We have measured the electric transport properties of TiN nanostrips with different widths. At zero magnetic field, the temperature-dependent resistance $R\left(T\right)$ saturates at a finite resistance toward low temperatures, which results from quantum phase slips in the narrower strips. We find that the current-voltage $\left(I\text{−}V\right)$ characteristics of the narrowest strips are equivalent to those of small Josephson junctions. Applying a transverse magnetic field drives the devices into a reentrant insulating phase, with $I\text{−}V$ characteristics dual to those in the superconducting regime. The results provide evidence that our critically disordered superconducting nanostrips behave like small self-organized random Josephson networks.

We solve analytically the problem of a finite-length Kitaev chain coupled to a quantum dot (QD) which extends the standard Kitaev chain problem to the quantum dot-semiconductor-superconductor (QD-SM-SC) nanowire heterostructure currently under intense investigation for possible occurrence of Majorana zero modes (MZMs). As a first step we obtain the analytical solution of the finite-length Kitaev chain without the quantum dot which to the best of our knowledge has also not appeared before. Our full solution of the Kitaev chain coupled to a quantum dot reveals the emergence of a robust near-zero-energy Andreev bound state (ABSs) localized in the quantum dot region as the generic lowest energy solution in the topologically trivial phase. By contrast in the Kitaev chain without the quantum dot such a solution does not exist. The robustness of the ABS in the topologically trivial phase is due to a partial spatial decoupling of the component Majorana bound states (MBSs) over the length of the dot potential. As a result the signatures of the ABS in measurements that couple locally to the quantum dot e.g. tunneling measurements are identical to the signatures of topologically protected MZMs which arise only in the topological superconducting (TS) phase of the Kitaev chain.

Motivated by the observation of superconductivity in ${\mathrm{SrTiO}}_3$ and Bi, we analyze phonon-mediated superconductivity in three-dimensional systems at low carrier density, when the chemical potential $\mu$ is comparable to or even smaller than the characteristic phonon frequency ${\omega }_L$. We consider the attractive part of the Bardeen-Pines pairing interaction, in which the frequency-dependent electron-phonon interaction is dressed by the Coulomb potential. This dressing endows the pairing interaction with momentum dependence. We argue that the conventional Migdal-Eliashberg approximation becomes invalid when $\mu \le {\omega }_L$, chiefly because the dominant contribution to pairing comes from electronic states away from the Fermi surface. We obtain the pairing onset temperature, which is equal to $T_c$ in the absence of phase fluctuations, as a function of $\mu /{\omega }_L$. We find both analytically and numerically that $T_c$ increases as the ratio $\mu /{\omega }_L$ becomes smaller. In particular, in the dilute regime, $\mu \to 0$, it holds that $T_c\propto {\omega }_L{\left(\frac{Ry}{{\omega }_L}\right)}^{\eta }$, where Ry is the Rydberg constant and $\eta \sim 0.2$.

We present a systematic study of the interplay between lattice parameters and the energy of the optical phonons as well as the antiferromagnetic coupling strength, $J$, in the high-$T_c$ superconducting cuprate ${R\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+\delta }$ ($R$-123, $R=\text{Y}$, Dy, Gd, Sm, Nd) with hole doping $p$ ($0.00<p\lesssim 0.04$). The energy of the $B_{1g}$ mode at ${\nu }_{B1g}\approx 335{\mathrm{cm}}^{-1}$ has been found to relate systematically to the inverse of the lattice parameter $a$. Our results confirm the temperature dependent phonon splitting for Nd-123 at low doping, which has been reported for optimally doped Nd-123. Surprisingly, $J$ is independent of $a$ for the first four $R$ families, and a general consistency between $T_c^{max}$ and $J$, as suggested in a previous investigation, could not be confirmed.

