The creation of moiré superlattices in twisted bilayers of two-dimensional crystals has been utilized to engineer quantum material properties in graphene and transition metal dichalcogenide semiconductors. Here, we examine the structural relaxation and electronic properties in small-angle twisted bilayers of metallic ${\mathrm{NbSe}}_2$. Reconstruction appears to be particularly strong for misalignment angles ${\theta }_{\mathrm{P}}<2.9^{\circ }$ and ${\theta }_{\mathrm{AP}}<1.2^{\circ }$ for parallel (P) and antiparallel (AP) orientations of monolayers' unit cells, respectively. Multiscale modeling reveals the formation of domains and domain walls with distinct stacking, for which density functional theory calculations are used to map the shape of the bilayer Fermi surface and the relative phase of the charge density wave (CDW) order in adjacent layers. We find a significant modulation of interlayer coupling across the moiré superstructure and the existence of preferred interlayer orientations of the CDW phase, necessitating the nucleation of CDW discommensurations at superlattice domain walls.

Brillouin scattering spectroscopy has been used to obtain an accurate (<1%) $\rho \text{−}\mathit{P}$ equation of state (EOS) of 1:1 and 9:1 ${\mathrm{H}}_2\text{−}\mathrm{He}$ molar mixtures from 0.5 to 5.4 GPa at 296 K. Our calculated equations of state indicate close agreement with the experimental data right to the freezing pressure of hydrogen at 5.4 GPa. The measured velocities agree on average, within 0.5%, of an ideal mixing model. The $\rho \text{−}\mathit{P}$ EOSs presented have a standard deviation of under 0.3% from the measured densities and under 1% deviation from ideal mixing. A detailed discussion of the accuracy, precision, and sources of error in the measurement and analyses of our equations of state is presented.

The change in physical properties of epitaxial $\mathrm{SrRu}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (SRO/STO) heterostructures occurring at the STO phase-transition temperature of 105 K has attracted enormous research interest. In this work, we have investigated the interfacial lattice structures in the SRO/STO heterostructure below and above 105 K. Based on the high-resolution structural analysis and ultrafast x-ray-diffraction experiments, we have revealed an interfacial lattice reconstruction of the SRO layer both in the real space and reciprocal space. Such an interfacial lattice reconstruction can be attributed to the cubic-to-tetragonal transition that occurs in structural transitions of the STO substrate.

Recently, Lee et al. claimed the experimental observation of room-temperature ambient-pressure superconductivity in a Cu-doped lead apatite $\left[{\mathrm{Pb}}_{10-x}{\mathrm{Cu}}_x{\left(\mathrm{P}{\mathrm{O}}_4\right)}_6\mathrm{O}\right]$ [S. Lee et al., arXiv:2307.12008 and arXiv:2307.12037]. The study revealed the Cu doping induces a chemical pressure, resulting in a structural contraction of one-dimensional Cu-O-Cu atomic column. This unique structure promotes a one-dimensional electronic conduction channel along the $c$ axis mediated by the O atoms, which may be related to superconductivity. These O atoms occupy 1/4 of the equivalent positions along the $c$ axis and exhibit a low diffusion activation energy of 0.8 eV, indicating the possibility of diffusion between these equivalent positions. Here, using a machine-learning based deep potential, we predict the pressure-induced fast diffusion of 1/4-occupied O atoms along the one-dimensional channel in ${\mathrm{Pb}}_{10}{\left(\mathrm{P}{\mathrm{O}}_4\right)}_6\mathrm{O}$ at 500 K, while the frameworks of Pb triangles and $\mathrm{P}{\mathrm{O}}_4$ tetrahedra remain stable. The calculation results also indicate Cu doping can provide appropriate effective chemical pressure, indicating the one-dimensional ion diffusion channel may appear in ${\mathrm{Pb}}_9\mathrm{Cu}{\left(\mathrm{P}{\mathrm{O}}_4\right)}_6\mathrm{O}$, even at ambient pressure. Our finding shows that the one-dimensional ion diffusion channel may be an important factor to affect the fabrication and electrical measurement of doped lead apatite.

