We study a multiorbital Hubbard model coupled to local Jahn-Teller phonons to investigate the superconducting state realized in fullerides. A weak-coupling approach is employed in combination with a local self-energy approximation. In addition to the normal and anomalous self-energies of the electrons, we consider the phonon self-energy, which allows a self-consistent treatment of the energetics. The frequency dependence of the self-energies and their characteristic coefficients, such as renormalization factors and dampings, are investigated in detail using numerical calculations. It is clarified that the anisotropic phonons play an important role in the stabilization of the superconducting state. By comparing the full results with those without phonon self-energies, we show that the superconductivity is stabilized by the softening of the phonon frequency. The effects of electronic fluctuations are also considered, and shown to lead to the coupling to orbitons, an analog of plasmons in the electron gas. This additional contribution further stabilizes the superconducting state.

Finding viable superconducting materials is of interest to the physics community, as the superconductors are the playground for manifesting many appealing quantum phenomena. In this paper, we exemplify an end-to-end materials discovery toward ${\mathrm{MgB}}_2$-like superconductors, starting from data-driven compound screening to experimental materialization. In addition to the known superconducting compounds, ${\mathrm{CaB}}_2 (T_c=9.4–28.6\mathrm{K}), {\mathrm{SrGa}}_2 (T_c=0.1–2.4\mathrm{K}), {\mathrm{BaGa}}_2 (T_c=0.3–3.3\mathrm{K}), {\mathrm{BaAu}}_2 (T_c=0.0–0.5\mathrm{K})$, and ${\mathrm{LaCu}}_2 (T_c=0.1–2.2\mathrm{K})$ are discovered, out of ∼182 000 starting structures, to be the most promising superconducting compounds that share similar atomic structures with ${\mathrm{MgB}}_2$. Moreover, ${\mathrm{BaGa}}_2$ is experimentally synthesized and measured to confirm that the compound is a Bardeen-Cooper-Schrieffer superconductor with $T_c=0.36\mathrm{K}$, in good agreement with our theoretical predictions. In this paper, we provide a study for the ${\mathrm{MgB}}_2$-like superconductors and showcase that it is feasible to discover materials via a data-driven approach.

We study the three-state antiferromagnetic Potts model on the simple-cubic lattice, paying attention to the surface critical behaviors. When the nearest-neighboring interactions of the surface is tuned, we obtain a phase diagram similar to the XY model, owing to the emergent O(2) symmetry of the bulk critical point. For the ordinary transition, we get $y_{h1}=0.780\left(3\right), {\eta }_{\parallel }=1.44\left(1\right)$, and ${\eta }_{\perp }=0.736\left(6\right)$; for the special transition, we get $y_s=0.59\left(1\right), y_{h1}=1.693\left(2\right), {\eta }_{\parallel }=-0.391\left(4\right)$, and ${\eta }_{\perp }=-0.179\left(5\right)$; in the extraordinary-log phase, the surface correlation function $C_{\parallel }\left(r\right)$ decays logarithmically with decaying exponent $q=0.60\left(2\right)$, however, the correlation $C_{\perp }\left(r\right)$ still decays algebraically with critical exponent ${\eta }_{\perp }=-0.442\left(5\right)$. If the ferromagnetic next-nearest-neighboring surface interactions are added, we find two transition points, the first one is a special point between the ordinary phase and the extraordinary-log phase, the second one is a transition between the extraordinary-log phase and the $Z_6$ symmetry-breaking phase, with critical exponent $y_{\mathrm{s}}=0.41\left(2\right)$. The scaling behaviors of the second transition is very interesting, the surface spin-correlation function $C_{\parallel }\left(r\right)$, and the surface squared staggered magnetization at this point decays logarithmically with exponent $q=0.37\left(1\right)$; however, the surface structure factor with the smallest wave vector and the correlation function $C_{\perp }\left(r\right)$ satisfy power-law decaying, with critical exponents ${\eta }_{\parallel }=-0.69\left(1\right)$ and ${\eta }_{\perp }=-0.37\left(1\right)$, respectively.

Using the time-dependent Ginzburg-Landau equations, we study vortex motion driven by an applied current in two-dimensional superconductors in the presence of a physical boundary. At smaller sourced currents the vortex lattice moves as a whole, with each vortex moving at the same velocity. At the larger sourced current, the vortex motion is organized into channels, with vortices in channels closer to the sample edges moving faster than those farther away from sample edges, opposite the Poiseuille flow of basic hydrodynamics in which the velocity is lowest at the boundaries. At intermediate currents, a stick-slip motion of the vortex lattice occurs in which vortices in the channel at the boundary break free from the Abrikosov lattice, accelerate, move past their neighbors, and then slow down and reattach to the vortex lattice, at which point the stick-slip process starts over. These effects could be observed experimentally, e.g., using fast scanning microscopy techniques.

