We study the possibility of superconductivity in a trilayer Ruddlesden-Popper nickelate ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ under pressure both theoretically and experimentally, making comparison with the recently discovered high $T_c$ superconductor ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$, a bilayer nickelate. Through DFT calculations, we find that a structural phase transition from monoclinic to tetragonal takes place around 10–15 GPa. Using the tetragonal crystal structure, we theoretically investigate the possibility of superconductivity, where a combination of fluctuation exchange approximation and linearized Eliashberg equation is applied to a six-orbital model constructed from first-principles band calculations. The obtained results suggests that ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ may also become superconducting under high pressure with $T_c$ comparable to some cuprates, although it is not as high as ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$. We also perform experimental studies using our polycrystalline samples of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{7.01}$ and ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{9.99}$. The superconducting transition of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{7.01}$, with a maximum onset $T_c$ of 67.0 K at a pressure of 26.5 GPa, is confirmed by a drop in the electrical resistance as well as the magnetic-field dependence of the resistance. Quite interestingly, similar temperature and magnetic field dependencies of the resistance are also observed for ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{9.99}$, where a drop in the resistance is observed at lower temperatures compared to ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{7.01}$, under pressures of 32.8 GPa and above. Given the theoretical expectation, the reduction in the resistance can most likely be attributed to the occurrence of superconductivity in ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{9.99}$. The temperature at which the resistance deviates from linear behavior, considered as the onset $T_c$, monotonically increases up to 23 K at 79.2 GPa, which is opposite to the pressure dependence of $T_c$ in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{7.01}$.

In a topological superconducting heterostructure comprising an $s$-wave superconductor and a semiconductor with Rashba spin-orbit coupling, several distinct modes emerge when an external magnetic field is applied: Majorana zero-energy modes, trivial Caroli–de Gennes–Matricon modes, and edge modes. We find three subtle properties of the modes under the fine tuning of the external magnetic field and Rashba spin-orbit coupling. (1) The spatial configuration of Majorana zero-energy modes undergoes a dramatic change when flipping the direction of magnetic field. (2) In the topological nontrivial regime, the Caroli–de Gennes–Matricon modes and edge modes exhibit spatial oscillatory behavior across the entire sample scale and approach zero energy as the strength of the Rashba spin-orbit coupling decreases. (3) In the topological trivial regime, another type of zero-energy mode may also manifest at the vortex center. The key distinction from the Majorana zero-energy mode is its lack of spatial coherence and no coexistence of edge modes. These properties highlight the modes' sensitivity to the tuning parameters and impose constraints on the experimental determination of Majorana zero-energy modes.

We show that spectral form factors of unconventional gapped superconductors have singularities occurring periodically in time. These are the superconductors whose gap function vanishes somewhere in momentum space (Brillouin zone) but whose fermionic excitation spectrum is fully gapped. Many, although not all, of these superconductors are topologically nontrivial. In contrast, conventional fully gapped superconductors have featureless spectral form factors which are analytic in time. Some gapless superconductors may also have singularities in their spectral form factors, but they are not as ubiquitous and their appearance may depend on the details of the interactions among fermionic particles which form the superconductor and on the underlying lattice where the particles move. This work builds on the prior publication [S. Gaur, V. Gurarie, and E. A. Yuzbashyan, Phys. Rev. B 106, L220506 (2022)] where Loschmidt echo of topological superconductors, related but not identical to spectral form factors, was studied. It follows that spectral form factors could be used as a test of the structure of the superconducting gap functions.

A quantum spin liquid (QSL) is a state of matter characterized by fractionalized quasiparticle excitations, quantum entanglement, and a lack of long-range magnetic order. However, QSLs have evaded definitive experimental observation. Several ${\mathrm{Yb}}^{3+}$-based triangular lattice antiferromagnets with effective $S=\frac{1}{2}$ have been suggested to stabilize the QSL state as the ground state. Here, we build a comprehensive magnetic temperature phase diagram of a high-quality single crystalline ${\mathrm{KYbSe}}_2$ via heat capacity and magnetocaloric effect down to 30 mK with magnetic field applied along the $a$ axis. At zero magnetic field, we observe the magnetic long-range order at $T_{\text{N}}=0.29$ K entering 120 degrees ordered state in heat capacity, consistent with neutron scattering studies. Analysis of the low-temperature $\left(T\right)$ specific heat $\left(C\right)$ at zero magnetic field indicates linear $T$ dependence of $C/T$ and a broad hump of $C/T$ in the proximate QSL region above $T_{\text{N}}$. By applying magnetic field, we observe the up-up-down phase with 1/3 magnetization plateau and oblique phases, in addition to two new phases. These observations strongly indicate that while ${\mathrm{KYbSe}}_2$ closely exhibits characteristics resembling an ideal triangular lattice, deviations may exist, such as the effect of the next-nearest-neighbor exchange interaction, calling for careful consideration for spin Hamiltonian modeling. Further investigations into tuning parameters, such as chemical pressure, could potentially induce an intriguing QSL phase in the material.

