Recently a relationship between the Debye temperature ${\mathrm{\Theta }}_D$ and the superconducting transition temperature $T_c$ of conventional superconductors has been proposed [Esterlis et al., npj Quantum Mater. 3, 59 (2018)]. The relationship indicates that $T_c\le A{\mathrm{\Theta }}_D$ for phonon-mediated BCS superconductors, with $A$ being a prefactor of order $\sim 0.1$. In order to verify this bound, we train machine learning (ML) models with 10 330 samples in the Materials Project database to predict ${\mathrm{\Theta }}_D$. By applying our ML models to 9860 known superconductors in the NIMS SuperCon database, we find that the conventional superconductors in the database indeed follow the proposed bound. We also perform first-principles phonon calculations for ${\mathrm{H}}_3\mathrm{S}$ and ${\mathrm{LaH}}_{10}$ at 200 GPa. The calculation results indicate that these high-pressure hydrides essentially saturate the bound of $T_c$ versus ${\mathrm{\Theta }}_D$.

Superconductivity in laser-excited correlated electron systems has attracted considerable interest due to reports of light-induced superconductinglike states. Here we explore the possibility of nonthermal superconducting order in strongly interacting multiorbital Hubbard systems, using nonequilibrium dynamical mean field theory. We find that a staggered $\eta$-type superconducting phase can be realized on a bipartite lattice in the high photodoping regime, if the effective temperature of the photocarriers is sufficiently low. The $\eta$ superconducting state is stabilized by Hund coupling—a positive Hund coupling favors orbital-singlet spin-triplet $\eta$ pairing, whereas a negative Hund coupling stabilizes spin-singlet orbital-triplet $\eta$ pairing.

In superconductors, the combination of broken time-reversal and broken inversion symmetries can result in a critical current being dependent on the direction of current flow. This phenomenon is known as superconducting diode effect (SDE) and has great potential for applications in future low-temperature electronics. Here, we investigate how magnetic textures such as domain walls or skyrmions on a racetrack can be used to control the SDE in a Josephson junction and how the SDE can be used as a low-temperature read-out of the data in racetrack memory devices. First, we consider a two-dimensional electron gas (2DEG) with strong spin-orbit interaction (SOI) coupled to a magnetic racetrack, which forms the weak link in a Josephson junction. In this setup, the exchange coupling between the magnetic texture and the itinerant electrons in the 2DEG breaks time-reversal symmetry and enables the SDE. When a magnetic texture, such as a domain wall or skyrmion enters the Josephson junction, the local exchange field within the junction is changed and, consequently, the strength of the SDE is altered. In particular, depending on the position and form of the magnetic texture, moving the magnetic texture can cause the SDE coefficient to change its sign, enabling a Josephson transistor effect with potentially fast switching frequencies. Further, we find that the SDE is enhanced if the junction lengthscales are comparable with the lengthscale of the magnetic texture. Furthermore, we show that, under certain circumstances, the symmetry breaking provided by particular magnetic textures, such as skyrmions, can lead to an SDE even in the absence of Rashba SOI in the 2DEG. Our results provide a proof-of-principle for forms of read-out in low-temperature memory devices as well as demonstrating how a Josephson transistor effect can be achieved even in the absence of an external magnetic field and intrinsic Rashba SOI.

