Quantum-mechanical calculations of electron magnetotransport in ideal graphene nanoribbons are presented. In noninteracting theory, it is predicted that an ideal ribbon that is attached to wide leads should reveal Fabry-Pérot conductance oscillations in magnetic field. In the theory with Coulomb interaction taken into account, the oscillation pattern should rather be determined by the Aharonov-Bohm interference effect. Both of these theories predict the formation of quasi-bound states, albeit of different structures, inside the ribbon because of strong electron scattering on the interfaces between the connecting ribbon and the leads. Conductance oscillations are a result of resonant scattering via these quasibound states.

The topological nontriviality of insulating phases of matter are by now well understood through topological K theory where the indices of the Dirac operators are assembled into topological classes. We consider in the context of the Kitaev chain a notion of a generalized Dirac operator where the associated Clifford algebra is centrally extended. We demonstrate that the central extension is achieved via taking rational operator powers of Pauli matrices that appear in the corresponding BdG Hamiltonian. Doing so introduces a pseudometallic component to the topological phase diagram within which the winding number is valued in $\mathbb{Q}$. We find that this phase hosts a mode that remains extended in the presence of weak disorder, motivating a topological interpretation of a nonintegral winding number. We remark that this is in correspondence with recent paper demonstrating that projective Dirac operators defined in the absence of ${\mathrm{spin}}^{\mathbb{C}}$ structure have rational indices.

The electronic structure of periodic lattice distortion (PLD) in ${\mathrm{NbTe}}_2$ was examined using low-temperature scanning tunneling spectroscopy and microscopy. The striped PLDs with $3\times 1$ and $1\times \frac{9}{2}$ superstructures were characterized in real and reciprocal space. The simultaneous formation of momentum-specific suppressions in the spectral weight and phase shifts of the wavefront related to the superstructures were observed at multiple energies. These unusual energy dependencies are well agreed with the charge density waves (CDWs) formed in the multiband electronic structure of ${\mathrm{NbTe}}_2$. Fermi-surface nesting and reconstruction of the reciprocal lattice were suggested in developing the multiband CDWs in ${\mathrm{NbTe}}_2$.

We study theoretically the temperature dynamics at the metal-dielectric interface excited by a strong pump surface plasmon polariton pulse propagating along it. We demonstrate that the inclusion of the interband transitions leads to much stronger temperature-induced modulation of the imaginary part of the dielectric constant induced by the pump. We show that in the visible part of the spectrum a strong pump field can even lead to the decrease in the extinction coefficient for the co- or contrapropagating probe wave.

Search for new topological quantum materials is the demand of time and the theoretical prediction plays a crucial role besides the obvious experimental verification. Divination of topological properties in already well-known narrow gap semiconductors is a flourishing area in quantum material. In this view we revisited the semiconductor compound in the chalcopyrite series, with a very small gap near the Fermi energy. Using the density functional theory-based first-principles calculations, we report a strong topologically nontrivial phase in chalcopyrite ${\mathrm{ZnGeSb}}_2$, which can act as a model system of strained HgTe. The calculations reveal the nonzero topological invariant ($Z_2$), the presence of Dirac cone crossing in the surface spectral functions with spin-momentum locked spin texture. We also study the interplay between the structural parameters and electronic properties, and report the tunable topological properties due to a very small band gap, from nontrivial to trivial phase under the application of moderate hydrostatic pressure within $\approx 7$ GPa. A small modification of a lattice parameter is enough to achieve this topological phase transition which is easily accomplished in an experimental laboratory. The calculations show that a discontinuity in the tetragonal distortion of noncentrosymmetric ${\mathrm{ZnGeSb}}_2$ plays a crucial role in driving this topological phase transition. Our results are further collaborated with a low energy $\mathbf{k}·\mathbf{p}$ model Hamiltonian to validate our $\mathit{ab}\mathit{initio}$ findings. We showed that the evaluation of the model band energy dispersion under the hydrostatic pressure is consistent with the obtained results.