We report an unusual canted magnetism due to $3d$ and $4f$ electrons, occupying two different crystallographic sites, with its consequence to electric dipole order. This is based on neutron powder diffraction measurements on ${\mathrm{Tb}}_2{\mathrm{BaNiO}}_5$ (orthorhombic, $Immm$ centrosymmetric space group), exhibiting Néel order below $\left(T_N=\right)63\mathrm{K}$, to understand multiferroic behavior below 25 K. The magnetic structure is made up of Ni and Tb magnetic moments, which are found to be mutually canted in the entire temperature range below $T_N$, though collinearity is seen within each sublattice, as known in the past. First-principles density functional theory calculations ($\mathrm{GGA}+\mathrm{SO}$ and $\mathrm{GGA}+\mathrm{U}+\mathrm{SO}$ approximations) support such a canted ground state. The intriguing finding, being reported here, is that there is a sudden increase in this Tb-Ni relative canting angle at the temperature (that is, at 25 K) at which spontaneous electric polarization sets in, with bond distance and bond angle anomalies. This finding emphasizes the need for a new spin-driven polarization mechanism—that is, a critical canting angle coupled with exchange striction—to induce multiferroicity in magnetic insulators with canted spins.

We report on the measurement of the intrinsic higher harmonic current-phase relation for epitaxial NbN/NiCu/NbN (SFS) Josephson junctions. We investigated the $\pi \text{−}0$ phase transition as a function of temperature for SFS Josephson junctions with a 3.2-nm-thick NiCu layer, and fitted the phase transition temperatures $T_{\pi \text{−}0}$ with a theoretical expression derived from the Usadel equations. We found that junctions on the same chip and with the same dimension showed different phase transition temperatures depending on the values of the fitting parameter $\alpha$. The fitting parameter $\alpha$ indicates the magnetic scattering intensity, which mainly derives from spin-flip or spin-flop scattering in the NiCu layer. At the $\pi \text{−}0$ crossover, the intrinsic positive second harmonic current-phase relation was confirmed in a junction with moderate magnetic scattering by the nonvanishing critical current, half-integer Shapiro steps, and half-periodic Fraunhofer modulation.

We study the quench dynamics on cross-stitch flat band networks by a sudden change of the intercell hopping strength $J$. For quench processes with $J$ changing as $J=0\to J\ne 0$, we give the analytical expression to the Loschmidt echo which possesses a series of zero points at critical times $t^{*}$, indicating where the dynamical quantum phase transitions occur. We further study the converse quench process with $J\ne 0\to J=0$, and find a nontrivial example that the prequench quantum state is not an eigenstate of the post-quench Hamiltonian, whereas the Loschmidt echo $\mathcal{L}\left(t\right)\equiv 1$ during this process. For both situations, these results are also illustrated numerically. Finally, we give a brief discussion on the possible experimental observation of these predictions in the system of ultracold atoms in optical lattices.

We describe a quench protocol that allows the rapid preparation of ground states of arbitrary interacting conformal field theories in $1+1$ dimensions. We start from the ground state of a related gapped relativistic quantum field theory and consider sudden quenches along the spacelike trajectories $t^2-x^2=T_0^2$ (parameterized by $T_0$) to a conformal field theory. Using only arguments of symmetry and conformal invariance, we show that the post-quench stress-energy tensor of the conformal field theory is uniquely constrained up to an overall scaling factor. Crucially, the geometry of the quench necessitates that the system approach the vacuum energy density over all space except the singular lines $x=\pm t$. The above arguments are verified using an exact treatment of the quench for the Gaussian scalar field theory (equivalently the Luttinger liquid), and numerically for the quantum $O\left(N\right)$ model in the large-$N$ limit. Additionally, for the Gaussian theory, we find in fact that even when starting from certain excited states, the quench conserves entropy, and is thus also suitable for rapidly preparing excited states. Our methods serve as a fast, alternative route to reservoir-based cooling to prepare quantum states of interest.

We report preferential excitation and detection of the precessional spin dynamics of individual nanomagnets via magnetoelastic (MEL) resonance excitation. Surface acoustic waves (SAWs) are generated via the ultrafast optical excitation of a nonmagnetic grating (Al bars) with lithographically defined acoustic eigenmodes. We show that the precessional spin dynamics in two identical, elliptical nanomagnets with orthogonal orientations can be selectively excited by the SAWs via control of the applied magnetic field. Furthermore, we observe that both the amplitude and damping of the magnetoelastically induced precession depend on the relative orientation of the SAW with respect to the nanomagnets. Using magnetomechanical simulations, we show that the acoustic excitation is most efficient when the spatial distributions of the natural and SAW-driven magnetic resonances overlap. These findings reveal a direct connection between the geometry and MEL excitation efficiency and have implications for the rational design of nanoscale magnetomechanical devices.

