Based on the phenomenological Ginzburg-Landau approach, we investigate phase solitonic states in a class of three-component superconductors for mesoscopic doubly connected geometry (thin-walled cylinder) in external magnetic fields. Analysis of the Gibbs free energy of the system shows that solitonic states in a three-component superconductor are thermodynamically metastable and separated from the ground state by a sizable energy. Our results demonstrate that despite the presence of a zoo of phase solitons states the earlier proposed method [Phys. Rev. B 96, 144513 (2017)] for the detection of BTRS (broken time-reversal symmetry) in multiband superconductors remains valid and useful.

We theoretically investigate the phase and voltage correlation dynamics, which includes both the deterministic contribution and stochastic fluctuations, under a current noise generated by a resistor including thermal and quantum fluctuations in a resistively and capacitively shunted Josephson junction. An external current is found to shift and intensify the deterministic contributions in phase and voltage. In addition to effects of external current, we observe the relaxation of autocorrelation functions of phase and voltage, which includes the variances due to the current noise, to finite values in the long-time limit. In particular, we find that the asymptotic correlations depend on the resistance as a consequence of quantum effects. We also find an earlier decay of coherence at a higher temperature in which thermal fluctuations dominate over quantum ones. These theoretical predictions can be tested in the next future experiments.

Many metallic transition-metal oxides turn insulating when grown as films that are only a few unit cells thick. The microscopic origins of these thickness-induced metal-to-insulator transitions, however, remain in dispute. Here, we simulate the extreme case of a monolayer of an inconspicuous correlated metal—the strontium vanadate $\mathrm{Sr}{\mathrm{VO}}_3$—deposited on a ${\mathrm{SrTiO}}_3$ substrate. Crucially, our system can have a termination to vacuum consisting of either a SrO or a ${\mathrm{VO}}_2$ top layer. While we find that both lead to Mott insulating behavior at nominal stoichiometry, the phase diagram emerging upon doping—chemically or through an applied gate voltage—is qualitatively different. Indeed, our many-body calculations reveal a cornucopia of nonlocal fluctuations associated with (in)commensurate antiferromagnetic, ferromagnetic, and stripe and checkerboard orbital ordering instabilities. Identifying that the two geometries yield crystal-field splittings of opposite signs, we elucidate the ensuing phases through the lens of the orbital degrees of freedom. Quite generally, our work highlights that interface and surface reconstruction and the deformation or severing of coordination polyhedra in ultrathin films drive rich properties that are radically different from the material's bulk physics.

Motivated by thermal conductivity experiments in spin chain compounds, we propose a phenomenological model to account for a ballistic magnetic transport coupled to a diffusive phononic one, along the line of the seminal two-temperature diffusive transport Sanders-Walton model. Although the expression for the effective thermal conductivity is identical to that of Sanders-Walton, the interpretation is entirely different, as the “magnetic conductivity” is replaced by an “effective transfer conductivity” between the magnetic and phononic component. This model also reveals the fascinating possibility of visualizing the ballistic character of magnetic transport, for appropriately chosen material parameters, as a two-peak counter-propagating feature in the phononic temperature. It is also appropriate for the analysis of any thermal transport experiment involving a diffusive component coupled to a ballistic one.

We study an effective one-dimensional quantum model that includes friction and spin-orbit coupling (SOC), and show that the model exhibits spin polarization when both terms are finite. Most important, strong spin polarization can be observed even for moderate SOC, provided that the friction is strong. Our findings might help to explain the pronounced effect of chirality on spin distribution and transport in chiral molecules. In particular, our model implies static magnetic properties of a chiral molecule, which lead to Shiba-like states when a molecule is placed on a superconductor, in accordance with recent experimental data.