The frustrated spin-chain (FSC) systems exhibit exotic ground states and distinct quantum phase transitions. The $S=\frac{1}{2}$ FSC is known to exhibit the Kosterlitz-Thouless transition from a commensurate gapless phase to a fully dimerized gapped phase upon the ratio of next-nearest-neighbor to nearest-neighbor coupling $(\alpha =\frac{J_2}{J_1})$ being tuned. On the other hand, the $S=\frac{5}{2}$ FSC system is known to show transitions from a commensurate gapless phase to partially dimerized and incommensurate floating phases [Chepiga, Affleck, and Mila, Phys. Rev. B 105, 174402 (2022)]. While a large region of the floating phase has been theoretically predicted for the $S=\frac{5}{2}$ FSC model when $\alpha >0.43$, it is yet to be explored experimentally. Here, we have investigated a compound ${\mathrm{K}}_3\mathrm{Fe}{\left({\mathrm{MoO}}_4\right)}_2\left({\mathrm{Mo}}_2{\mathrm{O}}_7\right)$, having well-separated $S=\frac{5}{2}$ FSCs. The electronic structure calculations show that the $\alpha =\frac{J_2}{J_1}$ is close to 0.9, being similar to another FSC compound ${\mathrm{Bi}}_3{\mathrm{FeMo}}_2{\mathrm{O}}_{12}$ $(\alpha \approx 1.1)$. No magnetic long-range order is found down to 0.09 K, despite the relatively sizable Curie-Weiss temperature ${\theta }_{CW}=-18\mathrm{K}$. The magnetic heat capacity shows the power-law behavior, indicating that the compound exhibits gapless excitations. Based on the experimental results and the theoretical calculations employed by density functional theory, we argue that the titled system is a possible candidate for exhibiting the floating phase.

We construct and explore two-band topological semimetals in different spatial dimensions that are protected by unstable homotopies. Dubbed “delicate semimetals,” they generically host nodal lines and are inspired by the example of such phases realized in four dimensions arising from maps from the three-torus $T^3$ [Brillouin zone of a three-dimensional (3D) crystal] to the two-sphere $S^2$ related to the Hopf map. In the four-dimensional example, a surface enclosing such a nodal line in the Brillouin zone carries a Hopf flux. These four-dimensional semimetals show a unique class of surface states: while some 3D surfaces host gapless Fermi-arc states and drumhead states, other surfaces have gapless Fermi surfaces. Gapless two-dimensional corner states are also present at the intersection of three-dimensional surfaces. We also demonstrate such semimetals realized in three dimensions in chiral class AIII, which arise from the unstable homotopies of maps from $T^2$ (Brillouin zone of a two-dimensional crystal) to $S^1$. These 3D semimetals also host nodal lines, accompanied by a rich collection of surface states, including drumhead type. This work provides a new framework to realize protected nodal line semimetals, particularly in synthetic quantum systems such as cold atoms, photonic, and topoelectric systems.

Tunability of near-room temperature insulator-to-metal transition (IMT) of ${\mathrm{VO}}_2$ is a prerequisite for its applications in switching and sensing devices. IMT in ${\mathrm{VO}}_2$ is accompanied with structural transition, monoclinic (insulating) to rutile (metallic), where the V-V dimer of the monoclinic phase becomes equidistant in the rutile phase. Tuning of the V-V dimer distances can result in dramatic changes in IMT characteristics. However, understanding of such processes has been limited due to intrigued relations between structure, electronic structure, and IMT of ${\mathrm{VO}}_2$. By utilizing the substrate and Cr doping-induced strain, we have grown ${\mathrm{VO}}_2$ thin films with distinct V-V dimers, which has enabled us to study the effect of these dimers on the structure, IMT, and electronic structures of ${\mathrm{VO}}_2$. In addition to the usual M1 phase of ${\mathrm{VO}}_2$, strain-mediated $T$ and M2 phases have been stabilized with the help of both Cr doping and tensile strain along the $c_R$ axis. We have observed that a small compressive strain ($\approx 0.19\%$) along the $c_R$ axis ($\approx \mathrm{monoclinic}{a}_M$ axis) lowers the transition temperature significantly (by $\approx 10{}^{\circ }\mathrm{C}$) compared to bulk. Temperature-dependent Raman spectroscopy measurement is used to track the exact structural transformation route followed by these insulating phases of ${\mathrm{VO}}_2$. Dependent on the nature of strain along the $c_R$ axis, IMT temperature is found to vary—increases (for tensile) or decreases (for compressive)—while Cr doping-induced strain has a less significant impact on the IMT temperature compared to the nature of the strain. X-ray absorption near-edge spectroscopy (XANES) has been utilized to examine the electronic structure of the grown ${\mathrm{VO}}_2$ thin films. Temperature variation of pre-edge features (vanadium $3d$ orbitals) in XANES directly scales with the insulator-to-metal transition, which suggests that the electronic structure of ${\mathrm{VO}}_2$ is strongly influenced by the nature (compressive/tensile) of strain, whereas minimal changes in electronic structure have been observed due to different insulating phases (M1, $T$, and M2). Our study underscores the important role of the nature of strain in tailoring the IMT in ${\mathrm{VO}}_2$.

