The inversion symmetry breaking in a Bernal-stacked graphene bilayer, e.g., by applying a perpendicular electric field, can open a band gap harboring the quantum valley Hall effect. The different valley Hall topologies induced by a spatially varying electric field lead to the formation of a one-dimensional topological conducting channel, also called the zero-line mode (ZLM), existing at the zero-field region. The ZLMs were theoretically predicted by an atomic model and experimentally realized in bilayer graphene. Although the atomic model has been extensively utilized to investigate the electronic properties of ZLMs, a comprehensive ab initio study that precisely characterizes the critical condition of ZLMs is still lacking. In this paper, by employing first-principles method, we systematically investigate the electronic properties in two realistic systems, i.e., bilayer graphene with different types of line defects and with dual-split gates. The characteristics of ZLMs in different systems are demonstrated. Interestingly, we find that bilayer graphene with a pentagon-heptagon type of line defect is the optimal geometry to realize ZLMs due to its minimum critical device width as well as a lower formation energy. Our first-principles study implements the research gap between the atomic model and the experimental realization of ZLMs, and provides a practical scheme for realizing ZLMs in bilayer graphene systems.

We theoretically study spin-valley quantum coherence dynamics in a hybrid system consisting of a monolayer of transition metal dichalcogenide (${\mathrm{WS}}_2$) and an Ag nanoantenna. For this we map the time evolution of the Bloch and Stokes vectors to investigate, respectively, the dynamics of valley excitons and the states of polarization of plasmon near fields. The results show the formation of two types of collective resonances in the time domain, i.e., valley and field plasmonic meta-resonances (VPMR and FPMR). VPMR is shown as a coherently time-delayed ultrafast rotation of the Block vector. FPMR, on the other hand, occurs as an abrupt change in the near-field polarization of the Ag nanoantenna. The “time of occurrence” (resonance time) of such meta-resonances can be tuned using the intensity and polarization of the laser field responsible for the excitation of the system. Our results show that FPMR can decorate near fields of the Ag nanoantenna at different locations with various types of coherent-field dynamics with nanoscale spatial resolution. VPMR, on the other hand, offers an avenue to control the spin-valley states of transition metal dichalcogenide monolayers using minuscule changes in the intensity and polarization of the incident light.

We study the effect of disorder and doping on the metal-insulator transition in a repulsive Hubbard model on a square lattice using the determinant quantum Monte Carlo method. First, with the aim of making our results reliable, we compute the sign problem with various parameters such as temperature, disorder, on-site interactions, and lattice size. We show that in the presence of randomness in the hopping elements, the metal-insulator transition occurs and the critical disorder strength differs at different fillings. We also demonstrate that doping is a driving force behind the metal-insulator transition.

We report thermoelectric studies of ${\mathrm{FeS}}_{1-x}{\mathrm{Se}}_x$ ($x=0,0.06$) superconducting single crystals that feature high irreversibility fields and critical current density $J_c$ comparable to materials with much higher superconducting critical temperatures ($T_c$'s). The ratio of $T_c$ to the Fermi temperature $T_F$ is very small, indicating weak electronic correlations. With a slight selenium substitution on sulfur site in FeS both $T_c/T_F$ and the effective mass $m^{*}$ rise considerably, implying increase in electronic correlation of the bulk conducting states. The first-principle calculations show rise of the density of states at the Fermi level in ${\mathrm{FeS}}_{0.94}{\mathrm{Se}}_{0.06}$ when compared to FeS, which is related not only to Fe but also to chalcogen-derived electronic states.