A comprehensive experiment-theory study on the magnetic field-induced spin-flip transitions ($H_{\mathrm{C}1}$ and $H_{\mathrm{C}2}$) and the field-temperature ($H-T$) phase diagram of nickel niobate ($\mathrm{Ni}{\mathrm{Nb}}_2{\mathrm{O}}_6$) polycrystals is reported. Magnetic susceptibility ($\chi$) measurements reveal the presence of quasi-one-dimensional magnetism with spin-1 ${\mathrm{Ni}}^{2+}$ Heisenberg spin chains, instead of Ising chains, along the easy axis ($c$ axis). The ferromagnetic intrachain exchange-interaction strength, $J_0/k_{\mathrm{B}}$ ($=4.20$ K), between ${\mathrm{Ni}}^{2+}$ ions in the $c$ axis is estimated from the molecular-field theory approximation. The interchain exchange interactions $J_1/k_{\mathrm{B}}=-1.335$ K and $J_2/k_{\mathrm{B}}=-1.298$ K have been determined from the experimentally obtained magnitudes of $H_{\mathrm{C}1}=10.8$ kOe and spin-saturated field $H_{\mathrm{S}}=38$ kOe. These magnitudes are consistent with the total magnetic exchange-interaction strength, $J/k_{\mathrm{B}}=7.00$ K obtained from $\chi \text{−}T$ data fitted with the Heisenberg linear chain model and our density-functional theory (DFT+$U$) calculations. The overall antiferromagnetic behavior of the $\mathrm{Ni}{\mathrm{Nb}}_2{\mathrm{O}}_6$ system, correctly captured within our DFT+$U$ calculations, occurs below $T_{\mathrm{N}}=5.59\pm 0.02$ K as determined from the analysis of the specific heat $\left(C_{\mathrm{P}}\text{−}T\right)$ data. The $C_{\mathrm{P}}$ vs $T$ data near $T_{\mathrm{N}}$ is fitted to power-law expression $C_{\mathrm{P}}=A{|T-T_{\mathrm{N}}|}^{-\alpha }$ yielding the critical exponent $\alpha =0.349$(2) [0.349(1)] for $T>T_{\mathrm{N}}$ ($T<T_{\mathrm{N}}$), respectively. Further, the $H-T$ phase diagram mapped for $H\left|\right|c$ axis using various critical fields acquired from the $\chi$(T, H) data including the field-induced transitions [$H_{\mathrm{C}1}\left(T\right)$ and $H_{\mathrm{S}}\left(T\right)$] yields a triple point, $T_{\mathrm{TP}}$ ($H, T$) $=$ (5.0 kOe, 5.50 K) that separates multiple distinct spin configurations in this interesting system and calls for additional studies near the triple point. The companion $H-T$ phase diagram for $H\left|\right|a$ axis is also presented using the data collected from published sources.

We present the magnetic and structural properties of $\left[\mathrm{Cu}{\left(\mathrm{pyrazine}\right)}_{0.5}\left(\mathrm{glycine}\right)\right]{\mathrm{ClO}}_4$ under applied pressure. As previously reported, at ambient pressure this material consists of quasi-two-dimensional layers of weakly coupled antiferromagnetic dimers which undergo Bose-Einstein condensation of triplet excitations between two magnetic field-induced quantum critical points (QCPs). The molecular building blocks from which the compound is constructed give rise to exchange strengths that are considerably lower than those found in other $S=1/2$ dimer materials, which allows us to determine the pressure evolution of the entire field-temperature magnetic phase diagram using radio-frequency magnetometry. We find that a distinct phase emerges above the upper field-induced transition at elevated pressures and also show that an additional QCP is induced at zero field at a critical pressure of $p_{\mathrm{c}}=15.7\left(5\right)$ kbar. Pressure-dependent single-crystal x-ray diffraction and density functional theory calculations indicate that this QCP arises primarily from a dimensional crossover driven by an increase in the interdimer interactions between the planes. While the effect of quantum fluctuations on the lower field-induced transition is enhanced with applied pressure, quantum Monte Carlo calculations suggest that this alone cannot explain an unconventional asymmetry that develops in the phase diagram.

