Vanadium dioxide is one of the most studied strongly correlated materials. Nonetheless, the intertwining between electronic correlation and lattice effects has precluded a comprehensive description of the rutile metal to monoclinic insulator transition, in turn triggering a longstanding “the chicken or the egg” debate about which comes first, the Mott localization or the Peierls distortion. Here, we suggest that this problem is in fact ill posed: The electronic correlations and the lattice vibrations conspire to stabilize the monoclinic insulator, and so they must be both considered to not miss relevant pieces of the ${\mathrm{VO}}_2$ physics. Specifically, we design a minimal model for ${\mathrm{VO}}_2$ that includes all the important physical ingredients: the electronic correlations, the multiorbital character, and the two components of the antiferrodistortive mode that condense in the monoclinic insulator. We solve this model by dynamical mean-field theory within the adiabatic Born-Oppenheimer approximation. Consistently with the first-order character of the metal-insulator transition, the Born-Oppenheimer potential has a rich landscape, with minima corresponding to the undistorted phase and to the four equivalent distorted ones, and which translates into an equally rich thermodynamics that we uncover by the Monte Carlo method. Remarkably, we find that a distorted metal phase intrudes between the low-temperature distorted insulator and high-temperature undistorted metal, which sheds new light on the debated experimental evidence of a monoclinic metallic phase.

Using spin-resolved time-of-flight momentum microscopy, the Rashba splitting of the Tamm surface state is investigated. This state resides at the Fermi level in a projected bulk band gap of the close-packed (0001) surface of hcp Re. The Rashba splitting amounts to $0.4{\AA }^{-1}$. The state with smaller parallel momentum is fully separated from bulk states, whereas the Rashba branch with larger momentum hybridizes with bulk states, which leads to a suppression of spin-momentum locking. We find a good agreement of the experimental results with one step photoemission calculations that are based on ab initio theory within the local density approximation. The spin polarization of the inner Rashba state is not complete, which manifests in the occurrence of quantum interference patterns as observed by scanning tunneling microscopy. A one-to-one agreement of scanning tunneling spectroscopy and photoemission results is observed, suggesting that the quantum interference pattern originates from the inner Rashba state.

We study the momentum-dependent magnetic and charge susceptibilities in the half-filled two-dimensional triangular Hubbard model within the ladder dual fermion approximation in the metallic, Mott insulating, and crossover regions of parameter space. In the insulating state, we find strong spin fluctuations at the $K$ point at low energy corresponding to the ${120}^{\circ }$ antiferromagnetic order. The spin spectra of the insulating state is consistent with the inelastic neutron spectra of the triangular compound ${\mathrm{Ba}}_8{\mathrm{CoNb}}_6{\mathrm{O}}_{24}$. We also find that the spin fluctuations at the $K$ point persist into the metallic phase and move to higher energy. These results are in agreement with studies of spin-lattice relaxation times in $\kappa \text{−}{\left(\mathrm{ET}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$. Finally, we present charge susceptibilities in different areas of parameter space, which should correspond to momentum-resolved electron-loss spectroscopy measurements on triangular compounds.

We analyze, analytically and numerically, a periodically driven Rashba nanowire proximity coupled to an $s$-wave superconductor using bosonization and renormalization group analysis in the regime of strong electron-electron interactions. Because of the repulsive interactions, the superconducting gap is suppressed, whereas the Floquet Zeeman gap is enhanced, resulting in a higher effective value of $g$ factor compared to the noninteracting case. The flow equations for different coupling constants, velocities, and Luttinger-liquid parameters explicitly establish that even for small initial values of the Floquet Zeeman gap compared to the superconducting proximity gap, the interactions drive the system into the topological phase and the interband interaction term helps to achieve larger regions of the topological phase in parameter space.

The discovery of topological Weyl semimetals has revealed opportunities to realize several extraordinary physical phenomena in condensed matter physics. Specifically, Weyl semimetals with strong spin-orbit coupling, broken inversion symmetry, and novel spin textures are predicted to exhibit a large spin Hall effect that can efficiently convert the charge current to a spin current. Here, we report a direct experimental observation of large spin Hall and inverse spin Hall effects in the Weyl semimetal ${\mathrm{WTe}}_2$ at room temperature obeying the Onsager reciprocity relation. We demonstrate the detection of a pure spin current generated by the spin Hall phenomenon in ${\mathrm{WTe}}_2$ by making a van der Waals heterostructure with graphene, taking advantage of its long spin coherence length and spin transmission at the heterostructure interface. These experimental findings, well supported by ab initio calculations, show a large charge-spin conversion efficiency in ${\mathrm{WTe}}_2$, which can pave the way for the utilization of spin-orbit-induced phenomena in spintronic memory and logic circuit architectures.