We report structure, vibrational properties, and weak antilocalization-induced quantum correction to magnetoconductivity in single-crystal ${\mathrm{Bi}}_2\mathrm{Ge}{\mathrm{Te}}_4$. Surface band-structure calculations show a single Dirac cone corresponding to topological surface states in ${\mathrm{Bi}}_2\mathrm{Ge}{\mathrm{Te}}_4$. An estimated phase coherence length, $l_{\varphi }\sim 143\mathrm{nm}$ and prefactor α∼−1.54 from Hikami-Larkin-Nagaoka fitting of magnetoconductivity describe the quantum correction to conductivity. An anomalous temperature dependence of $A_{1g}$ Raman modes confirms enhanced electron-phonon interactions. Our results establish that electrons of the topological state can interact with the phonons involving the vibrations of Bi-Te in ${\mathrm{Bi}}_2\mathrm{Ge}{\mathrm{Te}}_4$.

Using first-principles calculations, we identify the origin of the observed charge density wave (CDW) formation in a layered kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. It is revealed that the structural distortion of kagome lattice forming the trimeric and hexameric V atoms is accompanied by the stabilization of quasimolecular states, which gives rise to the opening of CDW gaps for the V-derived multibands lying around the Fermi level. We, thus, propose the Jahn-Teller-like instability having the local lattice distortion and its derived quasimolecular states as a driving force of the CDW order. Specifically, the saddle points of multiple Dirac bands near the Fermi level, located at the $M$ point, are hybridized to disappear along the $k_z$ direction, therefore, not supporting the widely accepted Peierls-like electronic instability due to Fermi surface nesting. It is further demonstrated that applied hydrostatic pressure significantly reduces the interlayer spacing to destabilize the quasimolecular states, leading to a disappearance of the CDW phase at a pressure of $\sim 2\mathrm{GPa}$. The presently proposed underlying mechanism of the CDW order in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ can also be applicable to other isostructural kagome lattices, such as ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ and ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$.

We analyze the subgap excitations and phase diagram of a quantum dot (QD) coupled to a semiconducting nanowire fully wrapped by a superconducting (S) shell. We take into account how a Little-Parks (LP) pairing fluxoid (a winding in the S phase around the shell) influences the proximity effect on the dot. We find that under axially symmetric QD-S coupling, shell fluxoids cause the induced pairing to vanish, producing instead a level renormalization that pushes subgap levels closer to zero energy and flattens fermionic parity crossings as the coupling strength increases. This fluxoid-induced stabilization mechanism has analoges in symmetric S-QD-S Josephson junctions at phase $\pi$, and can naturally lead to patterns of near-zero modes weakly dispersing with parameters in all but the zeroth lobe of the LP spectrum.

Recent experiments indicate that Bernal stacked graphene multilayers (BGMs) have an interaction-induced gapped (or pseudogapped) ground state. Here, we propose that, due to the electron correlation, the BGM can be induced into a half-metallic phase by applying a vertical electric field and doping. The half-metallic states in even-layer and odd-layer BGMs have totally different behaviors due to their different band structures. We systematically calculate the graphene tetralayer (4L-BGM) and trilayer (3L-BGM) as typical examples of even-layer and odd-layer BGMs, respectively. In 4L-BGMs, we find an interesting phenomenon of electric field-induced inversion of the spin-polarized bands, namely, in the half-metallic phase, the spin polarization of the conducting channel and the net magnetic moment are inversed when the applied electric field exceeds a critical value. In 3L-BGMs, a remarkable feature is that the inequivalence of the two sublattices will intrinsically break the degeneracy of the spin-up and spin-down bands even in the zero electric field case. Our results suggest that 4L-BGM could be an ideal platform to detect the proposed half-metallic phase in BGM systems.

We consider the effect of the coupling between two-dimensional (2D) quantum rotors near an XY ferromagnetic quantum critical point and spins of itinerant fermions. We analyze how this coupling affects the dynamics of rotors and the self-energy of fermions. A common belief is that near a $q=0$ ferromagnetic transition, fermions induce an $\mathrm{\Omega }/q$ Landau damping of rotors (i.e., the dynamical critical exponent is $z=3$) and Landau overdamped rotors give rise to non-Fermi liquid fermionic self-energy $\mathrm{\Sigma }\propto {\omega }^{2/3}$. This behavior has been confirmed in previous quantum Monte Carlo (QMC) studies. Here we show that for the XY case the behavior is different. We report the results of large-scale quantum Monte Carlo simulations, which show that at small frequencies $z=2$ and $\mathrm{\Sigma }\propto {\omega }^{1/2}$. We argue that the new behavior is associated with the fact that a fermionic spin is by itself not a conserved quantity due to spin-spin coupling to rotors, and a combination of self-energy and vertex corrections replaces $1/q$ in the Landau damping by a constant. We discuss the implication of these results to experiments.