Compared to the intensively studied niobium dichalcogenides, the physical properties of niobium monosulfide (NbS) have been less explored. Here we report the discovery of bulk superconductivity in orthorhombic NbS ($o$-NbS). First-principles results of $o$-NbS are also revealed. $o$-NbS crystallizes in a MnP-type structure ($Pnma$) with lattice parameters $a=\text{6.4450(1)}$ Å, $b=\text{3.3\hspace{0.17em}1770(4)}$ Å, and $c=\text{5.8629(1)}$ Å. It exhibits type-II superconductivity with a critical temperature ($T_{\mathrm{c}}$) of 6.0 K. Bulk superconductivity is confirmed by the magnetization and heat capacity measurements. The upper and lower critical fields are 7.4 T and 13.0 mT, respectively. Specific heat data suggest that $o$-NbS is a fully gapped $s$-wave superconductor, with ${\mathrm{\Delta }}_0/k_{\mathrm{B}}{T}_{\mathrm{c}}=1.75$ and $\mathrm{\Delta }{C}_{\mathrm{e}}/\gamma T_{\mathrm{c}}=1.45$, fitting perfectly within the BCS theory. First-principles calculations indicate that the $\mathrm{Nb}\text{−}4d$ states dominate the Fermi level ($E_{\mathrm{F}}$), and the density of states (DOS) on $E_{\mathrm{F}}$ is sensitive to niobium vacancies. Different from the quasi-two-dimensional $2H\text{−}{\mathrm{NbS}}_2$ and quasi-one-dimensional ${\mathrm{Nb}}_3{\mathrm{S}}_4$ superconductors, our discovery of superconductivity in three-dimensional $o$-NbS provides a different motif for both experimental and theoretical studies.

Certain systems feature phases that are dynamically unstable at 0 K but are stabilized by vibrations at higher temperatures. Treatment of these phases by conventional 0-K methods is not feasible and effective harmonic models introduce approximations. Here, we significantly advance the direct upsampling methodology [npj Comput. Mater. 9, 3 (2023)] to obtain free energies including the anharmonic contribution to full ab initio accuracy also for such dynamically stabilized phases. The centerpiece behind the procedure is accurate machine-learning potentials (moment tensor potentials) which are used to efficiently scan the volume-temperature space to uncover the stability regime and to perform thermodynamic integration on a dense grid within the stable window. We apply the methodology to the prototype systems Ti, Zr, and Hf and calculate hcp-bcc transition properties and thermodynamic properties of both phases. We find a very good agreement for the heat capacities with existing experimental/CALPHAD data, and an overall best agreement for Ti. The transition properties agree well on a relative temperature axis, where the temperature is scaled with respect to the transition temperature. Anharmonic free energies increase the transition temperature by up to one thousand kelvin. Electronic effects are smaller and bring down the transition temperature by as much as 172 K. We establish a new definition of the 0-K energy-volume curve for the dynamically stabilized bcc phase. Instead of using static lattice ab initio values, an extrapolation of the high-temperature high-accuracy free-energy surface to 0 K provides a physically more meaningful description. With this effective 0-K definition, discrepancies existing in the literature between CALPHAD and ab initio values are addressed.

The lattice thermal conductivity ($\kappa$) of two ceramic materials, cerium dioxide (${\mathrm{CeO}}_2$) and magnesium oxide (MgO), is computed up to 1500 K using first principles and the phonon Boltzmann transport equation (PBTE) and compared to time-domain thermoreflectance (TDTR) measurements up to 800 K. Phonon renormalization and the four-phonon effect, along with high-temperature thermal expansion, are integrated in our ab initio molecular dynamics calculations. This is done by first relaxing structures and then fitting to a set of effective force constants employed in a temperature-dependent effective potential method. Both three-phonon and four-phonon scattering rates are computed based on these effective force constants. Our calculated thermal conductivities from the PBTE solver agree well with the literature and our TDTR measurements. Other predicted thermal properties including thermal expansion, frequency shift, and phonon linewidth also compare well with the available experimental data. Our results show that high temperature softens phonon frequency and reduces four-phonon scattering strength in both ceramics. The temperature scaling law of $\kappa$ is $\sim T^{-1}$ for three-phonon scattering only and remains the same after phonon renormalization. This scaling for three- plus four-phonon scattering is $\sim T^{-1.2}$, but is weakened to $\sim T^{-1}$ by phonon renormalization. This indicates that four-phonon scattering can play an important role in systems where measured $\kappa$ decays with temperature as $\sim T^{-1}$, which was conventionally attributed to three-phonon only. Compared to MgO, we find that ${\mathrm{CeO}}_2$ has a weaker four-phonon effect and renormalization greatly reduces its four-phonon scattering rates.