We carry out a detailed theoretical study of the electronic and optical properties of bulk and monolayer bismuth triiodide (${\mathrm{BiI}}_3$), a layered metal halide, using the ab initio $\text{GW}+\text{BSE}$ scheme with a full spinorial formulation. We discuss in detail the effects due to the change of dimensionality and the role of spin-orbit coupling. Moreover, we compute the exciton dispersion by solving the BSE at finite momentum, also analyzing transverse and longitudinal excitons, and the L-T splitting at $\mathbf{q}\approx \mathrm{\Gamma }$. The results provide a reference for future experimental measurements. In addition, the interplay between spin-orbit coupling and large binding energy, together with the role of quantum confinement, confirm that ${\mathrm{BiI}}_3$ is an interesting material for optoelectronic applications and show that it is a good candidate for the study of exciton dynamics.

We investigate extensively the electronic and transport properties of twisted bilayer graphene when subjected to both an external perpendicular electric field and a magnetic field. Using a basic tight-binding model, we show the flat electronic band properties as well as the density of states (DOS), both without and with the applied electric field. In the presence of an electric field, the degeneracy at the Dirac points is lifted and a nonmonotonic behavior is found in the energy shift between the Dirac points of each layer, especially for twist angles below $3^{\circ }$. We also study the behavior of the Landau levels (LL) spectra for different twist angles within a very low energy range. These LL spectra get modified under the influence of the external electric field. Moreover, we calculate the dc Hall conductivity (${\sigma }_{xy}$) for a very large system using the kernel polynomial method. Interestingly, as we tune the twist angle from a higher to a lower value, ${\sigma }_{xy}$ undergoes a transition from a half-integer to an integer quantum Hall effect, i.e., the value of ${\sigma }_{xy}$ shifts from $\pm 4(n+1/2)(2e^2/h)$ ($n$ is an integer) to $\pm 2n(2e^2/h)$ around a small twist angle of $\theta =2.{005}^{\circ }$. At this angle, ${\sigma }_{xy}$ acquires a Hall plateau at zero Fermi energy. However, the behavior of ${\sigma }_{xy}$ remains unaltered when the system is exposed to the electric field, particularly at the magic angle where the bands in both layers can hybridize, and strong interlayer coupling plays a crucial role.

The thermal transport properties of single-wall carbon nanotubes (SWCNTs) are reinvestigated using the iterative solution of the Boltzmann transport equation by including four-phonon scattering. Using only three-phonon scattering, the flexural and twisted phonon modes are found to remain nonscattered via Umklapp processes. However, with four-phonon scattering, while longitudinal modes remain unaffected, the transverse modes undergo Umklapp scattering. The predicted thermal conductivity of SWCNTs using both three- and four-phonon scatterings is 4000 W/m-K at 300 K, and stays highest amongst all known materials.

We have investigated the electronic structure of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ within density functional theory using the generalized gradient approximation while considering strong Coulomb correlations in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. We have investigated x-ray absorption spectra, x-ray magnetic circular dichroism, and resonant inelastic x-ray scattering (RIXS) spectra at the Ir $K, L_3, M_3, M_5$, and O $K$ edges. The calculated results are in good agreement with experimental data. The RIXS spectrum of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ at the Ir $L_3$ edge possesses a sharp feature <1.5 eV corresponding to transitions within the Ir $t_{2g}$ levels. The excitation located from 2 to 5 eV is due to $t_{2g} \to e_g$ transitions. The third wide structure situated at $5–12\mathrm{eV}$ appears due to charge transfer transitions. We have found that the theory reproduces well the shape and energy position of the low-energy feature in the oxygen O $K$ RIXS spectrum, but to obtain the correct positions of the second and third peaks, a self-interaction-correction-like orbital-dependent potential $V_l$ must be included in the Hamiltonian to correct the position of the oxygen $2p$ band. We have found that the dependence of the RIXS spectrum at the oxygen $K$ edge on the incident photon energy and the momentum transfer vector Q is much stronger than the correspondent dependence at the Ir $L_3$ edge.