Electronic properties of FeSe can be tuned by various routes. Here, we present a comprehensive study on the evolution of the superconductivity and nematicity in FeSe with thickness from bulk single crystal down to bilayer $(\sim 1.1 \mathrm{nm})$ through exfoliation. With decreasing flake thickness, both the structural transition temperature $T_{\mathrm{s}}$ and the superconducting transition temperature $T_{\mathrm{c}}^{\mathrm{zero}}$ are greatly suppressed. The magnetic field $\left(B\right)$ dependence of Hall resistance $R_{xy}$ at 15 K changes from $B$-nonlinear to $B$-linear behavior up to 9 T, as the thickness $\left(d\right)$ is reduced to 13 nm. $T_{\mathrm{c}}$ is linearly dependent on the inverse of flake thickness $(1/d)$ when $d\le$ 13 nm, and a clear drop of $T_{\mathrm{c}}$ appears with thickness smaller than 27 nm. The $I\text{−}V$ characteristic curves in ultrathin flakes reveal the signature of Berezinskii-Kosterlitz-Thouless (BKT) transition, indicating the presence of two-dimensional superconductivity. Anisotropic magnetoresistance measurements further support 2D superconductivity in few-layer FeSe. Increase of disorder scattering, anisotropic strains, and dimensionality effect with reducing the thickness of FeSe flakes might be taken into account for understanding these behaviors. Our study provides systematic insights into the evolution of the superconducting properties, structural transition, and Hall resistance of a superconductor FeSe with flakes thickness and provides an effective way to find two-dimensional superconductivity as well as other 2D novel phenomena.

The recent discovery of superconductivity in Sr-doped ${\mathrm{NdNiO}}_2$, with a critical temperature of 10–15 K, suggests the possibility of a new family of nickel-based superconductors. ${\mathrm{NdNiO}}_2$ is the $n=\infty$ member of a larger series of layered nickelates with chemical formula ${\mathrm{R}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{2n+2}$ ($\mathrm{R}=\mathrm{La}$, Nd, Pr; $n=2,3,\cdots ,\infty$). The $n=3$ member has been experimentally and theoretically shown to be cupratelike and a promising candidate for superconductivity if electron doping could be achieved. The higher-order $n=4,5,$ and 6 members of the series fall directly into the cuprate dome area of filling without the need of doping, thus making them promising materials to study, but have not been synthesized yet. Here, we perform first-principles calculations on hypothetical $n=4,5,$ and 6 structures to study their electronic and magnetic properties and compare them with the known $n=\infty$ and $n=3$ materials. From our calculations, we find that the cupratelike character of layered nickelates increases from the $n=\infty$ to the $n=3$ members as the charge transfer energy and the self-doping effect due to R-$d$ bands around the Fermi level gradually decrease.

The correlation between electrons in different quantum wires is expected to affect the electronic properties of quantum electron-electron biwire systems. Here, we use the variational Monte Carlo method to study the ground-state properties of parallel, infinitely thin electron-electron biwires for several electron densities ($r_{\text{s}}$) and interwire separations ($d$). Specifically, the ground-state energy, the correlation energy, the interaction energy, the pair-correlation function (PCF), the static structure factor (SSF), and the momentum distribution (MD) function are calculated. We find that the interaction energy increases as $ln\left(d\right)$ for $d\to 0$ and it decreases as $d^{-2}$ when $d\to \infty$. The PCF shows oscillatory behavior at all densities considered here. As two parallel wires approach each other, interwire correlations increase while intrawire correlations decrease as evidenced by the behavior of the PCF, SSF, and MD. The system evolves from two monowires of density parameter $r_{\text{s}}$ to a single monowire of density parameter $r_{\text{s}}/2$ as $d$ is reduced from infinity to zero. The MD reveals Tomonaga-Luttinger (TL) liquid behavior with a power-law nature near $k_{\text{F}}$ even in the presence of an extra interwire interaction between the electrons in biwire systems. It is observed that when $d$ is reduced the MD decreases for $k<k_{\text{F}}$ and increases for $k>k_{\text{F}}$, similar to its behavior with increasing $r_{\text{s}}$. The TL liquid exponent is extracted by fitting the MD data near $k_{\text{F}}$, from which the TL liquid interaction parameter $K_{\rho }$ is calculated. The value of the TL parameter is found to be in agreement with that of a single wire for large separation between the two wires.