We study the spectral properties of and spectral crossovers between different random matrix ensembles [Poissonian, Gaussian orthogonal ensemble (GOE), Gaussian unitary ensemble (GUE)] in correlated spin-chain systems, in the presence of random magnetic fields, and the scalar spin-chirality term, competing with the usual isotropic and time-reversal invariant Heisenberg term. We have investigated these crossovers in the context of the level-spacing distribution and the level-spacing ratio distribution. We use random matrix theory (RMT) analytical results to fit the observed Poissonian-to-GOE and GOE-to-GUE crossovers, and examine the relationship between the RMT crossover parameter $\lambda$ and scaled physical parameters of the spin-chain systems in terms of a scaling exponent. We find that the crossover behavior exhibits universality, in the sense that, it becomes independent of lattice size in the large Hamiltonian matrix dimension limit. Moreover, the scaling exponent obtained from such a finite size scaling analysis, seems to be quite robust and independent of the type of crossover considered or the specific spectral correlation measure used.

Circularly polarized high harmonics (CHHs) have been observed in several solid targets under single-color circularly polarized excitation. However, experimental observations show that CHHs cannot be efficiently generated in monolayer molybdenum disulphide (${\text{MoS}}_2$) driving by a single-color circularly polarized laser, thus letting its unique valley-selective circular dichroism (VSCD) to remain unexploited in the high harmonic generation process. Here we demonstrate that the efficient generation of CHHs in monolayer ${\text{MoS}}_2$ driven by counterrotating bicircular (CRB) fields and the broken inversion symmetry of monolayer ${\text{MoS}}_2$ leads to the generation of forbidden $3n$ ($n\in \mathbb{N}$) harmonic orders. Interestingly, we find that the VSCD lead to a distinctive valley selection of harmonic orders, while the valley selection caused by the trefoil orientation relative to the lattice is also observed. Dynamical symmetry analyses show that the rotational symmetry of the crystals can be decoded by the combination of a linearly polarized excitation scheme and CRB excitation scheme via the lowest common multiple rule. Additionally, VSCD leaves unique fingerprints in the ellipticity of harmonics.

Interfacial superconductivity has manifested itself in various types of heterostructures: band insulator–band insulator, band insulator–Mott insulator, and Mott insulator–metal. We report the observation of high-temperature superconductivity (HTS) in a complementary and long-expected type of heterostructures, which consists of two Mott insulators, ${\mathrm{La}}_2{\mathrm{CuO}}_4$ (LCO) and ${\mathrm{PrBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (PBCO). By carefully controlling oxidization condition and selectively doping ${\mathrm{CuO}}_2$ planes with Fe atoms, which suppress superconductivity, we found that the superconductivity arises at the LCO side and is confined within no more than two unit cells (∼2.6 nm) near the interface. A phenomenon of “overcome the Fe barrier” will show up if excess oxygen is present during growth. Some possible mechanisms for the interfacial HTS have been discussed, and we attribute HTS to the redistribution of oxygen.

The kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, and Cs) have received intensive research interest due to the presence of charge density waves (CDWs), ${\mathbb{Z}}_2$ topological surface states, and intriguing pressure-dependent superconductivity. Using first-principles calculations, here we study the origin of the CDW order in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ and its superconducting properties under pressure up to 45 GPa. We reveal that the momentum-dependent electron-phonon coupling (EPC) effect plays an important role in the formation of the CDW order. Upon compression, the movement of the van Hove singularity, which induces the change in the density of states at the Fermi level, as well as the redistribution of the EPC, can explain the experimentally observed double superconducting domes. The main contribution to the EPC is varied from in-plane vibrational modes in superconducting area I (2–15 GPa) to out-of-plane modes in superconducting area II (15–45 GPa). Our observations clarify the origin of the CDW order and shed some light on understanding the experimental observations of pressure-dependent superconductivity in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$.