Ferromagnets, when out of equilibrium, have two sets of transverse variables: the tipped magnetization $d\vec{M}$, arising from the changed eigenstate, and the transverse component of the spin accumulation $\vec{m}$, arising from the nonequilibrium spin distribution function (where $\vec{m}=\vec{0}$ in equilibrium). For conductors, $\vec{m}$ is due a nonequilibrium distribution of electrons near the Fermi surface; for insulators, $\vec{m}$ is due to a nonequilibrium distribution of magnons. Applying irreversible thermodynamics to these four transverse degrees of freedom gives, in the small-amplitude limit, four static transverse surface modes (evanescent modes) of $d\vec{M}$ and $\vec{m}$, which couple both by uniform mean-field exchange $\lambda$ and by the recently noted mutual decay rate ${\tau }_{mM}^{-1}$. As prototype ferromagnets we consider static, pairwise degenerate modes for the insulator yttrium iron garnet (YIG) and the conductor permalloy (Py). YIG has much longer longitudinal and transverse decay lengths than Py. Numerical studies are made of the eigenmodes (ratio of $d\vec{M}$ to $\vec{m}$) and of the real and imaginary parts of the two eigenwave vectors; they reveal much about the dependence of the modes on field $H$, $\lambda$, ${\chi }_f$ (transverse spin accumulation susceptibility), ${\chi }_{\perp }$ (transverse magnetization susceptibility), and ${\tau }_{mM}$. For example, when ${\chi }_f$ is very small, as expected at low temperatures, both transverse modes are dominated by $d\vec{M}$. The transverse decay lengths typically are much shorter than the longitudinal (spin-flip) decay length, largely because they are subject to exchange-field precession. Thus, ultrathin samples can give a full transverse response (such as transverse “magnon-mediated” charge current “drag”), but thick samples can give neither longitudinal nor transverse response. With amplitudes determined by the boundary conditions on $\vec{m}$ and $\vec{M}$, these modes permit the interior of the ferromagnet to respond in a spatially varying fashion. The six transverse boundary conditions at an N/F interface are briefly discussed. For small enough sample thicknesses, relative to the transverse decay lengths, there should be a transverse “magnon drag” phenomenon, both for conductors and insulators.

We present a comprehensive study on the magnetic structure, dynamics, and phase evolution in the single phase double perovskite ${\mathrm{La}}_2{\mathrm{CoMnO}}_6$. The mixed valence state due to oxygen deficiency is verified by x-ray photoelectron spectroscopy and confirms a double ferromagnetic transition observed in dc magnetization. Neutron diffraction reveals that the magnetic structure is dominated by long-range ferromagnetic ordering, which is further corroborated by a critical exponents analysis of the paramagnetic to ferromagnetic phase transition. An analysis of the magnetization dynamics by means of linear and nonlinear ac magnetic susceptibilities marks the presence of two distinct cluster-glass-like states that emerge at low temperatures. The isothermal entropy change as a function of temperature and magnetic field ($H$) is exploited to investigate the mechanism of stabilization of the magnetic phases across the $H-T$ phase diagram. In the regime of the phase diagram where thermal energy is sufficiently low, regions of competing interactions due to local disorder become stabilized and display glasslike dynamics. The freezing mechanism of clusters is illustrated using a unique probe of transverse susceptibility that isolates the effects of the local anisotropy of the spin clusters. The results are summarized in a $H-T$ phase diagram of ${\mathrm{La}}_2{\mathrm{CoMnO}}_6$ from these data.