Twist angle modifies the band alignment, screening, and interlayer (IL) coupling in twisted bilayers (tBLs) of transition metal dichalcogenides. Intermediate misorientation (twist angles $>$ ${15}^{\circ }$) bilayers (BLs) offer a unique opportunity to tune excitonic behavior within these concurrent physical mechanisms but are seldom studied. In this paper, we measure many-body excitonic complexes in monolayer (ML), natural BL, and tBL ${\mathrm{WSe}}_2$. Neutral biexciton (XX) is observed in tBL, while being undetected in nonencapsulated ML and BL, demonstrating unique effects of disorder screening in tBLs. The XX as well as charged biexciton ($X{X}^{-}$) are robust to thermal dissociation and are controllable by electrostatic doping. Vanishing of momentum-indirect IL excitons with increasing electron doping is demonstrated in tBL, resulting from near alignment of $Q^{\prime }\text{−}K$ and $K\text{−}K$ valleys. Intermediate misorientation samples offer a high degree of control of excitonic complexes while offering possibilities of studying exciton-phonon coupling, band alignment, and screening.

Graphene+, a novel carbon monolayer with $s{p}^2\text{−}s{p}^3$ hybridization, was recently reported to exhibit unprecedented out-of-plane half-auxetic behavior and graphenelike Dirac properties [Yu et al., Cell Rep. Phys. Sci. 3, 100790 (2022)]. Herein, based on comprehensive state-of-the-art first-principles calculations, we reveal the significant effect of softened $s{p}^2\text{−}s{p}^3$ bonding on the lattice thermal transport in graphene+. At room temperature, the thermal conductivity (κ) of graphene+ is obtained as ∼170 W/mK, which is much lower than that of graphene (∼3170 W/mK). It is found that the softened $s{p}^2\text{−}s{p}^3$ bonding significantly suppresses the vibrations of acoustic phonons in graphene+, which leads to strong anharmonicity and weak phonon hydrodynamics. Thus, the large reduction in κ stems from the softened $s{p}^2\text{−}s{p}^3$ bonding network. Our study provides fundamental physical insights into the thermal transport properties of graphene+, which would provide prospective guidance for the promising application in the field of thermal management.

Respecting deformational constraints and predeformations poses a substantial challenge in the description of nonlinear elasticity. We here outline how group theory can play a beneficial role to overcome this challenge. Specifically, group theory guides us to generalized definitions of nonlinear shear deformation gradients and expressions of generalized elastic moduli in the nonlinear regime. Particularly, such achievements become important in the context of larger deformations under constraints and additional deformations on top of predeformations.

The existence of a net heat current of conductive thermal waves is demonstrated even in the absence of a mean temperature gradient. This effect, which we called heat shuttling, is generated by the temperature-dependent thermal conductivity of materials excited with a thermal excitation periodically modulated in time. We show that this modulation gives rise to a heat current superimposed on the one generated by the mean temperature gradient, which enhances the heat transport when the thermal conductivity increases with temperature. By contrast, if the thermal conductivity decreases as temperature increases, the thermal-wave heat current inverts its direction and reduces the total heat flux. The reported shutting effect is sensitive to the amplitude of the periodic thermal excitation, which can facilitate its observation and application to harvest energy from the temperature variations of the environment.

We study the Kitaev spin ladder with random couplings by using the real-space strong-disorder renormalization group (SDRG) technique. This model is the minimum model in Kitaev systems that has conserved plaquette fluxes, and its quasi-one-dimensional geometry makes it possible to study the strong-disorder fixed points for both spin- and flux-excitation gaps. In the Ising limit where the $x$-type Ising couplings compete with the $z$-type couplings and the $y$-type couplings are small but nonzero, the behavior of the spin gap is consistent with a random transverse-field Ising chain, but the flux gap is dominated by the $y$-type couplings. In the XX limit, while the $x$- and $y$-type couplings are locally equal and renormalized simultaneously, the $z$-type couplings are not renormalized drastically and lead to nonuniversal disorder criticality at low-energy scales. We show that different types of fractionalized degrees of freedom in the disordered Kitaev model can result in different critical behaviors, as long as its fluxes can survive the perturbative treatment.

Hund's pairing refers to Cooper pairing generated by onsite interactions that become attractive due to large Hund's exchange $J$. This is possible in multiorbital systems even when all local bare interactions are repulsive, since attractions in specific channels are given by certain linear combinations of interaction parameters. On the other hand, pairing processes such as the exchange of spin fluctuations are also present. We compare mean-field Hund's pairing and spin-fluctuation-mediated pairing using electronic bands appropriate for different classes of multiorbital systems over a wide range of interaction parameters. We find that, for systems without clear nesting features, the superconducting state generated by the Hund's mechanism agrees well with that from the full fluctuation exchange vertex when Hund's exchange and spin-orbit coupling are sufficiently large. On the other hand, for systems characterized by a peaked finite-momentum particle-hole susceptibility, spin-fluctuation pairing generally dominates over Hund's pairing. From this perspective Hund's pairing states appear unlikely to be realized in systems like ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and generic iron-based superconductors.

