing unconventional superconductivity.

We use pump-probe, spin- and angle-resolved photoemission spectroscopy (ARPES) with a 10.7 eV laser accessible up to the Brillouin zone edge, and reveal the entire band structure around the Fermi level, including the unoccupied side, for the elemental bismuth (Bi) with the spin-polarized surface states. Our data identify Bi as in a strong topological insulator phase ($Z_2=1)$ against the prediction of most band calculations. We unveil that the unoccupied topological surface states possess spin-polarized saddle points yielding the van Hove singularity. The unique feature provides an excellent platform for the future development of optospintronics.

Superclimbing modes are hallmark degrees of freedom of transverse quantum fluids describing wide superfluid one-dimensional interfaces and/or edges with negligible Peierls barrier. We report direct numeric evidence of quantum shape fluctuations, caused by superclimbing modes, in simple lattice models, as well as at the free edge of an incomplete solid monolayer of ${}^4\mathrm{He}$ adsorbed on graphite. Our data unambiguously reveals the defining feature of the superclimbing modes—canonical conjugation of the edge displacement field to the field of superfluid phase—and its unexpected implication, i.e., that superfluid stiffness can be inferred from density snapshots.

The magnetocaloric effect (MCE) is calculated in the framework of the Landau theory in a ferromagnet/antiferromagnet/ferromagnet (FM/AFM/FM) layered structure, where the AFM interlayer acts as a solid refrigerant. Exchange fields arise at the interfaces suppressing antiferromagnetic order in the interlayer which leads to an increase in its entropy. The direction of these exchange fields (the direction of magnetizations of the FM layers) can be changed by variation of an external magnetic field. The isothermal entropy change caused by a change in this direction from perpendicular to parallel relative to the easy axis of the AFM interlayer can reach up to $5\times {10}^5\text{erg}{\text{K}}^{-1}{\text{cm}}^{-3}$ $\left(13\text{J}{\text{kg}}^{-1}{\text{K}}^{-1}\right)$ in the ${\text{FM/MnF}}_2\text{/FM}$ structure if the field of $10\text{kOe}$ is applied. Large exchange enhancement of the MCE in this layered structure is demonstrated compared to a separate ${\text{MnF}}_2$ layer. The obtained results may be useful in thin-film coolers.

The collective tunneling of a Wigner necklace—a crystal-like state of a small number of strongly interacting electrons confined to a suspended nanotube and subject to a double-well potential—is theoretically analyzed and compared with experiments in Shapir et al. [Science 364, 870 (2019)]. Density matrix renormalization group computations, exact diagonalization, and instanton theory provide a consistent description of this very strongly interacting system, and show good agreement with experiments. Experimentally extracted and theoretically computed tunneling amplitudes exhibit a scaling collapse. Collective quantum fluctuations renormalize the tunneling, and substantially enhance it as the number of electrons increases.

Resonant photoelectron diffraction (RPED) and its use for studies of lanthanides are presented. To demonstrate the potential of this technique, we show how the ground-state properties of the heavy-fermion compound ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$ and its trivalent counterpart ${\mathrm{YbCo}}_2{\mathrm{Si}}_2$ can be determined from RPED measurements and modeling. By providing a modeling recipe and exploring RPED capabilities and limitations, we anticipate wide applications of this method for insights into the properties of lanthanide-based systems.

Iron, nickel, and their alloys are critically important materials for industrial and technological applications due to their unique magnetic properties, strength, and thermal expansion. In this study, lasers were used to compress and heat ${\mathrm{Fe}}_{36}\mathrm{Ni}$ alloy (36 wt% Ni) and pure nickel up to the melting temperature using a combination of shock and ramp compression. The structure was measured using nanosecond in situ x-ray diffraction, and simultaneous velocimetry was used to measure the pressure up to 454 GPa. A mixed face-centered-cubic (fcc) solid–liquid phase in ${\mathrm{Fe}}_{36}\mathrm{Ni}$ at 311 GPa provides experimental evidence that, compared with pure iron, the incorporation of nickel expands the stability field of the fcc phase to the melting curve. At lower temperatures, a mixed fcc and hexagonal-close-packed (hcp) phase is observed in ramp-compressed ${\mathrm{Fe}}_{36}\mathrm{Ni}$ at 278 GPa. At the higher compressions, a structure inconsistent with fcc, hcp, and body-centered cubic is observed. In the case of pure Ni, the fcc phase is stable under ramp compression up to 402 GPa.

