The onset of the pseudogap in high-$T_c$ superconducting cuprates (HTSC) is marked by the $T^{*}$ line in the doping-temperature phase diagram, which ends at a point $p^{*}$ at zero temperature within the superconducting dome. Although various theoretical and experimental studies indicate a competition between the pseudogap and superconductivity, there is no general consensus on the effects of the pseudogap within the superconducting phase. We use cluster dynamical mean-field theory on a three-band Hubbard model for the HTSC to study the superconducting phase at $T=0$, obtained when doping the charge-transfer insulator, for several values of $U$. We observe a first-order transition within the superconducting phase, which separates the underdoped and overdoped solutions. The transition to the underdoped solution is marked by a discontinuous increase in the spectral gap, and on further underdoping the spectral gap increases while the superconducting order parameter decreases. We conclude that this is due to the onset of the pseudogap in the underdoped region, which contributes to the increasing spectral gap; this is further consistent with the appearance of a pole in the normal component of the self-energy, in the antinodal region, in the underdoped solution. This is accompanied by a change in the source of the condensation energy from potential energy, in the overdoped region, to kinetic energy in the underdoped region. Further, we also observe that the $d$-wave node vanishes smoothly within the superconducting phase at low values of hole doping, within the underdoped region. We see this as a manifestation of Mott physics operating at very low doping. Various aspects of the results and their implications are discussed.

Nodal line states in electronic systems are the extended band crossings in three-dimensional (3D) momentum space, which recently has been widely explored in classical systems in analogs. With the Dirac cones in two-dimensional (2D) hexagonal lattices, the linear degeneracy points in the stacking 2D hexagonal lattices (3D lattices) are elongated into degeneracy lines in the momentum space. In this work, we show that by coupling the stacked hexagonal lattices with the time-reversal symmetry and inversion symmetry protected, degeneracy points will form a closed nodal ring in the momentum space in the strong coupling regime. We observe flat drumhead dispersion surfaces in the band gaps, which verifies the existence of the intriguing nodal line states. Based on full-wave simulations, we show the field confinement at the truncated surface and the field enhancement due to the large density of states in flat bands. Furthermore, topological robustness of the drumhead surface states is investigated against various randomly distributed defects, such as site disorders and hopping disorders. Our work may serve as the platform of the sonic-crystal based semimetal for versatile applications like sound trapping, vibration isolation, and absorption.

A dynamical signature of localization in quantum systems is the absence of transport which is governed by the amount of coherence that configuration space states possess with respect to the Hamiltonian eigenbasis. To make this observation precise, we study the localization transition via quantum coherence measures arising from the resource theory of coherence. We show that the escape probability, which is known to show distinct behavior in the ergodic and localized phases, arises naturally as the average of a coherence measure. Moreover, using the theory of majorization, we argue that broad families of coherence measures can detect the uniformity of the transition matrix (between the Hamiltonian and configuration bases) and hence act as probes to localization. We provide supporting numerical evidence for Anderson and many-body localization (MBL). For infinitesimal perturbations of the Hamiltonian, the differential coherence defines an associated Riemannian metric. We show that the latter is exactly given by the dynamical conductivity, a quantity of experimental relevance which is known to have a distinctively different behavior in the ergodic and in the many-body localized phases.

We study the magnon contribution to the gravitomagnetoelectric (gravito-ME) effect, in which the magnetization is induced by a temperature gradient, in noncentrosymmetric antiferromagnetic insulators. This phenomenon is totally different from the ME effect, because the temperature gradient is coupled to magnons but an electric field is not. We derive a general formula of the gravito-ME susceptibility in terms of magnon wave functions and find that a difference in $g$ factors of magnetic ions is crucial. We also apply our formula to a specific model. Although the obtained gravito-ME susceptibility is small, we discuss several ways to enhance this phenomenon.

We present a theoretical model of spin transitions in stacks of molecular layers. Our model captures the already established physics of these systems (thermal hysteretic transitions and crossovers) and suggests a way towards in situ control of this physics by means of an external electric field. Our results pave the way toward both temperature and voltage controllable organic memory.

On a ring, a single Jordan-Wigner transformation between the Kitaev model and the spin model suffers redundant degrees of freedom. However, we can establish an exact quaternary Jordan-Wigner mapping involving two Kitaev rings and two spin rings with periodic or antiperiodic boundary conditions. This mapping allows us to demonstrate exactly how a topological extended-kink (TEK) phase develops in the interacting Kitaev ring with odd number of lattice sites. The emergence of this new phase is attributed to the effect of geometrical ring frustration. Unlike the usual topological phases protected by an energy gap in noninteracting systems, the TEK phase is gapless. And because the spectra of low-energy excitations are quadratic, the specific heat per site approaches half of the Boltzmann constant near absolute zero temperature. More interestingly, the ground state is unique, immune to spontaneous symmetry breaking. It exhibits a long-range correlation function with a nonlocal factor, but no local order parameter can be defined. As a concomitant effect, a special kind of localized kink zero mode (KZM) takes place if we introduce a type of bond defect. We also show that the KZM is robust against moderate disorders.