We investigate the nature of the phase transitions in the quantum Ashkin-Teller chain in the presence of chiral perturbations. We locate the Lifshitz line separating a region of direct chiral transitions from the region where the transition is through an intermediate floating phase. Furthermore, we identify a small region in the vicinity of the four-state Potts point where chiral perturbations are irrelevant and where the transition remains conformal. Implications to Rydberg atom experiments are briefly discussed.

We study spin dynamics of ordered phases of ${\mathrm{Cs}}_2{\mathrm{CoBr}}_4$ in a magnetic field using electron-spin resonance (ESR) technique and theoretical analysis. This material hosts weakly interacting distorted-triangular-lattice planes of spin-$\frac{3}{2}{\mathrm{Co}}^{2+}$ ions which can be viewed as spin chains coupled by frustrating interactions. Strong single-ion anisotropy allows us to describe the low-energy spin dynamics of this system by an effective strongly anisotropic pseudospin-$\frac{1}{2}$ model. Our ESR data show up to seven branches of magnetic resonance in four magnetic phases arising due to subtle interplay of frustration, low dimensionality, and strong anisotropy. In particular, in the low-field collinear stripe phase, the field evolution of modes lying below 200 GHz is described reasonably well by spectra of spin-1 and spin-0 quasiparticles which we obtain using the bond-operator technique. These well-defined excitations can be treated as conventional magnons and bound states of two magnons, respectively. In contrast, numerous excitations lying above 200 GHz are not captured by our theory due to pronounced one-dimensional correlations inside spin chains which govern the spin dynamics at high enough energies. As was shown before, these modes can be most naturally interpreted as bound states of domain walls in individual chains and their sequence resembles the so-called “Zeeman ladder” in anisotropic Ising-like spin chains. Thus, ${\mathrm{Cs}}_2{\mathrm{CoBr}}_4$ is a system showing spin-dynamics in an ordered state characteristic of both two-dimensional and one-dimensional magnets.

${\mathrm{Cs}}_2{\mathrm{CoBr}}_4$ is a triangular-lattice antiferromagnet which can be viewed as weakly interacting spin chains due to spatially anisotropic frustrating exchange couplings. The spin-orbit interaction in ${\mathrm{Co}}^{2+}$ spin-$\frac{3}{2}$ ions leads to a strong easy-plane single-ion anisotropy which allows us to consider the low-energy spin dynamics of this system using an anisotropic pseudospin-$\frac{1}{2}$ model. By means of the electron spin resonance (ESR) technique, we study the spin dynamics of ${\mathrm{Cs}}_2{\mathrm{CoBr}}_4$ in magnetic field in a spin-liquid regime, i.e., above the Néel temperature of 1.3 K but below the temperature of the crossover to in-chain correlations of pseudospins ($\approx 6$ K). Our experiments reveal two bright branches of excitations which strongly differ both from excitations in the low-temperature ordered phases and from high-temperature paramagnetic resonance of uncorrelated pseudospins and spins. These two branches are interpreted as excitations with zero momentum of an anisotropic spin-$\frac{1}{2}$ chain. In addition, we observe several weak modes of unknown origin which arise mostly as satellites of one of the bright modes.

By molecular dynamics simulations, we study the spin Seebeck effect as a function of magnetic field in the prototype classical easy-axis antiferromagnetic chain, far out of equilibrium as well as linear response regime. We find distinct behavior in the low-field antiferromagnetic, middle-field canted, and high-field ferromagnetic phase. In particular, in the open boundary system at low temperatures, we observe a divergence of the spin current in the spin-flop transition between the antiferromagnetic and canted phase, accompanied by a change of sign in the generated spin current by the temperature gradient. These results are corroborated by a simple spin-wave phenomenological analysis and simulations in the linear response regime. They shed light on the spin current sign change observed in experiments in bulk antiferromagnetic materials.