Various $s{p}^3$-bond-rich noncrystalline carbon materials with extraordinary properties have been synthesized through structural transitions of $s{p}^2$-bonded amorphous carbon under high pressure and high temperature (HPHT). However, the specific role of pressure and temperature in the process of the structural transition remains elusive due to the lack of in situ investigation. Here, combining in situ HPHT synchrotron x-ray diffraction and first-principles simulations, we investigated the structural evolution of glassy carbon (GC) (an archetype amorphous carbon) under pressure and thermal annealing. We found GC transformed to a recoverable superstrong amorphous carbon with a considerable number of $s{p}^3$ bonds when compressed to ∼58 GPa and annealed at ∼728 K. Our results indicate that thermal annealing (even far below 1000 K) plays a key and unique role in stabilizing the $s{p}^3$ carbon bonds by lowering the free energy through local structural rearrangements. Otherwise, most pressure-induced $s{p}^3$ carbon bonds could not be preserved upon pressure release. Therefore, temperature is an essential tuning parameter for synthesizing $s{p}^3$-bond-rich amorphous carbon with tailorable properties. These results improve our fundamental understanding of the structural transition of amorphous carbon under HPHT and provide crucial guidance for synthesizing novel amorphous carbon materials.

The superfluid weight of an isolated flat band in multiorbital superconductors contains contributions from the band's quantum metric and a lattice geometric term that depends on the orbital positions in the lattice. Since the superfluid weight is a measure of the superconductor's energy fluctuation, it is independent of the lattice geometry, leading to the minimal quantum metric of a band [Phys. Rev. B 106, 014518 (2022)]. Here, a perturbation approach is developed to study the superfluid weight and its lattice geometric dependence for composite bands. When all orbitals exhibit uniform pairing, the quantum geometric term contains each band's contribution and an interband contribution between every pair of bands in the composite. Based on a band representation analysis, they provide a topological lower bound for the superfluid weight of an isolated composite of flat bands. Using this perturbation approach, an analytical expression of the lattice geometric contribution is obtained. It is expressed in terms of Bloch functions, providing a convenient formula to calculate the superfluid weight for multiorbital superconductors.

Materials that exhibit strongly coupled magnetic order and electronic properties are crucial for both fundamental research and technological applications. However, finding a material that not only shows remarkable magnetoresistive responses but also has an easily tunable ground state remains a challenge. Here, we report successful manipulation of the magnetic and transport properties of ${\mathrm{EuCd}}_2{\mathrm{P}}_2$, which is transformed from an A-type antiferromagnet $(T_{\mathrm{N}}=11\mathrm{K})$ exhibiting colossal magnetoresistance into a ferromagnet $(T_{\mathrm{C}}=47\mathrm{K})$ with metallic behavior. The dramatic alteration results from a low hole concentration of ${10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$ induced by changing the growth conditions. Electronic structure and total energy calculations confirm the tunability of magnetism with a small carrier concentration for ${\mathrm{EuCd}}_2{\mathrm{P}}_2$. It is feasible to switch between the magnetic states by using field-effect to control the carrier density, thereby changing the magneto-electronic response. The controllable magnetism and electrical transport of ${\mathrm{EuCd}}_2{\mathrm{P}}_2$ make it a potential candidate for spintronics.

This paper presents a comprehensive investigation of ${\mathrm{NaYbTe}}_2{\mathrm{O}}_7$ through a range of physical property measurements. A detailed examination of its crystal structure uncovers the formation of quasi-one-dimensional magnetic chains of ${\mathrm{Yb}}^{3+}$ ions along the $b$ axis, with a substantial separation of approximately 6.3 Å between these chains. This unique arrangement of Yb ions positions ${\mathrm{NaYbTe}}_2{\mathrm{O}}_7$ as an ideal candidate for exploring one-dimensional magnetism. Magnetic measurements confirm the absence of a long-range magnetic ordering down to 0.4 K; however, they indicate the emergence of magnetic correlations below 1 K. The analysis of the inverse magnetic susceptibility corroborates that the ground state can be described as a Kramers doublet, indicating $J_{\mathrm{eff}}=1/2$. Our findings from magnetization measurements align with the results from temperature and magnetic field-dependent heat capacity measurements, both of which suggest robust magnetic correlations among ${\mathrm{Yb}}^{3+}$ ions. Moreover, heat capacity measurements reveal no long-range magnetic ordering down to 0.285 K. The assertion of these magnetic correlations is further reinforced by thermal conductivity measurements, which confirm the scattering of phonons due to magnetic excitations at low temperatures. Considering the combination of low dimensionality, $J_{\mathrm{eff}}=1/2$, the absence of magnetic ordering down to 0.285 K, and the presence of strong magnetic correlation below 1 K, we suggest that these results are indicative of a potential quantum spin liquid (QSL) state in ${\mathrm{NaYbTe}}_2{\mathrm{O}}_7$.