We consider a finite-size topological Josephson junction formed at the edge of a two-dimensional topological insulator in proximity to conventional superconductors and study the impact of Cooper pair symmetries on the electron transport. We find that, due to the finite junction size, electron transport is highly tunable by the superconducting phase difference $\varphi$ across the junction. At zero frequency and $\varphi =\pi$, the setup exhibits vanishing local Andreev reflection and perfect normal transmission due to the interplay of finite junction size and formation of topological Andreev bound states in the middle of the junction. We reveal that this striking behavior enables odd-frequency Cooper pairs to become the only type of pairing inside the topological junction that contribute to transport. Our paper thus offers a highly tunable detection scheme for odd-frequency Cooper pairs.

We develop a formula to evaluate the purity of a series of thermal equilibrium states that can be calculated in numerical experiments without knowing the exact form of the quantum state a priori. Canonical typicality guarantees that there are numerous microscopically different expressions of such states, which we call thermal mixed quantum (TMQ) states. Suppose that we construct a TMQ state by a mixture of $N_{\mathrm{samp}}$ independent pure states. The weight of each pure state is given by its norm, and the partition function is given by the average of the norms. To qualify how efficiently the mixture is done, we introduce a quantum statistical quantity called “normalized fluctuation of partition function (NFPF).” For smaller NFPF, the TMQ state is closer to the equally weighted mixture of pure states, which means higher efficiency, requiring a smaller $N_{\mathrm{samp}}$. The largest NFPF is realized in the Gibbs state with purity-0 and exponentially large $N_{\mathrm{samp}}$, while the smallest NFPF is given for thermal pure quantum state with purity-1 and $N_{\mathrm{samp}}=1$. The purity is formulated using solely the NFPF and roughly gives $N_{\mathrm{samp}}^{-1}$. Our analytical results are numerically tested and confirmed by the two random sampling methods built on matrix-product-state-based wave functions.

The dynamics of several systems in nature occurs under some constraints and symmetries that ensure the appearance of constants of motion. In this work, we discuss the dynamics of the magnetic domain wall (DW) under the Walker regime (i.e., when its position oscillates as a function of time) in bent cylindrical magnetic nanowires (NWs) with constant curvatures. It is shown that the DW position sweeps, in relation to the curvature center, the same area for different NW curvatures. This phenomenon appears due to an exchange-driven curvature-induced interaction. The translational DW motion is accompanied by its rotation around the NW axis, leading to a periodic curvature-independent angular momentum, from which one obtains an area's law for the DW motion.

We theoretically study the scattering of spin waves from a sharp domain wall (DW) in an antiferromagnetic spin chain. While the continuum model for an antiferromagnetic material yields the well-known result that spin waves can pass through a wide DW with no reflection, here we show that, based on the discrete spin Hamiltonian, spin waves are generally reflected by a DW with a reflection coefficient that increases as the DW width decreases. Remarkably, we find that, in the interesting case of an atomically sharp DW, the reflection of spin waves exhibits strong dependence on the state of circular polarization of the spin waves, leading to mainly reflection for one polarization while permitting partial transmission for the other, thus realizing an atomic-scale spin-wave polarizer. The polarization of the transmitted spin wave depends on the orientation of the spin in the sharp DW, which can be controlled by an external field or spin torque. Our utilization of a sharp antiferromagnetic DW as an atomic-scale spin-wave polarizer leads us to envision that ultrasmall magnetic solitons such as DWs and skyrmions may enable realizations of atomic-scale spin-wave scatterers with useful functionalities.

The recent experiments on ultrathin superconductors (SCs), in proximity with some substrates, have unveiled the possible enhancement and suppression of superconducting order parameters; yet their mechanisms are still ambiguous. Here we examine the inverse proximity effect between an ultrathin SC and the surface state of a topological insulator. We show the effect of the renormalized electron-phonon ($e$-ph) coupling constant $\lambda$ on superconducting order parameters is determined by two important mechanisms: the modified density of states (DOS) $N\left({\epsilon }_{\mathrm{F}}\right)$ from the occupied bands near the Fermi surfaces, and the renormalized interaction $U_{\mathrm{eff}}$ from the unoccupied bands. We demonstrate the enhanced order parameter near the bottom of the double-well bands with appropriate interface coupling due to the enhanced DOS; while in the strong coupling regime we observe the suppressed order parameter even with finite occupation in the Fermi surface, which is the result of the conjunct influence of the suppressed DOS and the renormalized $U_{\mathrm{eff}}$. This picture is applied to multiple SC layers, showing that the oscillation of order parameter can be well ascribed to the oscillation of an $e$-ph coupling constant, instead of solely the DOS. Our results not only provide a theoretical basis for tuning of the order parameters in the SC by chemical potential and interface coupling, but also provide some new insight into the order parameters in a large number of SC-substrate hybrid systems via the inverse proximity effect.

