We describe a solvable model of a quantum transition in a single band model involving a change in the size of the electron Fermi surface without any symmetry breaking. In a model with electron density $1-p$, we find a “large” Fermi surface state with the conventional Luttinger volume $1-p$ of electrons for $p>p_c$, and a first-order transition to a “small” Fermi surface state with a non-Luttinger volume $p$ of holes for $p<p_c$. As required by extended Luttinger theorems, the small Fermi surface state also has fractionalized spinon excitations. The model has electrons with strong local interactions in a single band; after a canonical transformation, the interactions are transferred to a coupling to two layers of ancilla qubits, as proposed by Zhang and Sachdev [Phys. Rev. Research 2, 023172 (2020)]. Solvability is achieved by employing random exchange interactions within the ancilla layers, and taking the large $M$ limit with $\mathrm{SU}\left(M\right)$ spin symmetry, as in the Sachdev-Ye-Kitaev models. The local electron spectral function of the small Fermi surface phase displays a particle-hole asymmetric pseudogap, and maps onto the spectral function of a lightly doped Kondo insulator of a Kondo-Heisenberg lattice model. We discuss connections to the physics of the hole-doped cuprates: the asymmetric pseudogap observed in STM, and the sudden change from incoherent to coherent antinodal spectra observed recently in photoemission. A holographic analogy to wormhole transitions between multiple black holes is briefly noted.

We present studies of neutron diffraction, electrical resistivity, magnetic susceptibility, magnetization, and specific heat of noncentrosymmetric $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_2$. At normal pressure, $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_2$ is a moderate heavy-fermion compound with Kondo lattice. At 16 K we observe an anomaly in the temperature dependence of specific heat which is ascribed to an abrupt valence change of Yb ions. At pressures above 7 GPa, the valence change and the Kondo lattice state are suppressed, and near a temperature of 10 K we detect the appearance of magnetic order. Above 5 GPa, the temperature dependence of the resistivity behaves similarly to other compounds of the $\mathrm{Ce}\mathrm{Ni}{\mathrm{C}}_2$-type family, indicating the formation of charge density waves. This is attributed to the nesting properties of the Fermi surface (FS) found within the density functional $\mathrm{theory}+\mathrm{dynamical}$ mean field theory treatment. Our ab initio calculations also show that the valence of Yb in $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_2$ at normal conditions is 2.85, which increases with temperature and pressure. The FS of $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_2$ is anisotropic in shape in comparison with the 3D surface of $\mathrm{Yb}\mathrm{Co}{\mathrm{C}}_2$.

In this work the above-band-gap absorption spectrum in two-dimensional Dirac materials is calculated with account for the interaction between the photocarriers. Both the screened Rytova-Keldysh and pure Coulomb attraction potentials between the electron and hole are used in the study. We find that in the materials under consideration, the interaction enhances the absorbance in the narrow interband edge region, in sharp contrast to the band model with the parabolic free-carrier energy dispersion. We develop an approximation of the weak interaction, which allows us to reproduce the main features of the exactly calculated Sommerfeld factor. We show a substantial reduction of this factor at higher frequencies due to the single-particle energy renormalization.

In this work, we report for the first time the crucial role of topological anomalies like Stone-Wales (SW)–type bond rotations in tuning the optical properties of graphene quantum dots (GQDs). By means of first-principles calculations, we first show that the structural stability of GQDs strongly depends on the position of SW defects. Optical absorption spectra are then computed using electron-correlated methodology to demonstrate that SW-type reconstruction is responsible for the appearance of new defect-induced peaks below the optical gap and dramatically modifies the optical absorption profile. In addition, our investigations signify that electron correlation effects become more dominant for SW-defected GQDs. We finally establish that the introduction of SW defects at specific locations strongly enhances light absorption in the visible range, which is of prime importance for designing light harvesting, photocatalytic, and optoelectronic devices.

Trigonal tellurium with helical structure exhibits intriguing properties due to its inherent chirality, which has attracted considerable research interest recently. Using cryogenic scanning tunneling microscopy/spectroscopy and density functional theory calculations, we investigated quasiperiodic defects at particular edges of Te nanoribbons grown on graphene/SiC(0001) substrate. The detailed topography of edge defects reveals the chirality of Te nanoribbon, and the defects induce a quasiperiodic modulation of the electronic states along the edge. The average period of the edge defects varies with the twist angle between Te nanoribbon and substrate, implying the electronic modulation can be tuned by the interfacial interaction. Based on the morphology analyses and modeled calculations, we further propose the growth mechanism of the Te nanoribbons.

The ultraconfined light of plasmonic modes puts their effective wavelength close to the mean-free path of electrons inside the metal electron gas. The Drude model, which can not take the repulsive interactions of electrons into account, then clearly begins to show its limits. In an intermediate length scale where a full quantum treatment is computationally prohibitive, the semiclassical hydrodynamic model, instrinsically nonlocal, has proven successful. Here we generalize the expression for the absorption volume density and the reciprocity theorem in the framework of this hydrodynamic model. We validate numerically these generalized theorems and show that using classical expressions instead leads to large discrepancies.