Altermagnets are compensated magnets belonging to spin symmetry groups that allow alternating spin polarizations both in the coordinate space of the crystal and in the momentum space of the electronic structure. In these materials the anisotropic local crystal environment of the different sublattices lowers the symmetry of the system so that the opposite-spin sublattices are connected only by rotations. This results in an unconventional spin-polarized band structure in the momentum space. This low symmetry of the crystal structure is expected to be reflected in the anisotropy of the anomalous Hall effect. In this work, we study the anisotropy of the anomalous Hall effect in epitaxial thin films of ${\mathrm{Mn}}_5{\mathrm{Si}}_3$, an altermagnetic candidate material. We first demonstrate a change in the relative Néel vector orientation when rotating the external field orientation through systematic changes in both the anomalous Hall effect and the anisotropic longitudinal magnetoresistance. We then show that the anomalous Hall effect in this material is anisotropic with the Néel vector orientation relative to the crystal structure and that this anisotropy requires high crystal quality and unlikely stems from the magnetocrystalline anisotropy. Our results thus provide further systematic support to the case for considering epitaxial thin films of ${\mathrm{Mn}}_5{\mathrm{Si}}_3$ as an altermagnetic candidate material.

Antiferromagnets offer much faster dynamics compared to their ferromagnetic counterparts but their order parameter is extremely difficult to detect and control. So far, controlling the Néel order parameter electrically is limited to only very few materials where Néel spin-orbit torques are allowed by symmetry. In this work, we show that coupling a thin ferromagnet (permalloy) layer on top of an antiferromagnet $\left({\mathrm{Mn}}_2\mathrm{Au}\right)$ solves a major roadblock—the controlled reading, writing, and manipulation of antiferromagnetic domains. We confirm by atomistic spin dynamics simulations that the domain wall patterns in the ${\mathrm{Mn}}_2\mathrm{Au}$ are imprinted on the permalloy, therefore allowing for indirect imaging of the Néel order parameter. Our simulations show that the coupled domain wall structures in ${\mathrm{Mn}}_2\mathrm{Au}$-Py bilayers can be manipulated by either acting on the Néel order parameter via Néel spin-orbit torques or by acting on the magnetization (the ferromagnetic order parameter) via magnetic fields. In both cases, we predict ultrahigh domain wall speeds on the order of 8.5 km/s. Thus, employing a thin ferromagnetic layer has the potential to easily control the Néel order parameter in antiferromagnets even where Néel spin-orbit torques are forbidden by symmetry. The controlled manipulation of the antiferromagnetic order parameter provides a promising basis for the development of high-density storage and efficient computing technologies working in the THz regime.

We obtain an almost complete understanding of the mean-field phase diagram of the two-dimensional Hubbard model on a square lattice with a sizable next-nearest-neighbor hopping and a moderate interaction strength. In particular, we clarify the nature of the transition region between the spiral and the stripe phase. Complementing previous [Phys. Rev. B 108, 035139 (2023)] real-space Hartree-Fock calculations on large finite lattices, we solve the mean-field equations for coplanar unidirectional magnetic order directly in the thermodynamic limit, and we determine the nature of the magnetic states right below the mean-field critical temperature $T^{*}$ by a Landau free-energy analysis. While the magnetic order for filling factors $n\ge 1$ is always of Néel type, for $n\le 1$ the following sequence of magnetic states is found as a function of increasing hole-doping: Néel, planar circular spiral, multispiral, and collinear spin-charge stripe states. Multispiral states are superpositions of several spirals with distinct wave vectors, and lead to concomitant charge order. We finally point out that nematic and charge orders inherited from the magnetic order can survive even in the presence of fluctuations, and we present a corresponding qualitative phase diagram.