We show that elastic moduli of higher (third and fourth) order of prestressed single crystals at high pressure, obtained from the Gibbs free energy, enter the relation between the Cauchy stress and Lagrange finite-strain tensor the same way they do for the second-order elastic moduli and, therefore, directly describe the nonlinear elasticity of prestressed crystals.

We study the nonequilibrium dynamics of a one-dimensional complex Sachdev-Ye-Kitaev chain by directly solving for the steady state Green's functions in terms of small perturbations around their equilibrium values. The model exhibits non-Fermi-liquid behavior without quasiparticles and features diffusive propagation of both energy and charge. We explore the thermoelectric transport properties of this system by imposing uniform temperature and chemical potential gradients. We then expand the conserved charges and their associated currents to leading order in these gradients, which we can compute numerically and analytically for different parameter regimes. This allows us to extract the full temperature and chemical potential dependence of the transport coefficients. In particular, we uncover that the diffusivity matrix takes on a simple form in various limits and leads to simplified Einstein relations. At low temperatures, we also recover a previously known result for the Wiedemann-Franz ratio. Furthermore, we establish a relationship between diffusion and quantum chaos by showing that the diffusivity eigenvalues are upper bounded by the chaos propagation rate at all temperatures. Our work showcases an important example of an analytically tractable calculation of transport properties in a strongly interacting quantum system and reveals a more general purpose method for addressing strongly coupled transport.

Recently the graphene-SiC interface has emerged as a versatile platform for the epitaxy of otherwise unstable, monoelemental, two-dimensional (2D) layers via intercalation. Intrinsically capped into a van der Waals heterostructure with overhead graphene, they compose a new class of quantum materials with striking properties contrasting their parent bulk crystals. Intercalated silver presents a prototypical example where 2D quantum confinement and inversion symmetry breaking entail a metal-to-semiconductor transition. However, little is known about the associated unoccupied states, and control of the Fermi-level position across the band gap would be desirable. Here, we $n$-type dope a graphene/2D-Ag/SiC heterostack via in situ potassium deposition and probe its band structure by means of synchrotron-based angle-resolved photoelectron spectroscopy. While the induced carrier densities on the order of ${10}^{14}{\mathrm{cm}}^{-2}$ are not yet sufficient to reach the onset of the silver conduction band, the band alignment of graphene changes relative to the rigidly shifting Ag valence band and substrate core levels. We further demonstrate an ordered potassium adlayer ($2\times 2$ relative to graphene) with free-electron-like dispersion, suppressing plasmaron quasiparticles in graphene via enhanced metallization of the heterostack. Our results establish surface charge-transfer doping as an efficient handle to modify band alignment and electronic properties of a van der Waals heterostructure assembled from graphene and a novel type of monolayered quantum material.

We report observation of surface-defect-induced room-temperature multiferroicity—surface ferromagnetism ($M_S$ at 50 kOe $\approx 0.005$ emu/g), ferroelectricity ($P_R\approx 2\mathrm{nC}/{\mathrm{cm}}^2$), and significantly large magnetoelectric coupling (decrease in $P_R$ by $\approx 80\%$ under $\approx 15$ kOe field)—in nanorods (diameter $\approx 100$ nm) of double perovskite ${\mathrm{Y}}_2{\mathrm{NiMnO}}_6$ compound. In bulk form, this system exhibits multiferroicity only below its magnetic transition temperature $T_N\approx$ 70 K. On the other hand, the oxygen vacancies, formed at the surface region (thickness $\approx 10$ nm) of the nanorods, yield long-range magnetic order as well as ferroelectricity via Dzyloshinskii-Moriya exchange coupling interactions with strong Rashba spin-orbit coupling. Sharp drop in $P_R$ under magnetic field indicates strong cross coupling between magnetism and ferroelectricity as well. Observation of room-temperature magnetoelectric coupling in nanoscale for a compound which, in bulk form, exhibits multiferroicity only below 70 K underscores an alternative pathway for inducing magnetoelectric multiferroicity via surface defects and, thus, in line with magnetoelectric property observed, for example, in domain walls or boundaries or interfaces of heteroepitaxially grown thin films which do not exhibit such features in their bulk.