A macroscopic description of thermoelectric phenomena involves several tensorial transport coefficients. Textbook microscopic Kubo formulas for them are plagued with ambiguities in the definitions of the current operators and the magnetization. We derive a version of these formulas for lattice systems that is free from ambiguities but contains additional terms compared to the textbook results. For symmetric components of thermoelectric tensors, we identify a large class of lattice systems for which the additional terms vanish with a natural choice of the energy current. To eliminate ambiguities in the skew-symmetric components, one needs to interpret them as relative quantities: only their differences for pairs of materials are well-defined.

A combination of density functional theory (DFT) with experimental methods was used to study the electronic and crystal structure of ${\text{Sm}}_3{\text{Fe}}_5{\text{O}}_{12}$ (SmIG), which was synthesized using a modified sol-gel method. Computational studies were performed within the generalized gradient approximation (GGA), with and without the Hubbard-$U$ correction ($\mathrm{DFT}+U$), to analyze the influence of the on-site repulsion on the band structure and the density of states (DOS) of SmIG, as well as the structural parameters. The calculations were contrasted with experimental results from x-ray diffraction (XRD) and UV-Vis spectra. A Rietveld refinement returned a lattice parameter of 12.5231(3) Å. Synthesis methods seem to have a substantial effect in the band gap of SmIG, as our experimental value of 2.26–2.27 eV differs from the 2.02 eV value previously reported for samples prepared using the traditional solid-state method, despite similar lattice parameters. The DFT-calculated lattice parameters were within 1% of the experimental value. Analytically calculated effective Hubbard-$U$ values were 4.3092 eV for tetrahedral iron, and 6.0926 eV for octahedral iron. A model is proposed to calculate the band gap in ${\text{Sm}}_3{\text{Fe}}_5{\text{O}}_{12}$, taking into account the structure's ferrimagnetism and energy level distribution. A direct transition between minority spin states was found, resulting in a calculated band gap of 2.27 eV, close to the aforementioned value from sol-gel synthesis.

Electron- and hole-doped ${\mathrm{La}}_2{\mathrm{CoMnO}}_6$(LCMO) are investigated using first-principles DFT calculations. Hole and electron doping are achieved, respectively, by introducing ${\mathrm{Sr}}^{2+}$ at ${\mathrm{La}}^{3+}$ sites and by inducing O-site vacancies in LCMO. Electronic structure calculations suggest that hole doping alters the charge and valence state of Co ions, whereas electron doping influences the Mn ions. Introduction of defects is found to enhance antisite disorder (ASD) at Co/Mn site, which is expected to influence the magneto-dielectric properties of the system. Our calculations suggest that while ASD and/or hole doping induces half-metallicity in LCMO, electron doping restores its insulating state. Mean-field calculations performed using exchange constants obtained by mapping DFT total energies of different collinear spin configurations onto the Ising Hamiltonian find that defects tend to reduce the Curie temperature $\left(T_C\right)$. Interestingly, the calculated linear optical properties of the system suggest that the material becomes optically active with high values of birefringence in the presence of defects, a property that is highly sought-after in the optical communications and laser industry.

The negatively charged silicon vacancy (${\mathrm{V}}_{\text{Si}}^{-}$) in silicon carbide (SiC) is a paramagnetic and optically active defect in hexagonal SiC. The ${\mathrm{V}}_{\text{Si}}^{-}$ defect possesses $S=3/2$ spin with long spin coherence time, and it can be optically manipulated even at room temperature. Recently, electron spin resonance signals have been observed besides those associated with the ${\mathrm{V}}_{\text{Si}}^{-}$ defects in the $4H$ polytype of SiC. The corresponding centers share properties akin to those of the ${\mathrm{V}}_{\text{Si}}^{-}$ defects and thus they may be promising candidates for quantum technology applications. However, the exact origin of the new signals is unknown. In this paper, we report ${\mathrm{V}}_{\text{Si}}^{-}$-related pair defect models as possible candidates for the unknown centers. We determine the corresponding electronic structures and magneto-optical properties as obtained by density functional theory calculations. We propose models for the recently observed electron paramagnetic resonance centers, and we predict their optical signals for identification in future experiments.