We present a theoretical study of the structural, electronic, and topological properties of the ternary alloy $({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x{)}_2{\mathrm{Se}}_3$ at the rhombohedral phase through the generalized quasichemical approximation (GQCA) statistical approach and first-principles calculations, including approximate quasiparticle corrections with density functional theory (DFT)-1/2 method. Our results predict good miscibility along the entire composition range. The system exhibits an increase in the inverted energy gap with the increase of the antimony concentration in the alloy. We predict a maximum band gap of 0.28 eV for the Sb concentration $x=0.7$, as well its variation along the entire alloy composition. The inversion of the atomic orbital character of the valence and conduction band is observed for any considered composition, as well nontrivial ${\mathbb{Z}}_2$ topological invariant. The system $({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x{)}_2{\mathrm{Se}}_3$ is a promising candidate to be a topological insulator without topological phase transition, maintaining its topological order with ${\mathbb{Z}}_2=1$ for the entire alloy composition.

In this paper, the supercurrent of the superconductor (SC)–Weyl semimetal–SC Josephson junction hybrid system is studied based on the Landauer-Büttiker formula combined with the nonequilibrium Green's method. According to the different edge states, the Fermi arcs are divided into three regions: the symmetric, topological, and insulated regions, which are defined by the azimuthal angle $\theta$ and the transverse momentum $k_z$. Here, $\theta$ is the azimuthal angle between the symmetry axis of the Fermi arcs and the normal of the Josephson junction. It is found that the supercurrent is suppressed in the symmetric region. In the topological region, the supercurrent increases with increasing $\theta$. The supercurrent is almost zero in the insulated region due to the presence of the energy gap. The total supercurrent summed over all $k_z$ channels increases and then decreases with increasing $\theta$, that is, $\theta$ can control the value of the supercurrent. In addition, the magnetic field and the azimuthal angle $\theta$ can adjust the phase transition of the Josephson junction, allowing it to be a 0, $\pi$, or ${\phi }_0$ junction. These properties provide research ideas for controlling the value of the supercurrent and the Josephson junction phase transition and play an important role in the development and application of superconducting electronic devices.

The manipulation of the anomalous Hall effect (AHE) by controlling magnetization is of great interest in condensed matter physics due to its potential application for the practical design of spintronic devices. In this study, we report a combined experimental and theoretical investigation of the AHE in the MnSb manganese pnictide. Temperature-dependent magnetization measurement indicates spin reorientation transition (SRT) temperature at $\sim$ 120 K (${\mathrm{T}}_{\mathrm{SR}}$). Magnetotransport data shows that negative magnetoresistance increases from room temperature up to SRT temperature 120 K, then decreases and becomes positive at very low temperatures. The anomalous Hall conductivity (AHC) shows temperature-independent behavior from room temperature to ${\mathrm{T}}_{\mathrm{SR}}$ followed by a drop and sign reversal at low temperatures. Detailed scaling analysis of anomalous Hall data suggests that the AHE above ${\mathrm{T}}_{\mathrm{SR}}$ is primarily governed by the intrinsic Berry curvature and the obtained value of intrinsic AHC is about 310 S/cm. In contrast, below ${\mathrm{T}}_{\mathrm{SR},}$ the extrinsic skew scattering becomes the dominant contributor to the AHE compared to the intrinsic Berry curvature and the obtained value of intrinsic AHC is about–28 S/cm. The first-principles calculations reveal that changes in the sign and magnitude of the intrinsic AHC are attributed to modifications in the Berry curvature when the magnetic moment undergoes rotation from the $c$-axis to the $ab$-plane. Our study yields a compound exhibiting large AHC and offers an insightful comprehension of the anisotropic behavior of AHE due to the modification of Berry curvature.

We investigate one-dimensional (1D) and 2D cross-stitch lattices with hard-core bosons and analytically construct exact ground states that feature macroscopic degeneracy. The construction relies on the presence of a flat band in the single particle spectrum and the orthogonality of the associated compact localized states (CLS). Up to filling fraction $\nu =1/2$, the ground state is constructed by occupying the CLS. Exactly at $\nu =1/2$, the ground state becomes a Wigner crystal. For higher filling fractions, the ground state is constructed by filling the CLS sites completely one by one. Macroscopic degeneracy arises from the multiple choices available when occupying or filling the CLS sites. An occupied CLS acts as an impenetrable barrier for bosons both in 1D and 2D, leading to Hilbert space fragmentation. A similar phenomenology also holds for hard-core bosons on the diamond chain and its higher dimensional generalizations. We also discuss the mapping of these hard-core models onto spin models with quantum many-body scars.