${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ has often been described via a simple, one-band pseudospin-$\frac{1}{2}$ model subject to electron-electron interactions on a square lattice, fostering analogies with cuprate superconductors believed to be well described by a similar model. In this work we argue—based on a detailed study of the low-energy electronic structure by circularly polarized spin and angle-resolved photoemission spectroscopy combined with dynamical mean-field theory calculations—that a pseudospin-$\frac{1}{2}$ model fails to capture the full complexity of the system. We show instead that a realistic multiband Hubbard Hamiltonian, accounting for the full correlated $t_{2g}$ manifold, provides a detailed description of the interplay between spin-orbital entanglement and electron-electron interactions and yields quantitative agreement with experiments. Our analysis establishes that the $j_{3/2}$ states make up a substantial percentage of the low-energy spectral weight, i.e., approximately 74% as determined from the integration of the $j$-resolved spectral function in the 0 to $-1.64\mathrm{eV}$ energy range. The results in our work are of relevance not only to Ir-based materials but also more generally to multiorbital materials with closely spaced energy scales.

Unconventional ferromagnetism and superconductivity in the imbalanced kagome-lattice Hubbard model are investigated by the mean-field theory and determinant quantum Monte Carlo method. Due to the asymmetric band structure of kagome lattice, the spin-$z$ ferromagnetic order intrinsically exists in the system, which is first enhanced by the interaction, and then continuously destructed after reaching a maximum at a moderate interaction strength. In contrast, the $xy$-plane ferromagnetism develops only above a critical interaction, which is estimated to be $U_c/t=3.65\pm 0.05$ by finite-size scaling. We further verify the nature of the above transverse magnetic transition, and demonstrate it belongs to the three-dimensional $XY$ universality class. Finally, we study the superconducting property, and reveal the possible superconducting state has a triplet $f$-wave pairing symmetry. Our results uncover the exotic quantum states induced by the interactions on kagome lattice, and provide important insights regarding the interplay between electronic correlations and geometry frustrations.

Twisted bilayer transition metal dichalcogenides are ideal platforms to study flat-band phenomena. In this paper, we investigate flat-band plasmons in the hole-doped twisted bilayer ${\mathrm{MoS}}_2$ (tb-${\mathrm{MoS}}_2$) by employing a full tight-binding model and the random phase approximation. When considering lattice relaxations in tb-${\mathrm{MoS}}_2$, the flat band is not separated from remote valence bands, which makes the contribution of interband transitions in transforming the plasmon dispersion and energy significantly different. In particular, low-damped and quasiflat plasmons emerge if we only consider intraband transitions in the doped flat band, whereas a $\sqrt{q}$ plasmon dispersion emerges if we also take into account interband transitions between the flat band and remote bands. Furthermore, the plasmon energies are tunable with twist angles and doping levels. However, in a rigid sample that suffers no lattice relaxations, lower-energy quasiflat plasmons and higher-energy interband plasmons can coexist. For rigid tb-${\mathrm{MoS}}_2$ with a high doping level, strongly enhanced interband transitions quench the quasiflat plasmons. Based on the lattice relaxation and doping effects, we conclude that two conditions, the isolated flat band and a proper hole-doping level, are essential for observing the low-damped and quasiflat plasmon mode in twisted bilayer transition metal dichalcogenides. We hope that our study on flat-band plasmons can be instructive for studying the possibility of plasmon-mediated superconductivity in twisted bilayer transition metal dichalcogenides in the future.

The Majumder-Ghosh (MG) point is a point in parameter space for a one-dimensional (1D) frustrated system where $\alpha =J_1/J_2=0.5$ and the ground state is shown to be a superposition of singlet states. This leads to no magnetic order, instead the ground state is dominated by electronic dynamics, much like that of a spin liquid. Szenicsite is a natural mineral that is predicted to have isolated 1D ${\mathrm{Cu}}^{2+}$ chains and lies close to or on the MG point. In this work we use muon spin spectroscopy to demonstrate that szenicsite does not magnetically order down to 100 mK and there is an absence of a spin gap, with 1D magnetic excitations dominating. Therefore, we believe szenicsite shows the properties that would put it on the MG point and, as such, it is an interesting system for studying the properties of this quantum mechanical state.

We report phase coherent electron transport in PbTe nanowire networks with a loop geometry. Magnetoconductance shows Aharonov-Bohm (AB) oscillations with periods of $h/e$ and $h/2e$ in flux. The amplitudes of $h/2e$ oscillations are enhanced near zero magnetic field, possibly due to interference between time-reversal paths. Temperature dependence of the AB amplitudes suggests a phase coherence length $\sim 812\mu \mathrm{m}$ at 50 mK. This length scale is larger than the typical geometry of PbTe-based hybrid semiconductor-superconductor nanowire devices.