We revealed the electrical transport through surface ferromagnetic states of a nonmagnetic metal $\mathrm{PdCo}{\mathrm{O}}_2$. Electronic reconstruction at the Pd-terminated surface of $\mathrm{PdCo}{\mathrm{O}}_2$ induces Stoner-like ferromagnetic states, which could lead to spin-related phenomena among the highly conducting electrons in $\mathrm{PdCo}{\mathrm{O}}_2$. Fabricating a series of nanometer-thick $\mathrm{PdCo}{\mathrm{O}}_2$ thin films, we detected a surface-magnetization-driven anomalous Hall effect via systematic thickness- and termination-dependent measurements. Besides, we discuss that finite magnetic moments in electron doped $\mathrm{Co}{\mathrm{O}}_2$ triangular lattices may have given rise to additional unconventional Hall resistance.

In the context of field theory in curved space-times, it is known that suitable background space-time geometries can trigger instabilities of fields, leading to exponential growth of their (quantum and classical) fluctuations, a phenomenon called vacuum awakening in the quantum context, which in some classical scenarios seeds spontaneous scalarization or vectorization. Despite its conceptual interest, an actual observation in nature of this effect is uncertain since it depends on the existence of fields with appropriate masses and couplings in strong-gravity regimes. Here, we propose analogues for this gravity-induced instability based on nonlinear optics of metamaterials which could, in principle, be observed in laboratory.

In the area of non-Hermitian physics, there has been increasing research interest in photonics. Recently, this interest has expanded to topological systems in which symmetry and topology intertwine with non-Hermiticity, giving rise to many intriguing physical effects. One of the major tasks in exploring topological systems is unveiling the bulk-boundary correspondence in the presence of non-Hermiticity. Several proposals have been put forward in this vein, including non-Bloch bulk-boundary correspondence and the non-Hermitian skin effect. However, its practical realization has remained elusive thus far. In this paper, we demonstrate a feasible design of a one-dimensional non-Hermitian Su-Schrieffer-Heeger model based on photonic coupled resonant optical waveguides (CROWs). We show that non-Hermitian asymmetric coupling can be realized by the judicious design of optical gain and loss elements into unidirectional coupling link rings. The phase transition points of a technically achievable CROW open chain are different from those of the periodic boundary, thus revealing the non-Bloch bulk-boundary correspondence. Moreover, the field distribution is found to be exponentially localized at the ends of an open-boundary chain, which demonstrates the non-Hermitian skin effect. Our results underpin the fundamental importance as well as potential applications in various optical devices such as optical couplers, beam splitters, lasers, optical trapping, etc.

In heavy-fermion systems, the competition between the local Kondo physics and intersite magnetic fluctuations results in unconventional quantum critical phenomena which are frequently addressed within the Kondo lattice model (KLM). Here we study this interplay in the $\mathrm{SU}\left(N\right)$ symmetric generalization of the two-dimensional half-filled KLM by quantum Monte Carlo simulations with $N$ up to 8. While the long-range antiferromagnetic (AF) order in $\mathrm{SU}\left(N\right)$ quantum spin systems typically gives way to spin-singlet ground states with spontaneously broken lattice symmetry, we find that the $\mathrm{SU}\left(N\right)$ KLM is unique in that for each finite $N$ its ground-state phase diagram hosts only two phases—AF order and the Kondo-screened phase. The absence of any intermediate phase between the $N=2$ and large-$N$ cases establishes adiabatic correspondence between both limits and confirms that the large-$N$ theory is a correct saddle point of the KLM fermionic path integral and a good starting point to include quantum fluctuations. In addition, we determine the evolution of the single-particle gap, quasiparticle residue of the doped hole at momentum $(\pi ,\pi )$, and spin gap across the magnetic order-disorder transition. Our results indicate that increasing $N$ modifies the behavior of the coherence temperature: while it evolves smoothly across the magnetic transition at $N=2$ it develops an abrupt jump—of up to an order of magnitude—at larger but finite $N$. We discuss the magnetic order-disorder transition from a quantum-field-theoretic perspective and comment on implications of our findings for the interpretation of experiments on quantum critical heavy-fermion compounds.

We consider two species of hard-core bosons with density-dependent hopping in a one-dimensional optical lattice, for which we propose experimental realizations using time-periodic driving. The quantum phase diagram for half-integer filling is determined by combining different advanced numerical simulations with analytic calculations. We find that a reduction of the density-dependent hopping induces a Mott-insulator to superfluid transition. For negative hopping, a gauge-dressed superfluid state is found where one species induces a gauge phase of the other species, which leads to a superfluid phase of gauge-paired particles. The corresponding experimental signatures are discussed.