Cobalt is a magnetic material that finds extensive use in various applications, ranging from magnetic storage to ultrafast spintronics. Usually, it exists in two phases with different crystal lattices, namely in hexagonal-close-packed (hcp) or face-centered-cubic (fcc) structure. The crystal structure of Co films significantly influences their magnetic and spintronic properties. We report on the thickness dependence of the structural and magnetic properties of sputter-deposited Co on a Pt seed layer. It grows in an hcp lattice at low thicknesses, while for thicker films it becomes a mixed hcp-fcc phase due to a stacking fault progression along the growth direction. The x-ray-based reciprocal space map technique has been employed to distinguish and confirm the presence of both phases. Moreover, the precise determination of Landé's $g$-factor by ferromagnetic resonance provides valuable insights into the structural properties. In our detailed experiments, we observe that a structural variation results in a nonmonotonic variation of the magnetic anisotropy along the thickness. This careful study reveals the fundamental physics, but also provides important insight for potential applications of thin Co films with perpendicular magnetic anisotropy.

We study the phase diagram of a resistively shunted Josephson junction (RSJJ) in the framework of the boundary sine-Gordon model. Using the nonperturbative functional renormalization group (FRG) we find that the transition is not controlled by a single fixed point but by a line of fixed points, and compute the continuously varying critical exponent $\nu$. We argue that the conductance also varies continuously along the transition line. In contrast to the traditional phase diagram of the RSJJ, an insulating ground state when the shunt resistance $R$ is larger than $R_q=h/{\left(2e\right)}^2$ and a superconducting one when $R<R_q$, the FRG predicts the transition line in the plane $(\alpha ,E_J/E_C)$ to bend in the region $\alpha =R_q/R<1$ but we cannot discard the possibility of a vertical line at $\alpha =1$ ($E_J$ and $E_C$ denote the Josephson and charging energies of the junction, respectively). Our results regarding the phase diagram and the nature of the transition are compared with Monte Carlo simulations and numerical renormalization group results.

The state-of-the-art first-principles quantum transport theory and modeling are based on carrying out self-consistent atomistic calculations within the Keldysh nonequilibrium Green's function (NEGF) formalism. The atomistic model of the device can be at the tight-binding (TB) or the density functional theory levels, and NEGF determines the nonequilibrium carrier distribution under external bias and gate voltages. In this work, we report an end-to-end automatic differentiable NEGF simulator (AD-NEGF) within the NEGF-TB framework. AD-NEGF calculates gradient information by automatic differentiation (AD) and the implicit layer technique while guaranteeing the correctness of forward simulation. The gradient information enables accurate calculations of transport properties that depend on the derivatives of the transmission coefficient and/or charge current. More interestingly, AD-NEGF can be applied to the extremely interesting inverse design problem; namely, with a desired transport property, AD-NEGF inversely finds a possible device Hamiltonian that would produce such a property.

A single Fermi node in ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ protected by time reversal and inversion symmetry resulting from an interplay between strong electronic correlations and spin-orbit coupling (SOC) has attracted significant attention. Here, we report a detailed temperature-dependent Raman study of the “quadratic band touching Luttinger semimetal” ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$. In addition to phonons, we observe an underlying flat electronic continuum due to electronic Raman scattering (ERS) that is dominant below a characteristic temperature $T_Q$. The observed ERS is understood using the Raman susceptibility of a non-Fermi liquid expected in a quadratic band touching system. The phonon frequencies and linewidths show anomalous temperature dependence below $T_Q\sim 150$ K attributed to strong electron-phonon coupling (EPC) in a quadratic band touching (QBT) state. The Ir-O-Ir bending $E_g$ mode shows pronounced Fano asymmetry as well as anomalous linewidth broadening below $T_Q$. Further, a Raman line at $\sim 460{\mathrm{cm}}^{-1}$ involving crystal field transition couples strongly to the $T_{2g}$ phonon at $\sim 350{\mathrm{cm}}^{-1}$. Our results establish the enhanced scale of electron-electron correlations and electron-phonon coupling in the QBT state of metallic ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$.