We identify a mapping between two-dimensional (2D) electron transport in a minimally twisted graphene bilayer and a one-dimensional (1D) quantum walk, where one spatial dimension plays the role of time. In this mapping, a magnetic field $B$ perpendicular to the bilayer maps onto an electric field. Bloch oscillations due to the periodic motion in a 1D Bloch band can then be observed in purely DC transport as magnetoconductance oscillations with periodicity set by the Bloch frequency.

Recent experiments have shown that electronic Mach-Zehnder interferometers of unprecedented fidelities could be built using a graphene $pn$ junction in the quantum Hall regime. In these junctions, two different edge states corresponding to two different valley configurations are spatially separated and form the two arms of the interferometer. The observed separation, of several tens of nanometers, has been found to be abnormally high and thus associated to unrealistic values of the exchange interaction. In this work, we show that, although the separation is due to exchange interaction, its actual value is entirely governed by the sample geometry and independent of the value of the exchange splitting. Our analysis follows the lines of the classical work of Chklovski-Shklovskii-Glazman on electrostatically induced edge state reconstruction and includes quantitative numerical calculations in the experimental geometries.

Noble gas elements have been illustrated to exhibit chemical activity to form unconventional compounds at high pressure. In this paper, we report combined structure prediction and first-principles calculations to propose an unexpected stoichiometry of ${\left({\mathrm{NH}}_3\right)}_2\mathrm{Xe}$ that becomes energetically stable >11 GPa. Here, ${\mathrm{NH}}_3$ in the compound remains in its molecular form up to at least 300 GPa, indicating that the incorporation of Xe could suppress the ionization of ${\mathrm{NH}}_3$. Ab initio molecular dynamics simulations reveal that ${\left({\mathrm{NH}}_3\right)}_2\mathrm{Xe}$ transforms from a solid to a superionic, and finally to a fluid as the temperature increases. The superionic phase remains stable in the pressure and temperature region that covers the extreme conditions of the layer outside the core of planets such as Uranus, Neptune, Venus, and Earth. This suggests that ${\left({\mathrm{NH}}_3\right)}_2\mathrm{Xe}$ is a possible constituent of planetary interiors. The current results provide theoretical evidence that Xe could be trapped inside planets during their evolution and could help to update models of planetary interiors.

We propose and study generic multi-Weyl semimetal (mWSM) lattice Hamiltonians that break particle-hole symmetry. These models fall into two categories: model I (model II) where the gap and tilt terms are coupled (decoupled) can host type-I and type-II Weyl nodes simultaneously (separately) in a hybrid phase (type-I and type-II phases, respectively). We concentrate on the question of how anisotropy and nonlinearity in the dispersions, gaps and tilt terms influence diffusive second-order transport quantities, namely, the circular photogalvanic effect (CPGE) and the Berry curvature dipole (BCD) as well as first-order Magnus Hall effect (MHE) in the ballistic limit. The signatures of topological charges are clearly imprinted in the quantized CPGE response for the hybrid mWSM phase in model I. Such a quantization is also found in the type-I WSM phase for model II, however, the frequency profiles of the CPGE in these two cases is distinctively different owing to their different band dispersion irrespective of the identical topological properties. The contributions from the vicinity of Weyl nodes and away from the WNs are clearly manifested in the BCD response, respectively, for models I and II. The Fermi surface properties for the activated momentum lead to a few hallmark features on the MHE for both models. Furthermore, we identify distinguishing signatures of the above responses for type-I, type-II, and hybrid phases to provide an experimentally viable probe to differentiate these WSMs phases.

Two-dimensional materials with intrinsic magnetism have provoked comprehensive attentions due to their potential application in spintronics. In this work, a class of stable transition metal triborides, $\mathrm{TM}{\mathrm{B}}_3$ ($\mathrm{TM}=\mathrm{Ti}$, V, Cr, Mn, Fe) with various electronic and magnetic features were predicted by density functional theory. Based on static energy, phonon spectrum, and $abinitio$ molecular dynamics simulations, all the $\mathrm{TM}{\mathrm{B}}_3$ monolayers have thermal and kinetic stability. Meanwhile, $\mathrm{TM}{\mathrm{B}}_3\mathrm{s}$ exhibit mechanical anisotropy reflected by Young's modulus and Poisson's ratio. Moreover, $\mathrm{Ti}{\mathrm{B}}_3$ and $\mathrm{Fe}{\mathrm{B}}_3$ are robust ferromagnetic metals with the respective Curie temperatures of 248 and 367 K. $\mathrm{Cr}{\mathrm{B}}_3$ is antiferromagnetic semiconductor with direct bandgap of 513.6 meV and magnetic transform temperatures of as high as 378 K. Finally, it is predicted that the magnetic properties can be further modulated by exerting external strains. Our results may provide new clues for searching and designing two-dimensional magnetic materials.