We study theoretically the generation of high-order harmonics in graphene quantum dots placed in the field of an elliptically polarized ultrashort pulse. The generated high-order harmonics are sensitive to the pulse's ellipticity and its amplitude. The intensities of high-order harmonics become very sensitive to the ellipticity of an incident pulse when its polarization gets close to a circular one, and some high-order harmonics become strongly suppressed for a circularly polarized incident pulse. The suppressed harmonic orders depend on the symmetry of the quantum dot systems. For triangular quantum dots, which have $D_{3h}$ symmetry, every third harmonic is suppressed, while for hexagonal quantum dots with $D_{6h}$ symmetry, such suppression is observed for every sixth harmonic, and the even-order harmonics are suppressed for all ellipticities of the incident pulse due to an additional inversion symmetry of the hexagonal quantum dots. The ellipticities of the generated high-order harmonics also show strong nonmonotonic dependence on the ellipticity of an incident pulse, in which the dependence becomes stronger for high pulse amplitudes.

In accordance with the trajectory of the reflected hole, local Andreev reflection (AR) in three dimensions can be classified into three distinct types, namely retro AR, specular AR, and anomalous AR, which collectively constitute a complete set of local AR. Expanding this classification to crossed Andreev reflection (CAR), we categorize it into forward CAR, negative CAR, and anomalous CAR, which involve the reversal of zero, two, and one lateral velocity components, respectively, with respect to the incident electron. Forward CAR and negative CAR are commonly observed in materials with spherical Fermi surfaces. In this work, we investigate the CAR in a nodal line semimetal-superconductor-nodal line semimetal junction, and find that the presence of a torus-shaped Fermi surface in nodal line semimetals leads to the occurrence of rare anomalous CAR. Notably, in the presence of the interface barriers, the anomalous CAR is found to dominate nonlocal transport within a specific energy range. We show that the anomalous CAR visibly manifests itself as a ridge structure in the spatially resolved conductance, serving as a distinct signature of this peculiar anomalous CAR phenomenon.

Photonic time crystals (PTCs) are spatially homogeneous materials with a dielectric parameter that is modulated periodically in time, paving a new way in electromagnetic wave manipulation and field enhancement. Modeling of time-varying media requires the involvement of a dispersion effect, which is widely present in materials capable of dynamic modulation. Surface waves are another important phenomenon in strongly dispersive systems, appearing at the material interface. Their intrinsic characteristics of subwavelength resolution and local-field enhancement are desirable in constructing PTCs in finite-size structures. In this work, by introducing a nonlocal dispersive PTC-air interface model, we analyze the dispersion relationship of the time-modulated bulk plasma and the surface-wave amplification at the PTC-air interface. It is found that a momentum gap appears under slight modulation, which is due to the low group velocity of surface-plasmon polaritons (SPPs) at the interface. In addition, we also consider the magnetic bias as a degree of freedom in the model, which allows for shifting frequency corresponding to the momentum gap. Finally, we investigate the spatiotemporal evolution of the surface wave at the PTC-air interface excited by a point source and observe a narrow-wave-vector amplification in the momentum gap. The research on the surface wave of the PTC-air interface model not only enables systematic investigation of tunable nonresonant wave amplification in the subwavelength structure, but also offers a platform for studying plenty of novel electromagnetic phenomena in time-varying and spatially finite systems.

Bound states in the continuum hold extraordinary optical topological properties and can significantly enhance the interaction between light and matter including the circular dichroism (CD) in chiral resonant metasurfaces. In this paper, we demonstrate the split and displacement of topological charges through tilted double-elliptical half-disk metasurfaces. It enables the feasibility to attain perfect intrinsic chiroptic responses, with a CD exceeding 0.99. The ellipticity (χ) of the intrinsic chiroptic response, induced by vertical incidence, is also ∼1. In addition to linear chirality CD, significant near-field enhancement contrast is observed at the positions of half-integer topological charge, leading to remarkable nonlinear CD approaching a nearly perfect value of 1. The proposed metasurface simultaneously possesses perfect intrinsic chirality and nonlinear chirality, providing an avenue for the multifaceted development of planar optical devices.