Ferromagnet-dielectric nanostructures possessing voltage-controlled magnetic anisotropy provide valuable opportunities for the excitation and regulation of magnetization dynamics by an electric field created in the dielectric counterpart. Here, we theoretically investigate the electrically driven motion of ${180}^{\circ }$ domain walls (DWs) in multilayer ferromagnetic nanotracks with a voltage-induced gradient of the perpendicular magnetic anisotropy (PMA) and the interfacial Dzyaloshinskii-Moriya interaction (DMI). Our paper includes micromagnetic simulations of the DW motion in the Pt/Co/MgO/Si and Pt/Fe/MgO/Si nanostructures and analytical calculations of the DW velocity in the frame of a collective coordinate model. It is shown that the PMA gradient created under the Si gate of a finite length $L_{\mathrm{Si}}$ gives rise to the DW movement over significant distances, which may strongly exceed $L_{\mathrm{Si}}$ in the presence of substantial DMI. Remarkably, the electrically driven DW motion over micrometer distances with the peak velocities up to 100 m/s can be achieved in the defect-free Pt/Fe/MgO/Si nanostructure. The numerical results are explained by the analytical model, which predicts that the DW velocity should rise with increasing PMA gradient and become higher in ferromagnets with a low magnetic damping. Similar predictions hold for the motion of the ${180}^{\circ }$ DWs in antiferromagnetic nanostructures with the voltage-controlled PMA. Our findings provide guidelines for the development of energy-efficient spintronic devices employing mobile DWs.

We show that the amplitude or Higgs mode of a superconductor with strong spin-orbit coupling and an exchange field, couples linearly to the electromagnetic field. Furthermore, by coupling such a superconductor to an LC resonator, we demonstrate that the Higgs resonance becomes a regular mode at frequencies smaller than the quasiparticle energy threshold $2{\mathrm{\Delta }}_0$. We finally propose and discuss a possible experiment based on microwave spectroscopy for an unequivocal detection of the Higgs mode. Our approach may allow visualizing Higgs modes also in more complicated multiband superconductors with a coupling between the charge and other electronic degrees of freedom.

We study a class of multiorbital models based on those proposed by Venderbos et al. [Phys. Rev. B 98, 235160 (2018)] which exhibit an interplay of topology, interactions, and fermion incoherence. In the noninteracting limit, these models exhibit trivial and Chern insulator phases with Chern number $C\ge 1$ bands as determined by the relative angular momentum of the participating orbitals. These quantum anomalous Hall insulator phases are separated by topological transitions protected by crystalline rotation symmetry, featuring Dirac or quadratic band-touching points. Here we study the impact of Sachdev-Ye-Kitaev (SYK) type interactions on these lattice models. Given the random interactions, these models display average symmetries upon disorder averaging, including a charge conjugation symmetry, so they behave as interacting models in topological class $\mathbf{D}$ enriched by crystalline rotation symmetry. The phase diagram of this model features a non-Fermi liquid at high temperature and an exciton condensate with nematic transport at low temperature. We present results from the free energy, spectral functions, and the anomalous Hall resistivity as a function of temperature and tuning parameters. Our results are broadly relevant to correlated topological matter in multiorbital systems, and may also be viewed, with a suitable particle hole transformation, as an exploration of strong interaction effects on mean-field topological superconductors.

Ideal Weyl semimetals, with minimum Weyl nodes located far away from each other in reciprocal space and near the Fermi level in the energy space, are considered to be crucial for revealing the intrinsic physical properties of Weyl semimetals. Based on first-principles calculations, we demonstrate that elemental tellurium, under application of a broad range of shear strain, is a candidate for realizing ideal Weyl semimetals. Particularly, the sheared elemental tellurium has only four Weyl nodes in the Brillioun zone, which is the minimum number of Weyl nodes that can be achieved by a system with time-reversal symmetry. More importantly, by adjusting the strength of the strain, we obtain abundant topological phases, i.e., Weyl semimetals, strong topological insulators, composite Weyl semimetals, and weak topological insulators in elemental tellurium. Notably, this paper proposes implementing a weak topological insulator phase in a space group with only time-reversal symmetry. The complete topological phase diagram of sheared tellurium is given and the topological properties of different topological nontrivial phases are characterized, respectively.