${\mathrm{CaBi}}_2$ is a recently discovered type-I superconductor with $T_c=2\mathrm{K}$ and a layered crystal structure. In this work electronic structure, lattice dynamics, and electron-phonon interaction are studied, with special attention paid to the influence of the spin-orbit coupling (SOC) on the above-mentioned quantities. We find that in the scalar-relativistic case (without SOC), electronic structure and electron-phonon interaction show quasi-two-dimensional character. Strong Fermi surface nesting is present, which leads to the appearance of the Kohn anomaly in the phonon spectrum and enhanced electron-phonon coupling for the phonons propagating in the Ca-Bi atomic layers. However, strong spin-orbit coupling in this material changes the topology of the Fermi surface and reduces the nesting, and the electron-phonon coupling becomes weaker and more isotropic. The electron-phonon coupling parameter $\lambda$ is reduced by SOC to almost half, from 0.94 to 0.54, giving an even stronger effect on the superconducting critical temperature $T_c$, which drops from 5.2 K (without SOC) to 1.3 K (with SOC). Relativistic values of $\lambda$ and $T_c$ remain in good agreement with experimental findings, confirming the general need for including SOC in the analysis of the electron-phonon interaction in materials containing heavy elements.

We reexamine the nature of the ground states of bilayer graphene at odd integer filling factors within a simplified model of nearly degenerate $n=0$ and $n=1$ Landau levels. Previous Hartree-Fock studies have found that ferroelectric states with orbital coherence can be stabilized by tuning the orbital splitting between these levels. These studies indicated that, in addition to a uniform ferroelectric state, a helical ferroelectric phase with spontaneously broken translational symmetry is possible. By performing exact diagonalization on the torus, we argue that the system does not have a uniform coherent state but instead transitions directly from the uniform incoherent state into the ferroelectric helical phase. We argue that there is a realistic prospect to stabilize the helical ferroelectric state in bilayer graphene by tuning the interlayer electric field in a model that includes all single-particle corrections to its zero energy eightfold multiplet of Landau levels.

We study the spectrum and statistical properties of photons scattered from a semiconductor-quantum-dot–metal-nanoparticle system under monochromatic and bichromatic excitations. We rely on the Bloch equation to describe the evolution of the density matrix of the quantum dot. We pay attention to the self-interaction of the quantum dot in the presence of the nanoparticle. Going beyond the dipole approximation, we show that the system exhibits optical responses of different character in different regions of the quantum dot dipole moment versus the nanoparticle radius phase diagram. In the strong transition and bistability regions, upon changing the initial state, a pronounced fluorescence spectrum may become a faint one, and an oscillatory intensity-intensity correlation may become a monotonically increasing one. The amplitudes, frequencies, and phases of the laser fields tailor the number, position, height, and width of the peaks of the fluorescence spectrum. The antibunched light as well as the sub-Poissonian light can be generated. Our results suggest that in view of solid-state-based sources of nonclassical light, a hybrid quantum-dot–nanoparticle system may be superior to an isolated quantum dot.

A recent experiment shows that the $K^{\prime }K$ valley degeneracy can be lifted in monolayer ${\mathrm{WSe}}_2$ deposited on a layered ferromagnetic substrate of ${\mathrm{CrI}}_3$. In this work, we take a van der Waals heterostructure ${\mathrm{WSe}}_2/{\mathrm{CrI}}_3$ to model the monolayer ${\mathrm{WSe}}_2$ on the ${\mathrm{CrI}}_3$ substrate and investigate the effects underpinning the $K^{\prime }K$ valley splitting based on first-principles calculations. We demonstrate that the interfacial atom superposition plays an important role and a W-Cr superposition is essential for a relatively large valley splitting. The results indicate inevitable sample-to-sample variations in the $K^{\prime }K$ valley splitting in the ${\mathrm{WSe}}_2/{\mathrm{CrI}}_3$. Furthermore, we show that the $K^{\prime }K$ valley splitting can be tuned in the trilayer ${\mathrm{CrI}}_3/{\mathrm{WSe}}_2/{\mathrm{CrI}}_3$ from nearly zero to more than two times of that in the bilayer ${\mathrm{WSe}}_2/{\mathrm{CrI}}_3$ by manipulating the layer alignment.