We investigate the electronic properties in the Bloch electron on a two-dimensional lattice with vacancies in the uniform magnetic field. We show that a single vacancy site introduced to the system creates a defect energy level in every single innumerable fractal energy gap in the Hofstadter butterfly. The wave functions of different defect levels have all different localization lengths depending on their fractal generations, and they can be described by a single universal function after an appropriate fractal scaling. We also show that each defect state has its own characteristic orbital magnetic moment, which is exactly correlated to the gradient of the energy level in the Hofstadter diagram. Probing the spatial nature of the defect-localized states provides a powerful way to elucidate the fractal nature of the Hofstadter butterfly.

We calculate the electronic structure in quasiperiodic double moiré systems of graphene sandwiched by hexagonal boron nitride (hBN) and identify the characteristic integers of energy gaps. We find that the electronic spectrum contains a number of minigaps, and they exhibit a recursive fractal structure similar to the Hofstadter butterfly when plotted against the twist angle. Each of the energy gaps can be characterized by a set of integers, which are associated with an area in momentum space. The corresponding area is geometrically interpreted as a quasi-Brillouin zone, which is a polygon enclosed by multiple Bragg planes of the composite periods and can be uniquely specified by the plain wave projection in the weak-potential limit.

Using first-principles modeling we investigate how phonon transport evolves in layered/van der Waals materials when going from 3D to 2D, or vice versa, by gradually pulling apart the atomic layers in graphite to form graphene. Focus is placed on identifying the features impacting thermal conductivity that are likely shared with other layered materials. The thermal conductivity $\kappa$ of graphite is found to be lower than that of graphene in large part due to changes in the phonon dispersion driven by van der Waals coupling. Specifically, as the atomic layers are brought closer together, the acoustic flexural phonons in graphene form low-energy optical flexural phonons in graphite that possess lower in-plane velocities, density of states, and phonon occupation, thus reducing $\kappa$. Similar dispersion changes, and impact on thermal conductivity, can be expected in other van der Waals materials when transitioning from 2D to 3D. Our findings also indicate that the selection rules in graphene, which reduce phonon-phonon scattering and contribute to its large $\kappa$, effectively hold as the atomic layers are brought together to form graphite. While the selection rules do not strictly apply to graphite, in practice reduced scattering is observed due in part to the weak interlayer coupling. This suggests that some bulk van der Waals materials may have lower phonon-phonon scattering rates than other nonlayered bulk materials.

Spin-momentum locking arising from strong spin-orbit coupling is one of the key natures of topological materials. Since charge can induce a spin polarization due to spin-momentum locking, the search for materials that exhibit this feature has become one of the top priorities in the field of spintronics. In this paper, we report the electrical detection of the spin-transport properties of ${\mathrm{ZrTe}}_5$ nanowires, using a nonlocal geometry measurement. A clear hysteresis voltage signal, which depends on the relative orientations between the magnetization of the ferromagnetic electrodes and the carrier spin polarization, has been observed. The hysteresis voltage states can be reversed by altering the electron movement direction, providing direct evidence of the spin-momentum locking feature of ${\mathrm{ZrTe}}_5$ nanowires and revealing its topological nature. Furthermore, the current-dependent measurement suggests that the charge (spin) current is induced by thermal effect, which utilizes the thermoelectric properties of ${\mathrm{ZrTe}}_5$. Using the thermal effect to control the spin-polarized current protected by spin-momentum locking offers possibilities for small-sized devices based on the topological materials.