Single-spin qubits in solid-state materials are important fundamental platforms for quantum information applications. Traditionally, coherent control of solid-state defects is realized under Stokes excitation. Little is known about the coherent control of a single defect spin under anti-Stokes excitation. In this work, we experimentally verify that the mechanism of anti-Stokes excitation of the divacancy in silicon carbide is a phonon-assisted single-photon absorption process and provide the confocal microscopy anti-Stokes photoluminescence scanning image of an isolated single divacancy. Moreover, the optically detected magnetic resonance measurement and coherent control of the single divacancy spin under anti-Stokes excitation are realized at room temperature. We further reveal that the spin readout contrast under anti-Stokes excitation is more robust than that under Stokes excitation at elevated temperatures. Our work establishes a nontraditional protocol for the optical addressing and coherent control of single-spin qubits and expands the boundary of quantum information technology.

The exploration of novel and stable two-dimensional (2D) materials holds considerable significance in the development of optoelectronic devices and photocatalytic water splitting. Herein, $\mathit{MNXY}(M\text{/}N=\mathrm{Al},\mathrm{Ga},X\text{/}Y=\mathrm{N},\mathrm{P},\text{As})$ monolayers are constructed based on 2D double-layer honeycomb structures. Six stable semiconductor types were screened through first-principles calculations, and their mechanical, electrical, optical, and transport properties as well as their applications in photocatalytic water splitting were investigated. We found that the six stabilized MNXY monolayers had band gaps of 0.846–3.806 eV. The mechanical properties indicate that ${\mathrm{AlGaN}}_2$ has a Young's modulus exceeding 100 N/m, whereas the remaining five monolayers exhibit values below this threshold. The hole carrier mobility of the ${\mathrm{AlGaN}}_2$ monolayer along the armchair direction reaches an ultrahigh value of 3649.21 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$; thus it has the potential for application in optoelectronic devices. Furthermore, we observed that ${\mathrm{AlGaP}}_2$ monolayers exhibit band-edge potentials spanning the redox potential of water, a considerable difference in electron-hole carrier mobility, strong visible light absorption, and a high solar to hydrogen efficiency (17.51%) in the absence of strain, making them suitable for photocatalytic water splitting. We expect that our results will pave the way forward for material selection for next-generation optoelectronic devices and photocatalysts.

In the framework of semiclassical Boltzmann transport theory and by first principles calculations, we study the intrinsic resistivity (arising from the electron-phonon scattering) of two MBene materials: ${\mathrm{Ti}}_2{\mathrm{B}}_2$ and ${\mathrm{Mo}}_2{\mathrm{B}}_2$. We find that the two kinds of MBenes are two-dimensional (2D) good conductors, with lower resistivity than other typical two-dimensional metals. For example, their intrinsic resistivity at room temperature is both significantly lower than the high-buckled plumbene and 2D ${\text{Ti}}_3{\text{C}}_2$, noting that the latter possesses the maximal conductivity among MXene metals accessible so far. The impressive low resistivity of the two MBenes can be attributed to their sizable Fermi surface and relatively weaker electron-phonon (el-ph) interaction. What is more, we find the temperature dependence of the intrinsic resistivity of the two MBenes, i.e., the $\rho \text{−}T$ relation, disagrees with the prediction of conventional Bloch-Grüneisen theory in both the low and high temperature limits. On one hand, the nontrivial umklapp scattering causes a notable deviation of the $\rho \text{−}T$ relation of the two MBenes from the $T^4$ law, a general decay rate of 2D metals in the low temperature limit. On the other hand, on the high temperature side, the temperature dependence of the el-ph interaction's spectral function must be taken into account for describing correctly the $\rho \text{−}T$ relation of actual materials with a sharply varying density of states of electrons around Fermi surfaces such as the case of the two MBenes, though it was often ignored in the relevant literature.

The alkali-doped fulleride ${\mathrm{Cs}}_3{\mathrm{C}}_{60}$, crystallized in the space group $Fm\overline{3}m$ or $Pm\overline{3}n$, exhibits unconventional $s$-wave superconductivity under pressure with a maximum $T_c\sim 38$ K. Recently, a new primitive-cubic-structured ${\mathrm{Cs}}_3{\mathrm{C}}_{60}$ phase corresponding to the space group $Pa\overline{3}$ has been reported (Peng et al., arXiv:2208.09429), and the authors observed superconductivity at ambient pressure. Using density functional theory (DFT) calculations, we show that the proposed $Pa\overline{3}$ structure is not stable under ionic relaxation, but transforms into the fcc structure. We study the normal and superconducting states of the stable fcc phase at different temperatures and volumes using DFT plus dynamical mean-field theory in the Nambu formalism. As the temperature increases, the transition between superconductor and normal metal (Mott insulator) at small (big) volume is found to be second (first) order. The recently developed maximum entropy analytic continuation method for the anomalous self-energy is used to study the momentum-resolved spectra and optical conductivity.