Unconventional superconductivity has been suggested to be present at the interface between bismuth and nickel in thin-film bilayers. In this work, we study the structural, magnetic, and superconducting properties of sputter deposited Bi/Ni bilayers. As-grown, our films do not display a superconducting transition; however, when stored at room temperature, after about 14 days our bilayers develop a superconducting transition up to 3.8 K. To systematically study the effect of low temperature annealing on our bilayers, we perform structural characterization with x-ray diffraction and polarized neutron reflectometry, along with magnetometry and low-temperature electrical transport measurements on samples annealed at ${70}^{\circ }\mathrm{C}$. We show that the onset of superconductivity in our samples is coincident with the formation of ordered ${\mathrm{NiBi}}_3$ intermetallic alloy, a known $s$-wave superconductor. We calculate that the annealing process has an activation energy of $(0.86\pm 0.06)\mathrm{eV}$. As a consequence, gentle heating of the bilayers will cause formation of the superconducting ${\mathrm{NiBi}}_3$ at the Ni/Bi interface, which poses a challenge to studying any distinct properties of Bi/Ni bilayers without degrading that interface.

Nodal semimetals are a unique platform to explore topological signatures of the unusual band structure that can manifest by accumulating a nontrivial phase in quantum oscillations. Here we report a study of the de Haas–van Alphen oscillations of the candidate topological nodal line semimetal CaAgAs using torque measurements in magnetic fields up to 45 T. Our results are compared with calculations for a toroidal Fermi surface originating from the nodal ring. We find evidence of a nontrivial $\pi$ phase shift only in one of the oscillatory frequencies. We interpret this as a Berry phase arising from the semiclassical electronic Landau orbit which links with the nodal ring when the magnetic field lies in the mirror ($ab$) plane. Furthermore, additional Berry phase accumulates while rotating the magnetic field for the second orbit in the same orientation which does not link with the nodal ring. These effects are expected in CaAgAs due to the lack of inversion symmetry. Our study experimentally demonstrates that CaAgAs is an ideal platform for exploring the physics of nodal line semimetals and our approach can be extended to other materials in which trivial and nontrivial oscillations are present.

Magnetic skyrmions are topologically stable objects that are made with a bunch of spins tightly arranged in a smooth fashion. Their topological nature provides unusual and complex transport properties, such as the skyrmion Hall effect. Extensive Hall data have further revealed asymmetry between skyrmion and antiskyrmion Hall angles, which cannot be accounted by known mechanisms. Here, we explain this asymmetry by utilizing another universal transport coefficient called “Hall viscosity,” extensively studied in quantum Hall systems. Hall viscosity is modeled in steady-state skyrmions motion by generalizing the Thiele equation with a transverse velocity component and is independent of the skyrmion charge. Our analyses, based on available asymmetric Hall angle data, reveal this transverse force amounts to 3%–5.4% of the force due to the skyrmion Hall effect. Further clarification of Hall viscosity will be essential for designing next-generation storage devices properly, not to mention for our deeper understanding of the fundamental properties of nature.

We present a theoretical study of the shift current in a noncentrosymmetric polytype of graphitic ${\mathrm{BC}}_2\mathrm{N}$. We find that the photoconductivity near the fundamental gap is strongly anisotropic due to the vanishing of particular tensor components not foretold by point-group symmetry arguments; this is a consequence of dipole selection rules imposed by mirror symmetry, which imply that the relative parities between valence and conduction bands are key for determining the directionality of the band-edge response. In addition, the band-edge photoconductivity turns out to be rather large, with the peak value occurring in an energy range suitable for optical manipulation.

Pure spin currents have been ubiquitous in contemporary spintronics research. Despite its profound physical and technological significance, the detection of pure spin current has largely remained indirect, which is usually achieved by probing spin-transfer torque effects or spin-to-charge conversions. By using scanning transmission x-ray microscopy, we report the direct detection and spatial mapping of spin accumulation in a nonmagnetic Cu layer without any direct charge current injection. Such a pure spin current is induced by spin pumping from a ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ layer and is not accompanied by concomitant charge motion. The observed frequency dependence indicates that the signal is dominated by a coherent, pure spin current, but the magnitude of the spin accumulation suggests also possible additional thermal contributions. Our technique takes advantage of the x-ray magnetic circular dichroism and the synchronization of microwave with x-ray pulses, which together provide a high sensitivity for probing transient magnetic moment. From the detected x-ray signals, we observe two distinct resonance modes induced by spin pumping, which, based on micromagnetic simulations, we attribute to nonlinear microwave excitations. Our result provides a new pathway for detecting pure spin currents that originate from many spintronics phenomena, such as spin Hall and spin Seebeck effects, and which can be applied to both metal and insulator spin current sources.