Inducing a robust long-range magnetic order in diamagnetic graphene remains a challenge. While nitrogen-doped graphene is reported to be a promising candidate, the corresponding exchange mechanism remains unclear and is essential to tune further and manipulate magnetism. Within the first-principles calculations, we systematically investigate the local moment formation and the concurrent interaction between various nitrogen-containing defect complexes. The importance of adatom diffusion on the differential defect abundance is discussed. The itinerant magnetism in the graphitic N defects is found to be fundamentally different from the more localized moments in the vacancy-containing N complexes. The magnetic interaction between the complexes is found to depend on the concentration, complex type, sublattice, separation, and orientation. We propose that the direct exchange mechanism between the delocalized magnetic moment originating from the itinerant $\pi$ electron at the prevalent graphitic complexes to be responsible for the observed ferromagnetism. We show that B codoping further improves ferromagnetism. Present results will assist in the microscopic understanding of the current experimental results and motivate new experiments to produce robust magnetism following the proposed synthesis strategy.

Theoretical progress in graphene physics has largely relied on the application of a simple nearest-neighbor tight-binding model capable of predicting many of the electronic properties of this material. However, important features that include electron-hole asymmetry and the detailed electronic bands of basic graphene nanostructures (e.g., nanoribbons with different edge terminations) are beyond the capability of such a simple model. Here we show that a similarly simple plane-wave solution for the one-electron states of an atom-based two-dimensional potential landscape, defined by a single fitting parameter (the scattering potential), performs better than the standard tight-binding model, and levels to density-functional theory in correctly reproducing the detailed $\pi$-band structure of a variety of graphene nanostructures. In particular, our approach identifies the three hierarchies of nonmetallic armchair nanoribbons, as well as the doubly-degenerate flat bands of free-standing zigzag nanoribbons with their energy splitting produced by symmetry breaking. The present simple plane-wave approach holds great potential for gaining insight into the electronic states and electro-optical properties of graphene nanostructures and other two-dimensional materials with intact or gapped Dirac-like dispersions.

The motion of doped electrons or holes in an antiferromagnetic lattice with strong on-site Coulomb interactions touches one of the most fundamental open problems in contemporary condensed matter physics. The doped charge may strongly couple to elementary spin excitations, resulting in a dressed quasiparticle which is subject to confinement. This “spin polaron” possesses internal degrees of freedom with a characteristic “ladder” excitation spectrum. Despite its fundamental importance for understanding high-temperature superconductivity, clear experimental spectroscopic signatures of these internal degrees of freedom are scarce. Here, we present scanning tunneling spectroscopy results of the spin-orbit-induced Mott insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. Our spectroscopy data reveal distinct shoulder-like features for occupied and unoccupied states beyond a measured Mott gap of $\mathrm{\Delta }\approx 620$ meV. Using the self-consistent Born approximation we assign the anomalies in the unoccupied states to the spin-polaron ladder spectrum with excellent quantitative agreement and estimate the Coulomb repulsion $U=2.05...2.28$ eV in this material. These results confirm the strongly correlated electronic structure of this compound and underpin the previously conjectured paradigm of emergent unconventional superconductivity in doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$.

We present a comprehensive angle-resolved photoemission spectroscopy study of ${\mathrm{Ca}}_{1.8}{\mathrm{Sr}}_{0.2}{\mathrm{RuO}}_4$. Four distinct bands are revealed and along the Ru-O bond direction their orbital characters are identified through a light polarization analysis and comparison to dynamical mean-field theory calculations. Bands assigned to $d_{xz},d_{yz}$ orbitals display Fermi liquid behavior with fourfold quasiparticle mass renormalization. Extremely heavy fermions—associated with a predominantly $d_{xy}$ band character—are shown to display non-Fermi-liquid behavior. We thus demonstrate that ${\mathrm{Ca}}_{1.8}{\mathrm{Sr}}_{0.2}{\mathrm{RuO}}_4$ is a hybrid metal with an orbitally selective Fermi liquid quasiparticle breakdown.

We show that any interacting integrable model possesses a class of initial states for which the leading corrections to ballistic transport are subdiffusive rather than diffusive. These initial states are natural to realize experimentally and include the domain-wall initial condition that has been the object of much recent scrutiny. Upon performing numerical matrix product state simulations in the spin-$1/2$ XXZ chain, we find that such states can exhibit subdiffusive $t^{1/3}$ scaling of fronts of spin, energy, and entanglement entropy across the entire range of anisotropies. This demonstrates that Tracy-Widom scaling is not incompatible with model interactions, as was previously believed.