Two-dimensional (2D) ferrovalley materials, displaying coexistence of spontaneous spin and valley polarizations, have recently attracted significant attention due to their potential for applications in the fields of spintronics and valleytronics. However, unfortunately, to date only a few 2D ferrovalley materials exist. Here, first-principles calculations predict a 2D Janus ferrovalley material lanthanum bromiodide (LaBrI). It is found that LaBrI is a stable ferromagnetic electride, whose magnetic moment originates mainly from the interstitial anionic electrons. The magnetic transition temperature of LaBrI monolayers is estimated to be far beyond room temperature, and a sizable magnetic anisotropy with easy in-plane magnetization is also present. Most importantly, LaBrI monolayers exhibit a large valley polarization due to the concurrent broken space- and time-reversal symmetries, with the calculated valley polarization reaching up to 59 meV. This value is comparable to that of the ferrovalley materials reported to date. Intriguingly, the inequivalent Berry curvature at the two valleys takes opposite values, giving rise to an anomalous valley Hall effect, where a valley current with an accompanying net charge Hall current may be induced.

Magnons and phonons are two fundamental neutral excitations of magnetically ordered materials which can significantly dominate the low-energy thermal properties. In this work we study the interplay of magnons and phonons in honeycomb and kagome lattices. When the mirror reflection with respect to the magnetic ordering direction is broken, the symmetry-allowed in-plane Dzyaloshinskii-Moriya (DM) interaction will couple the magnons to the phonons and the magnon-polaron states are formed. Besides, both lattice structures also allow for an out-of-plane DM interaction rendering the uncoupled magnons to be topological. Our aim is to study the interplay of such topological magnons with phonons. We show that the hybridization between magnons and phonons can significantly redistribute the Berry curvature among the bands. Especially, we found that the topological magnon band becomes trivial while the hybridized states at lower energy acquire Berry curvature strongly peaked near the avoided crossings. As such the thermal Hall conductivity of topological magnons shows significant changes due to coupling to the phonons.

We develop a microscopic theory for plasmon excitations of $\text{graphene}/\alpha \text{−}\mathrm{Ru}{\mathrm{Cl}}_3$ heterostructures. Within a Kondo-Kitaev model with various interactions, a heavy Fermi liquid hosting flat bands emerges in which the itinerant electrons of graphene effectively hybridize with the fractionalized fermions of the Kitaev quantum spin liquid. We find novel correlated plasmon bands induced by the interplay of flat bands and interactions and argue that our theory is consistent with the available experimental data on $\text{graphene}/\alpha \text{−}\mathrm{Ru}{\mathrm{Cl}}_3$ heterostructures. We predict novel plasmon branches beyond the long-wavelength limit and discuss the implications for probing correlation phenomena in other flat band systems.

Employing a large-$N$ scheme of the layered $t-J$ model with the long-range Coulomb interaction, which captures the fine details of the charge excitation spectra recently observed in cuprate superconductors, we explore the role of charge fluctuations on the electron self-energy. We fix the temperature at zero and focus on quantum charge fluctuations. We find a pronounced asymmetry of the imaginary part of the self-energy $\mathrm{Im}\mathrm{\Sigma }(\mathbf{k},\omega )$ with respect to $\omega =0$, which is driven by strong electron correlation effects. The quasiparticle weight is reduced dramatically, which occurs almost isotropically along the Fermi surface. Concomitantly, an incoherent band and a sharp side band are generated and acquire sizable spectral weight. All these features are driven by the usual on-site charge fluctuations, which are realized in a rather high-energy region and yield plasmon excitations. On the other hand, the low-energy region with the scale of the superexchange interaction $J$ is dominated by bond-charge fluctuations. Surprisingly, compared with the effect of on-site charge fluctuations, their effect on the electron self-energy is much weaker, even if the system approaches close to bond-charge instabilities. Furthermore, quantum charge dynamics does not produce a clear kink nor a pseudogap in the electron dispersion.