We combined a spin resolved photoemission spectrometer with a high-harmonic generation (HHG) laser source in order to perform spin, time, and angle resolved photoemission spectroscopy (STARPES) experiments on the transition metal dichalcogenide bulk ${\mathrm{WTe}}_2$, a possible Weyl type-II semimetal. Measurements at different femtosecond pump-probe delays and comparison with spin resolved one-step photoemission calculations provide insight into the spin polarization of electrons above the Fermi level in the region where Weyl points of ${\mathrm{WTe}}_2$ are expected. We observe a spin accumulation above the Weyl points region, which is consistent with a spin-selective bottleneck effect due to the presence of spin-polarized conelike electronic structure. Our results support the feasibility of STARPES with HHG, which despite being experimentally challenging provides a unique way to study spin dynamics in photoemission.

Discovery of novel spin-orbital entangled quantum ground states paves an important avenue for controllable quantum materials via unique couplings to the lattice and other external perturbations. In this work, motivated by recent experiments on cubic heavy fermion materials with multipolar local moments, we theoretically investigate strongly interacting spin-orbital entangled quantum ground states in multipolar quantum impurity systems. Here itinerant electrons are interacting with the local moments carrying quadrupolar and octupolar moments, in contrast to the conventional Kondo problem with dipolar local moments. Using perturbative renormalization group methods, we uncover a number of non-Fermi-liquid ground states, which are characterized by an absence of well-defined quasiparticles and singular power-law behaviors in physical properties. We show that the non-Fermi-liquid states found here are outside the known categories of non-Fermi-liquid states in the conventional multi-channel Kondo problem. This work lays a novel ground for the identification of unexpected non-Fermi-liquid phases in many strongly spin-orbital coupled quantum materials.

Classical chaotic systems exhibit exponentially diverging trajectories due to small differences in their initial states. The analogous diagnostic in quantum many-body systems is an exponential growth of out-of-time-ordered correlation functions (OTOCs). These quantities can be computed for various models, but their experimental study requires the ability to evolve quantum states backward in time, similar to the canonical Loschmidt echo measurement. In some simple systems, backward time evolution can be achieved by reversing the sign of the Hamiltonian; however, in most interacting many-body systems, this is not a viable option. Here we propose a family of protocols for OTOC measurement that do not require backward time evolution. Instead, they rely on ordinary time-ordered measurements performed in the thermofield double (TFD) state, an entangled state formed between two identical copies of the system. We show that, remarkably, in this situation the Lyapunov chaos exponent ${\lambda }_L$ can be extracted from the measurement of an ordinary two-point correlation function. As an unexpected bonus, we find that our proposed method yields the so-called “regularized” OTOC—a quantity that is believed to most directly indicate quantum chaos. According to recent theoretical work, the TFD state can be prepared as the ground state of two weakly coupled identical systems and is therefore amenable to experimental study. We illustrate the utility of these protocols on the example of the maximally chaotic Sachdev-Ye-Kitaev model and support our findings by extensive numerical simulations.

We theoretically study dilute superfluidity of spin-1 bosons with antiferromagnetic interactions and synthetic spin-orbit coupling (SOC) in a one-dimensional lattice. Employing a combination of density matrix renormalization group and quantum field theoretical techniques we demonstrate the appearance of a robust superfluid spin-liquid phase in which the spin sector of this spinor Bose-Einstein condensate remains quantum disordered even after introducing quadratic Zeeman and helical magnetic fields. Despite remaining disordered, the presence of these symmetry-breaking fields lifts the perfect spin-charge separation and thus the nematic correlators obey power-law behavior. We demonstrate that, at strong coupling, the SOC induces a charge density wave state that is not accessible in the presence of linear and quadratic Zeeman fields alone. In addition, the SOC induces oscillations in the spin and nematic expectation values as well as the bosonic Green's function. These nontrivial effects of an SOC are suppressed under the application of a large quadratic Zeeman field. We discuss how our results could be observed in experiments on ultracold gases of ${}^{23}\mathrm{Na}$ in an optical lattice.

Polarons are physical objects of material science that are hard to capture from first-principles calculations. ${\mathrm{WO}}_3$ is a paradigmatic system to study polarons and here we present calculations of a single self-trapped single polaron in ${\mathrm{WO}}_3$ from density functional theory calculations. Our calculations show that the single polaron is at a higher energy than the fully delocalized solution, in agreement with the experiments where a single polaron is an excited state of ${\mathrm{WO}}_3$. The symmetry-adapted mode decomposition of the polaron distortions shows that, among numerous modes, a polar zone center mode has the largest contribution and can be at the origin of the observed weak ferroelectricity of ${\mathrm{WO}}_3$.