We present magnetostriction and thermal expansion measurements on multiferroic ${\left({\mathrm{Ni}}_{0.93}{\mathrm{Co}}_{0.07}\right)}_3{\mathrm{V}}_2{\mathrm{O}}_8$. The high field phase diagrams up to 33 T along the $a$, $b$, and $c$ directions are built. For $H\parallel a$, as the magnetic field increasing, two intermediate phases appear between the incommensurate phase and the paramagnetic phase at about 7 K, and then a magnetically induced phase appears above the paramagnetic phase. For $H\parallel b$, a thermal expansion measurement indicates a mutation in the spin lattice coupling of the high field phases. The interlaced phase boundary suggests a mixed state in the optical high field phase. For $H\parallel c$, an intermediate phase between the commensurate phase and the incommensurate phase is detected. A nonlinear boundary between the intermediate phase and the low-temperature incommensurate phase, and a clear boundary between the commensurate phase and the paramagnetic phase are found. These results indicate that doping ${\mathrm{Co}}^{2+}$ breaks the weak ferromagnetic moment of the commensurate phase, which exists in the parent compound ${\mathrm{Ni}}_3{\mathrm{V}}_2{\mathrm{O}}_8$ and ${\left({\mathrm{Ni}}_{0.9}{\mathrm{Co}}_{0.1}\right)}_3{\mathrm{V}}_2{\mathrm{O}}_8$. This nonlinear influence reflects complicated spin modulation in ${\mathrm{Ni}}_3{\mathrm{V}}_2{\mathrm{O}}_8$ by doping ${\mathrm{Co}}^{2+}$.

In 1992, Kennedy and Tasaki constructed a nonlocal unitary transformation that maps between a ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ spontaneously symmetry breaking phase and the Haldane gap phase, which is a prototypical symmetry-protected topological (SPT) phase in modern framework, on an open spin chain. In this work, we propose a way to define it on a closed chain by sacrificing unitarity. The operator realizing such a nonunitary transformation satisfies noninvertible fusion rule and implements a generalized gauging of the ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ global symmetry. These findings connect the Kennedy-Tasaki transformation to numerous other concepts developed for SPT phases and opens a way to construct SPT phases systematically using the duality mapping.

In bulk superconductors or Josephson junctions formed in materials with spin-orbit interaction, the critical current can depend on the direction of current flow or of applied magnetic field, an effect known as the superconducting (SC) diode effect. Here, we consider the SC diode effect in Josephson junctions in nanowire devices. We find that the $4\pi$-periodic contribution of Majorana bound states (MBSs) to the current phase relation (CPR) of single junctions results in a significant enhancement of the SC diode effect when the device enters the topological phase and can therefore be used as an indicator of the phase transition. Crucially, this enhancement of the SC diode effect is independent of the parity of the junction and therefore protected from parity-altering events, such as quasiparticle poisoning, which have hampered efforts to directly observe the $4\pi$-periodic CPR of MBSs. We show that this effect can be generalized to SQUIDs and that, in such devices, the parity-protected SC diode effect can provide a highly controllable probe of the topology in a Josephson junction.

We explore the effects of chemical doping and photodoping in the strain-induced multiferroic ${\mathrm{EuTiO}}_3$ grown on ${\mathrm{DyScO}}_3$ substrates. The polar order is probed experimentally using second harmonic generation (SHG) and modeled using ab initio calculations. At low photodoping concentrations, we observe a reduction in SHG signal, indicating a destructive coupling between charge carriers and polar order in accordance with our simulations and expectations for a second-order Jahn-Teller driven ferroelectric. However, under increased photodoping the reduction in SHG plateaus at 84% of its original magnitude, indicating resilience of the polar order in the presence of a high concentration of delocalized electrons. This behavior stands in contrast with our first-principles simulations, indicating that ${\mathrm{EuTiO}}_3$ undergoes a transition from ferroelectric to polar metallic character under photodoping. We suggest several hypotheses for the mechanism behind this change in distortion character.