Motivated by recent studies of three-state Potts nematic states in magic-angle twisted bilayer graphene and doped-${\mathrm{Bi}}_2{\mathrm{Se}}_3$, we analyze the impact of critical nematic fluctuations on the low-energy properties of phonons. In this study, we propose how to identify the three-state Potts nematic fluctuations by ultrasound attenuation. The Gaussian fluctuation analysis shows that the Landau damping term becomes isotropic due to fluctuations of the $C_3$-breaking bond-order, and the nematoelastic coupling is also shown to be isotropic. These two features lead to an isotropic divergence of the transverse sound attenuation coefficient and an isotropic lattice softening, in contrast to the case of the $C_4$-breaking bond-order, which shows strong anisotropy. Moreover, we use a mean-field approximation and discuss the impurity effects. The transition temperature takes its maximum near the filling of the van Hove singularity, and the large density of states favors the nematic phase transition. It turns out that the phase transition is of weak first-order in the wide range of filling and, upon increasing the impurity scattering, the first-order transition line at low temperatures gradually shifts towards the second-order line, rendering the transition a weak first-order in a wider range of parameters. Furthermore, it is confirmed that the enhancement of the ultrasound attenuation coefficient will be clearly observed in experiments in the case of a weak first-order phase transition.

The valley-polarized quantum anomalous Hall effect (VQAHE) attracts intensive interest since it uniquely combines valleytronics and spintronics with nontrivial band topology. Here, based on first-principles calculations and the Wannier-function-based tight-binding model, we reveal that valley-based Hall effects and especially the VQAHE induced by external magnetic fields can occur in two-dimensional (2D) ferromagnetic van der Waals heterostructures (vdWHs). The results show that valley splitting derived from the Zeeman exchange energy drives these vdWHs generating the valley anomalous Hall effect and then the VQAHE. The chiral edge states and quantized Hall conductance are utilized to confirm the presence of VQAHE. Besides, it is also found that applying external electric fields (or tuning interlayer distances) can facilitate the realization of VQAHE, and thus we present a phase diagram in a broad parameter regime of magnetic fields and electric fields (or interlayer distances). Our work not only offers a class of ferromagnetic vdWHs to realize various valley-based Hall phases, but also can guide advancements for designing topological devices with spin-valley filtering effects based on the VQAHE.

The theory of composite fermion provides a simple and unified picture to understand a vast amount of phenomenology in the quantum Hall regime. However it has remained challenging to formulate this concept properly within a single Landau level, which provides the relevant degrees of freedom in the limit of strong magnetic field. Recently a low-energy noncommutative field theory for bosons at Landau-level filling factor $\nu =1$ has been formulated by Dong and Senthil [Z. Dong and T. Senthil, Phys. Rev. B 102, 205126 (2020)]. In the limit of long-wavelength and small-amplitude gauge fluctuation, they found it reduces to the celebrated Halperin-Lee-Read theory of a composite fermion liquid. In this work we consider a Bose-Fermi mixture at total filling factor $\nu =1$. Different from previous work, the number density of composite fermions in the mixture and corresponding Fermi momentum can be tuned by changing the filling factor of bosons, ${\nu }_b=1-{\nu }_f$. This tunability enables us to study the dilute limit ${\nu }_b\ll 1$, which allows for a controlled and asymptotically exact calculation of the energy dispersion and effective mass of composite fermions. Furthermore, the approximation of the low-energy description by a commutative field theory is manifestly justified. Most importantly, we demonstrate that gauge fluctuations acquire a Higgs mass due to the presence of a composite boson condensate, as a result of which the system behaves like a genuine Landau- Fermi liquid. Combined with the irrelevance of four-fermion interaction in the dilute limit, we are able to obtain asymptotically exact properties of this composite fermion Fermi liquid. In the opposite limit of ${\nu }_f\ll 1$, the Higgs mass goes to zero and we find crossover between Fermi liquid and non-Fermi liquid as temperature increases. Observing these properties either experimentally or numerically provides unambiguous evidence of not only the composite fermions and the Fermi surface they form, but also the presence of emergent gauge fields and their fluctuations due to strong correlation.