Recently, a quantum anomalous Hall state at odd-integer filling in moiré stacked ${\mathrm{MoTe}}_2/{\mathrm{WSe}}_2$ was convincingly interpreted as a topological Mott insulator state appearing due to strong interactions in band basis [P. Mai, J. Zhao, B. E. Feldman, and P. W. Phillips, Nat. Commun. 14, 5999 (2023)]. In this work, we aim to analyze the formation of a topological Mott insulator due to interactions in orbital basis instead, being more natural for systems where interactions originate from the character of $f$ or $d$ orbitals rather than band flatness. For that reason, we study an odd-integer filled Anderson lattice model incorporating odd-parity hybridization between orbitals with different degrees of correlations introduced in the Hatsugai-Kohmoto spirit. We demonstrate that a topological Mott insulating state can be realized in a considered model only when weak intra- and interorbital correlations involving dispersive states are taken into account. Interestingly, we find that all topological transitions between trivial and topological Mott insulating phases are not accompanied by a spectral gap closing, consistent with a phenomenon called first-order topological transition. Instead, they are signaled by a kink developed in spectral function at one of the time reversal invariant momenta. We believe that our approach can provide insightful phenomenology of topological Mott insulators in spin-orbit coupled $f$ or $d$ electron systems.

While charge carriers can typically be designated as either electron or hole type, depending on the sign of the Hall coefficient, some materials defy this straightforward classification. Here, we find that ${\mathrm{LaRh}}_6{\mathrm{Ge}}_4$ goes beyond this dichotomy, where the Hall resistivity is electronlike for magnetic fields along the $c$ axis but holelike in the basal plane. Together with first-principles calculations, we show that this direction-dependent switching of the carrier type arises within a single band, where the special geometry leads to charge carriers on the same Fermi surface orbiting as electrons along some directions, but holes along others. The relationship between the Fermi surface geometry and occurrence of a Hall sign reversal is further generalized by considering tight-binding model calculations, which show that this type of Fermi surface corresponds to a more robust means of realizing this phenomenon, suggesting an important route for tailoring direction-dependent properties for advanced electronic device applications.

Although condensed matter systems usually do not have higher-form symmetries, we show that, unlike 0-form symmetry, higher-form symmetries can emerge as exact symmetries at low energies and long distances. In particular, emergent higher-form symmetries at zero temperature are robust to arbitrary local UV perturbations in the thermodynamic limit. This result is true for both invertible and noninvertible higher-form symmetries. Therefore emergent higher-form symmetries are exact emergent symmetries: they are not UV symmetries but constrain low-energy dynamics as if they were. Since phases of matter are defined in the thermodynamic limit, this implies that a UV theory without higher-form symmetries can have phases characterized by exact emergent higher-form symmetries. We demonstrate this in three lattice models, the quantum clock model and emergent ${\mathbb{Z}}_N$ and $\mathrm{U}\left(1\right)p$-gauge theory, finding regions of parameter space with exact emergent (anomalous) higher-form symmetries. Furthermore, we perform a generalized Landau analysis of a $2+1\mathrm{D}$ lattice model that gives rise to ${\mathbb{Z}}_2$ gauge theory. Using exact emergent 1-form symmetries accompanied by their own energy/length scales, we show that the transition between the deconfined and Higgs/confined phases is continuous and equivalent to the spontaneous symmetry-breaking transition of a ${\mathbb{Z}}_2$ symmetry, even though the lattice model has no symmetry. Also, we show that this transition line must always contain two parts separated by multi-critical points or other phase transitions. We discuss the physical consequences of exact emergent higher-form symmetries and contrast them to emergent 0-form symmetries. Lastly, we show that emergent 1-form symmetries are no longer exact at finite temperatures, but emergent $p$-form symmetries with $p\ge 2$ are.