Many conductors, including recently studied Dirac materials, show saturation of coherence length on decreasing temperature. This surprising phenomenon is assigned to external noise, residual magnetic impurities, or two-level systems specific to noncrystalline solids. Here, by considering the SnTe-class of compounds as an example, we show theoretically that breaking of mirror symmetry deteriorates Berry's phase quantization, leading to additional dephasing in weak-antilocalization magnetoresistance (WAL-MR). Our experimental studies of WAL-MR corroborate these theoretical expectations in (111) ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Se}$ thin film with Sn contents $x$ corresponding to both topological crystalline insulator and topologically trivial phases. In particular, we find the shortening of the phase coherence length in samples with intentionally broken mirror symmetry. Our results indicate that the classification of quantum transport phenomena into universality classes should encompass, in addition to time-reversal and spin-rotation invariances, spatial symmetries in specific systems.

We study theoretically spin relaxation during phonon-assisted tunneling of a single electron in self-assembled InAs/GaAs quantum-dot molecules formed by vertically stacked dots. We find that the spin-flip tunneling rate may be as high as $1\%$ of the spin-conserving one. By studying the dependence of spin-relaxation rate on external fields, we show that the process is active at a considerable rate even without the magnetic field, and scales with the latter differently than the relaxation in a Zeeman doublet. Utilizing a multiband $\mathit{k}·\mathit{p}$ theory, we selectively investigate the impact of various spin-mixing terms in the electron energy and carrier-phonon interaction Hamiltonians. As a result, we identify the main contribution to come from the Dresselhaus spin-orbit interaction, which is responsible for the zero-field effect. At magnetic fields above $\sim 15\mathrm{T}$, this is surpassed by other contributions due to the structural shear strain. We also study the impact of the sample morphology and determine that the misalignment of the dots may enhance relaxation rate by over an order of magnitude. Finally, via virtual tunneling at nonzero temperature, the process in question also affects stationary electrons in tunnel-coupled structures and provides a Zeeman-doublet spin-relaxation channel even without the magnetic field.

We apply an intense infrared laser pulse in order to perturb the electronic and vibrational states in the three-dimensional charge density wave material $1T\text{−}{\mathrm{VSe}}_2$. Ultrafast snapshots of the light-induced hot carrier dynamics and nonequilibrium quasiparticle spectral function are collected using time- and angle-resolved photoemission spectroscopy. The hot carrier temperature and time-dependent electronic self-energy are extracted from the time-dependent spectral function, revealing that incoherent electron-phonon interactions heat the lattice above the charge density wave critical temperature on a timescale of $(200\pm 40)$ fs. Density functional perturbation theory calculations establish that the presence of hot carriers alters the overall phonon dispersion and quenches efficient low-energy acoustic phonon scattering channels, which results in a new quasiequilibrium state that is experimentally observed.

In disordered two-dimensional Chern insulators, a single bulk extended mode per band is predicted to exist up to a critical disorder strength, with all other bulk modes localized. This behavior contrasts with topologically trivial two-dimensional phases, whose modes are all immediately localized by disorder. Using a tight-binding model of a realistic photonic Chern insulator, we show that delocalized bulk eigenstates can be observed in an experimentally realistic setting. This requires the selective use of resonator losses to suppress topological edge states and acquiring sufficiently large ensemble sizes using variable resonator detunings.

We show that strained Weyl semimetals exhibit a directional asymmetry of the heat transfer along the strain axis. The forward/backward asymmetry of the thermal conductivity appears as a result of the axial anomaly, which leads to the emergence of unidirectionally propagating “chiral” sound modes. The chiral phonons modify, in a directionally asymmetric manner, the spectrum of the acoustic lattice vibrations, and both types of phonons contribute to the thermal asymmetry of the material. We estimate the magnitude of the effect and discuss its possible experimental detection.

We derive the anomalous Hall contributions arising from dipolar interactions to diffusive spin transport in magnetic insulators. Magnons, the carriers of angular momentum in these systems, are shown to have a nonzero Berry curvature, resulting in a measurable Hall effect. For yttrium iron garnet (YIG) thin films we calculate both the anomalous and magnon spin conductivities. We show that for a magnetic field perpendicular to the film the anomalous Hall conductivity is finite. This results in a nonzero Hall signal, which can be measured experimentally using Permalloy strips arranged like a Hall bar on top of the YIG thin film. We show that electrical detection and injection of spin is possible, by solving the resulting diffusion-relaxation equation for a Hall bar. We predict the experimentally measurable Hall coefficient for a range of temperatures and magnetic field strengths. Most strikingly, we show that there is a sign change of the Hall coefficient associated with increasing the thickness of the film.

In a type-II superconductor, the vortex core (VC) behaves like a normal metal. Consequently, the single-particle density of states in the VC of a conventional type-II superconductor remains either flat or (for very clean single crystals) exhibits a peak at zero bias due to the formation of the Caroli–de Gennes–Matricon bound state inside the core. Here, we report an unusual observation from scanning tunneling spectroscopy measurements in a weakly pinned thin film of the conventional $s$-wave superconductor $a$-MoGe, namely, that a soft gap in the local density of states continues to exist even at the center of the VC. We ascribe this observation to rapid fluctuation of vortices about their mean position that blurs the boundary between the gapless normal core and the gapped superconducting region outside. Analyzing the data as a function of magnetic field, we show that the variation of fluctuation amplitude is consistent with quantum zero-point motion of vortices.