The strong coupling between magnetic, structural, and electronic degrees of freedom is a prominent feature of Fe-based superconductors. However, the relationship between the degrees of freedom and vortex dynamics remains unclear. In this paper, we measure the vortex pinning energy $U_0/k_{\mathrm{B}}$ within the thermally activated flux-flow regime with respect to the pressure-induced phases in a FeSe single crystal. We show that the dependence of $U_0/k_{\mathrm{B}}$ on pressure, measured at a magnetic field higher than the crossover field $H_{\mathrm{cr}}$, follows a trend like those of critical temperature $T_c\left(p\right)$ and the average Fermi velocity ${\overline{v}}_{\mathrm{F}}\left(p\right)$. On the other hand, at magnetic fields lower than $H_{\mathrm{cr}}, U_0/k_{\mathrm{B}}$ increases remarkably before reaching the pressure at which antiferromagnetic (AFM) long-range order occurs. Our results suggest the presence of additional pinning sites, possibly AFM domain boundaries, correlated with the enhanced $U_0/k_{\mathrm{B}}$. In addition, in this paper, we provide a universal description of the vortex dynamics in FeSe.

Warm dense matter is a highly active research area both at the frontier and interface of material science and plasma physics. We assess the performance of the commonly used exchange-correlation (XC) approximation (LDA, PBE, PBEsol, and AM05) in the spin-polarized inhomogeneous electron gas under warm dense conditions based on exact path-integral quantum Monte Carlo calculations. This extends our recent analysis on the relevance of inhomogeneities in the spin-unpolarized warm dense electron gas [Moldabekov et al., J. Chem. Phys. 155, 124116 (2021)]. We demonstrate that the predictive accuracy of these XC functionals deteriorates with (1) a decrease in density (corresponding to an increase in the interelectronic correlation strength) and (2) an increase of the characteristic wave number of the density perturbation. We provide recommendations for the applicability of the considered XC functionals at conditions typical for warm dense matter. Furthermore, we hint at future possibilities for constructing more accurate XC functionals under these conditions.

Antimonene has attracted much attention for its high carrier mobility and suitable band gap for electronic, optoelectronic, and even spintronic devices. To tailor its properties for such applications, strain engineering may be adopted. However, such two-dimensional (2D) crystals may prefer to exist in the rippled form because of the instability of long-range orders, and rippling has been shown to have a contrasting, significant impact on the electronic properties of various 2D materials, which complicates the tuning process. Hence, the effects of rippling on the electronic properties of antimonene under strain are herein investigated by comparing antimonene in its rippled and flat forms. Density functional theory calculations are performed to compute the structural and electronic parameters, where uniaxial compression of up to 7.5% is applied along the armchair and zigzag directions to study the anisotropic behavior of the material. Highly stable properties such as the work function and band gap are obtained for the rippled structures, where they are fully relaxed, regardless of the compression level, and these properties do not deviate much from those of the pristine structure under no strain. In contrast, various changes are observed in their flat counterparts. The mechanisms behind the different results are thoroughly explained by analyses of the density of states and structure geometry. The out-of-plane dipole moments of the rippled structures are also presented to give further insights into potential applications of rippled antimonene in sensors, actuators, triboelectric nanogenerators, etc. This work presents extensive data and thorough analysis on the effect of rippling on antimonene. The identification of optimal ripple amplitudes for which the electronic properties of the pristine condition can be recovered will be highly significant in guiding the rational design and architecture of antimonene-based devices.