Twisted $N$-layer graphene (TNG) moiré structures have recently been shown to exhibit robust superconductivity similar to twisted bilayer graphene (TBG). In particular for $N=4$ and $N=5$, the phase diagram features a superconducting pocket that extends beyond the nominal full filling of the flat band. These observations are seemingly at odds with the canonical understanding of the low-energy theory of TNG, wherein the TNG Hamiltonian consists of one flat-band sector and accompanying dispersive bands. Using a self-consistent Hartree-Fock treatment, we explain how the phenomenology of TNG can be understood through an interplay of in-plane Hartree and inhomogeneous layer potentials, which cause a reshuffling of electronic bands. We extend our understanding beyond the case of $N=5$ realized in experiment so far. We describe how the Hartree and layer potentials control the phase diagram for devices with $N>5$ and tend to preclude exchange-driven correlated phenomena in this limit. To circumvent these electrostatic constraints, we propose a flat-band paradigm that could be realized in large-$N$ devices by taking advantage of two nearly flat sectors acting together to enhance the importance of exchange effects.

Electron dynamics of anatase ${\mathrm{TiO}}_2$ under the influence of ultrashort and intense laser field is studied using the real-time time-dependent density functional theory (TDDFT). Our findings demonstrate the effectiveness of TDDFT calculations in modeling the electron dynamics of solids during ultrashort laser excitation, providing valuable insights for designing and optimizing nonlinear photonic devices. We analyze the perturbative and nonperturbative responses of ${\mathrm{TiO}}_2$ to 30 fs laser pulses at 400 and 800 nm wavelengths, elucidating the underlying mechanisms. At 400 nm, ionization via single photon absorption dominates, even at very low intensities. At 800 nm, we observe ionization through two-photon absorption within the intensity range of $1\times {10}^{10}$ to $9\times {10}^{12} \mathrm{W}/{\mathrm{cm}}^2$, with a transition from multiphoton to tunneling ionization occurring at $9\times {10}^{12} \mathrm{W}/{\mathrm{cm}}^2$. We observe a sudden increase in energy and the number of excited electrons beyond $1\times {10}^{13} \mathrm{W}/{\mathrm{cm}}^2$, leading to their saturation and subsequent laser-induced damage. We estimate the damage threshold of ${\mathrm{TiO}}_2$ for 800 nm to be $0.1 \mathrm{J}/{\mathrm{cm}}^2$. In the perturbative regime, induced currents exhibit a phase shift proportional to the peak intensity of the laser pulse. This phase shift is attributed to the intensity-dependent changes in the number of free carriers, indicative of the optical Kerr effect. Leveraging the linear dependence of phase shift on peak intensities, we estimate the nonlinear refractive index ($n_2$) of ${\mathrm{TiO}}_2$ to be $3.54\times {10}^{-11}{\mathrm{cm}}^2/\mathrm{W}$.

Spatial confinement of electronic topological surface states (TSSs) in topological insulators poses a formidable challenge because TSSs are protected by time-reversal symmetry. In previous works formation of a gap in the electronic spectrum of TSSs has been successfully demonstrated in topological insulator/magnetic material heterostructures, where ferromagnetic exchange interactions locally lift the time-reversal symmetry. Here we report experimental evidence of exchange interaction between a topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and a magnetic insulator EuSe. Spin-polarized neutron reflectometry reveals a reduction of the in-plane magnetic susceptibility within a 2 nm interfacial layer of EuSe, and the combination of superconducting quantum interference device (SQUID) magnetometry and Hall measurements points to the formation of an interfacial layer with a suppressed net magnetic moment. This suppressed magnetization survives up to temperatures five times higher than the Néel temperature of EuSe. Its origin is attributed to the formation of an interfacial antiferromagnetic state. Abrupt resistance changes observed in high magnetic fields are consistent with antiferromagnetic domain reconstruction affecting transport in a TSS via exchange coupling. The high-temperature local control of TSSs with zero net magnetization unlocks new opportunities for the design of electronic, spintronic, and quantum computation devices, ranging from quantization of Hall conductance in zero fields to spatial localization of non-Abelian excitations in superconducting topological qubits.