Motivated by recent experimental progress on iron-based ladder compounds, we study the doped two-orbital Hubbard model for the two-leg ladder ${\mathrm{BaFe}}_2{\mathrm{S}}_3$. The model is constructed by using ab initio hopping parameters and the ground state properties are investigated using the density matrix renormalization group method. We show that the $(\pi ,0)$ magnetic ordering at half filling, with ferromagnetic rungs and antiferromagnetic legs, becomes incommensurate upon hole doping. Moreover, depending on the strength of the Hubbard $U$ coupling, other magnetic patterns, such as $(0,\pi )$, are also stabilized. We found that the binding energy for two holes becomes negative for intermediate Hubbard interaction strength, indicating hole pairing. Due to the crystal-field split among orbitals, the holes primarily reside in one orbital, with the other one remaining half filled. This resembles orbital selective Mott states. The formation of tight hole pairs continues with increasing hole density, as long as the magnetic order remains antiferromagnetic in one direction. The study of pair-pair correlations indicates the dominance of the intraorbital spin-singlet channel, as opposed to other pairing channels. Although in a range of hole doping pairing correlations decay slowly, our results can also be interpreted as corresponding to a charge density wave made of pairs, a precursor of eventual superconductivity after interladder couplings are included. Such a scenario of intertwined orders has been extensively discussed before in the cuprates, and our results suggest a similar physics could exist in ladder iron-based superconductors. Finally, we also show that a robust Hund's coupling is needed for pairing to occur.

In general, a magnon does not interact directly with an atom. Here we present a scheme to generate coherent coupling between two magnons and a superconducting artificial atom via exchange of virtual photons in dispersive cavities. By probing into the induced magnon-atom interactions, we identify the two-magnon process, which causes almost perfect two-mode squeezing and entanglement of the two magnons for specific conditions. The quantum entanglement of the magnon modes in two massive ferrimagnets survives at the steady state and represents genuinely macroscopic quantum state. Our work offers an interesting alternative in designing experiments for generating the coherent coupling between the magnons and the atom. It finds broad applications in the investigation of macroscopic quantum effects and the preparation of quantum devices.

Solid-state superconducting heterostructures can provide a possible source of entangled electrons for the purpose of quantum-information processing, and shot noise can indicate when this occurs. Shot noise cross-correlations in a ballistic NSNSN (normal-superconductor-normal-superconductor-normal) is normally negative for electron currents, but becomes positive for entangled electrons that result from the breakup of Cooper pairs. A one-to-one correspondence is found between the energies of the quasibound states and regions of positivity in the cross-correlated shot noise. The quasibound states of the NSNSN system are associated with poles of the NSNSN scattering matrix. We find that regions of positive cross-correlated shot noise distributions, and the associated emission of entangled electrons, exist over a wide range of system sizes and in the presence of multiple quasibound states. We also find that, at the quasibound-state energies, the Andreev approximation is not adequate to describe the key physical processes in the NSNSN device.

We develop an algorithm to construct high-quality numerical atomic orbital (NAO) basis sets suitable for large-scale, efficient density-functional calculations. The key idea behind this algorithm is that, in addition to fitting the reference wavefunctions themselves generated by plane-wave based calculations of chosen target systems, the first derivatives of the reference wavefunctions are also taken into account as the fitting target. By doing so, the quality of the generated NAO basis sets is significantly improved in the sense that the same level of numerical precision can be achieved with smaller basis set sizes or with reduced cutoff radii of the NAOs.

In a recent study [Phys. Rev. X 10, 021042 (2020)], we showed using large-scale density matrix renormalization group (DMRG) simulations on infinite cylinders that the triangular lattice Hubbard model has a chiral spin liquid phase. In this work, we introduce hopping anisotropy in the model, making one of the three distinct bonds on the lattice stronger or weaker compared with the other two. We implement the anisotropy in two inequivalent ways, one which respects the mirror symmetry of the cylinder and one which breaks this symmetry. In the full range of anisotropy, from the square lattice to weakly coupled one-dimensional chains, we find a variety of phases. Near the isotropic limit we find the three phases identified in our previous work: metal, chiral spin liquid, and ${120}^{\circ }$ spiral order; we note that a recent paper suggests the apparently metallic phase may actually be a Luther-Emery liquid, which would also be in agreement with our results. When one bond is weakened by a relatively small amount, the ground state quickly becomes the square lattice Néel order. When one bond is strengthened, the story is much less clear, with the phases that we find depending on the orientation of the anisotropy and on the cylinder circumference. While our work is to our knowledge the first DMRG study of the anisotropic triangular lattice Hubbard model, the overall phase diagram we find is broadly consistent with that found previously using other methods, such as variational Monte Carlo and dynamical mean field theory.