Transitions between quantum spin liquids (QSLs) are fundamental problems lying beyond the Landau paradigm and requiring a deep understanding of the entanglement structures of QSLs called topological orders. The novel concept of anyon condensation has been proposed as a theoretical mechanism, predicting various possible transitions between topological orders, but it has long been elusive to confirm the mechanism in quantum spin systems. Here, we introduce a concrete spin model that incarnates the mechanism of the anyon condensation transition. Our model harbors two topological QSLs in different parameter regions, a non-Abelian Kitaev spin liquid (KSL) bilayer state and a resonating valence bond (RVB) state. The bilayer-KSL-to-RVB transition indeed occurs by the mechanism of anyon condensation, which we identify by using parton theories and exact diagonalization studies. Moreover, we observe “anyon confinement” phenomena in our numerical results, akin to the quark confinement in high-energy physics. Namely, non-Abelian Ising anyons of the bilayer KSL are confined in the transition to the RVB state. Implications and extensions of this study are discussed in various aspects such as (i) anyon-condensed multilayer construction of Kitaev's 16-fold way of anyon theories, (ii) an additional vison condensation transition from the RVB to a valence bond solid in the Kitaev bilayer system, (iii) dynamical anyon condensation in a non-Hermitian Kitaev bilayer, (iv) generalizations of our model to other lattice geometries, and (v) experimental realizations. This work puts together the two fascinating QSLs that are extensively studied in modern condensed matter and quantum physics into a concrete spin model, offering a comprehensive picture that unifies the anyon physics of the Kitaev spin liquids and the resonating valence bonds.

We study the ground-state phase diagram of a spin-$\frac{1}{2}$ XXZ model with a chirality-chirality interaction (CCI) on a two-leg ladder. This model offers a minimal setup to study an interplay between spin and chirality degrees of freedom. The spin-chirality duality transformation allows us to relate the regimes of weak and strong CCIs. By applying the Abelian bosonization and the duality, we obtain a rich phase diagram that contains distinct gapped featureless and ordered phases. In particular, Néel and vector chiral orders appear for easy-axis anisotropy, while two distinct symmetry-protected topological (SPT) phases appear for easy-plane anisotropy. The two SPT phases can be viewed as twisted variants of the Haldane phase. We also present an effective description in terms of (spinor) hard-core bosons, which reveals critical behavior on the self-dual line in the easy-axis and easy-plane regimes. We perform numerical simulations to confirm the predicted phase structure and critical properties. We further demonstrate that the two SPT phases and a trivial phase are distinguished by topological indices in the presence of certain symmetries. A similar phase structure is expected in a spin-$\frac{1}{2}$ XXZ ladder with four-spin ring exchange.

Using variable temperature atomic pair distribution function analysis, we study the emergence of charge density wave (CDW) order in $1T\text{−}{\mathrm{TiSe}}_2$ and find that it takes place via a two-step transition. First, upon decreasing temperature to about 235(3) K, CDW-related lattice distortions emerge in the individual ${\mathrm{TiSe}}_2$ layers alone. Then, upon further decreasing the temperature, the two-dimensional distortions in the layers couple and the widely recognized three-dimensional $2a_0\times 2a_0\times 2c_0$ superstructure emerges at about 205(3) K. Because two different band gaps are known to emerge at the same temperatures, the finding indicates the presence of strong electron-phonon coupling. The transient phase between the two steps lacks inversion symmetry and may serve as a precursor of the debated chiral 1T-${\mathrm{TiSe}}_2$ phase. Our findings are important for the understanding of the enigmatic CDW transition in 1T-${\mathrm{TiSe}}_2$ and CDW instabilities in van der Waals materials in general.

While it is commonly accepted that the disorder induced by magnetic ion doping in quantum magnets usually generates a rugged free-energy landscape resulting in slow or glassy spin dynamics, the disorder/distortion effects associated with nonmagnetic ion sites doping are still illusive. Here, using AC susceptibility measurements, we show that the mixture of Sn/Ti on the nonmagnetic ion sites of pyrochlore ${\mathrm{Yb}}_2{\left({\mathrm{Ti}}_{1-x}{\mathrm{Sn}}_x\right)}_2{\mathrm{O}}_7$ induces an antiferromagnetic ground state despite both parent compounds, ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Yb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$, order ferromagnetically. Local structure studies through neutron total scattering reveals the local distortion in the nonmagnetic ion sites and its strong correlation with the magnetic phase switching. Our study demonstrates the local distortion as induced by the nonmagnetic ion site mixture could be a new path to achieve magnetic phase switching, which has been traditionally obtained by external stimuli such as temperature, magnetic field, pressure, strain, light, etc.