We present a Raman study of ${\mathrm{MoS}}_2$ powders and ${\mathrm{MoS}}_2$ individual layers covalently functionalized with organic molecules. In ${\mathrm{MoS}}_2$ powders, the defect-induced “LA” Raman mode shows evidence for successful functionalization. Increasing temperature induces oxidation of both functionalized and nonfunctionalized ${\mathrm{MoS}}_2$ into ${\mathrm{MoO}}_3$. In contrast, mechanically exfoliated individual ${\mathrm{MoS}}_2$ layers do not transfer into ${\mathrm{MoO}}_3$ under the same conditions. Instead, the Raman spectra show that the procedure of covalent functionalization leads to a partial transition from the 2H into the 1T' crystallographic phase in few-layer ${\mathrm{MoS}}_2$. We support the identification of the 1T' phase by DFT calculations of the corresponding vibrational modes in the mono- and bilayer 1T'-${\mathrm{MoS}}_2$.

Spin rectification effect (SRE) is an important electrical detection tool employed to probe magnetic dynamics and spin-orbit torques, and it is also closely related to spin pumping measurements. The SRE has multiple physical origins, and the entanglement among them makes it difficult to derive quantitative information. Herein, we present a systematic study of the SRE in ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ and Ni microstrips with out-of-plane-dominant microwave magnetic field excitation. The SREs from different physical mechanisms can be quantitatively decomposed via a symmetry consideration. The validity of the method is further supported by angular-dependent analyses. We observe both the anisotropic magnetoresistance induced SRE from the longitudinal induction current, and the planar Hall effect as well as the anomalous Hall effect induced SREs from the transverse induction current. Our work provides a comprehensive understanding of the SREs in ferromagnetic metal microstrips, and it is beneficial for quantitative analyses in microwave-related studies in spintronics.

Helical spin chains, consisting of magnetic (ad)atoms, on the surface of bulk superconductors are predicted to host Majorana bound states (MBSs) at the ends of the chain. Here, we investigate the prevalence of trivial zero-energy bound states in these helical spin-chain systems. The existence of trivial zero-energy bound states can prevent the conclusive identification of MBSs and, given the limited tunability of atomic spin-chain systems, could present a major experimental roadblock. First, we show that the Hamiltonian of a helical spin chain with varying nonuniform rotation rate between neighboring magnetic moments on a superconductor can be mapped to an effective Hamiltonian reminiscent of a ferromagnetic chain with strong Rashba spin-orbit coupling and with smooth nonuniform chemical potential, reminding a Rashba nanowire setups. Previously it has been found that trivial zero-energy states are abundant in nanowire systems with smoothly changing potentials. Therefore, we perform an extensive search for zero-energy bound states in helical spin-chain systems with varying rotation rates. Although bound states with near zero energy do exist for certain dimensionalities and rotation profiles, we find that zero-energy bound states are far less prevalent than in semiconductor nanowire systems with equivalent nonuniformities. In particular, utilizing varying rotation rates, we do not find zero-energy bound states in the most experimentally relevant setup consisting of a one-dimensional helical spin chain on the surface of a three-dimensional superconductor, even for profiles that produce near zero-energy states in equivalent one- and two-dimensional systems. Although our findings do not rule them out, the much reduced prevalence of zero-energy bound states in long nonuniform helical spin chains compared with equivalent semiconductor nanowires, as well as the ability to measure states locally via scanning tunneling microscopy, should reduce the experimental barrier to identifying MBSs in such systems.

We discuss a scheme for performing Jordan-Wigner transformation for various lattice fermion systems in two and three dimensions which keeps internal and spatial symmetries manifest. The correspondence between fermionic and bosonic operators is established with the help of auxiliary Majorana fermions. The current construction is applicable to general lattices with even coordination numbers and an arbitrary number of fermion flavors. The approach is demonstrated on the single-orbital square, triangular, and cubic lattices for spin-$1/2$ fermions. We also discuss the relation to some quantum spin liquid models.