We employ the interaction distance to characterize the physics of a one-dimensional extended XXZ spin model, whose phase diagram consists of both integrable and nonintegrable regimes, with various types of ordering, e.g., a gapless Luttinger liquid and gapped crystalline phases. We numerically demonstrate that the interaction distance successfully reveals the known behavior of the model in its integrable regime. As an additional diagnostic tool, we introduce the notion of “integrability distance” and particularize it to the XXZ model to quantity how far the ground state of the extended XXZ model is from being integrable. This distance provides insight into the properties of the gapless Luttinger liquid phase in the presence of next-nearest-neighbor spin interactions which break integrability.

Indium atom manipulation was achieved on the (110) cross-sectional surface of a quantum structure consisting of InAs/GaSb multilayer using a scanning tunneling microscope (STM) at low temperature. Positioning, removing, and lateral displacement by means of tip-induced hopping was performed on both InAs and GaSb layers. Electron tunneling via the electron orbital of the manipulated atom (adatom) was observed not only on the InAs layer but also on the GaSb layer, even when the energy of the orbital was in the GaSb band gap. The tunneling process from the STM tip to the InAs conduction band via the adatom and the GaSb band gap is explained well by the single electron tunneling model through the double-barrier tunneling junction. In the case of the adatom being very close to the InAs/GaSb heterointerface, interference with the electron tunneling to the InAs conduction electronic states through the GaSb barrier with/without passing the adatom was observed. The peak energy in the tunneling spectrum corresponding to the orbital of the adatom depends on the adatom position on the GaSb, demonstrating the fundamental interactions between the surface atomic-scale structures and semiconductor heterostructures.

We report on the successful synthesis and low-temperature electron transport investigations of a new form of material—$\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_2\mathrm{Se}$ semiconducting nanowires. Gate-tunable 0- and $\pi \text{−}h/e(h$ is the Planck constant and $e$ the elementary charge) periodic resistance oscillations in longitudinal magnetic field were observed unexpectedly, demonstrating novel quasiballistic, phase-coherent surface states in $\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_2\mathrm{Se}$ nanowires. By reaching a very good agreement between the calculated density of states and the experimental data, we clarified the mechanism to be the one-dimensional subbands formed along the circumference of the nanowire rather than the usually considered Aharonov-Bohm interference. A qualitative physical picture based on downward band bending associated with the complex band structure is proposed to describe the formation of the surface states.

Quadratic detection in linear mesoscopic transport systems produces cross terms that can be viewed as interference signals reflecting statistical properties of charge carriers. In electronic systems these cross-term interferences arise from exchange effects due to Pauli principle. Here we demonstrate fermionic Hanbury Brown and Twiss (HBT) exchange phenomena due to indistinguishability of charge carriers in a diffusive graphene system. These exchange effects are verified using current-current cross-correlations in combination with regular shot noise (autocorrelation) experiments at microwave frequencies. Our results can be modeled using semiclassical analysis for a square-shaped metallic diffusive conductor, including contributions from contact transparency. The experimentally determined HBT exchange factor values lie between the calculated ones for coherent and hot electron transport.

Charge and thermoelectric transport should be closely correlated with each other; however, little is known regarding the origin of thermopower in semicrystalline $\pi$-conjugated polymers, particularly those doped with molecular dopants. It is controversial whether the well-established Mott formula is valid for such conducting polymers, which inevitably have a finite structural disorder. We show that a truly metallic regime that can be realized in a highly crystalline domain gives rise to thermopower, which is demonstrated unambiguously by the observation of a linear temperature dependence of the Seebeck coefficient in semicrystalline polythiophene-based conducting polymers. The presence of onset metallicity is also verified comprehensively by the Hall effect and Drude optical response, which indicates that the Mott formula, which is frequently used for degenerated semiconductors and metals, can be applicable to highly crystalline conjugated polymers. This provides insight into the structure-thermoelectric property relationships in semicrystalline conducting polymers.

${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ is supposed to be a simple semiconductor in which excitonic instability of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ is suppressed due to its large band gap. However, the Ni $2p$ core-level photoemission of ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ exhibits a satellite indicating Ni $3d$ orbitals are mixed into its conduction band as expected in an excitonic insulator. The valence band does not show dispersion flattening and spectral sharpening which are fingerprints of an excitonic insulator. Instead, Ni $3p-3d$ resonant photoemission indicates Mottness of the Ni $3d$ electron in ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ with negative charge-transfer energy. The present results show that ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ can be viewed as a valence bond insulator with a band gap exceeding the exciton binding energy.

The ability to localize and manipulate individual quasiparticles in mesoscopic structures is critical in experimental studies of quantum mechanics and thermodynamics, and in potential quantum information devices, e.g., for topological schemes of quantum computation. In strong magnetic field, the quantum Hall edge modes can be confined around the circumference of a small antidot, forming discrete energy levels that have a unique ability to localize fractionally charged quasiparticles. Here, we demonstrate a Dirac fermion quantum Hall antidot in a graphene, where charge-transport characteristics can be adjusted through the coupling strength between the contacts and the antidot, from Coulomb blockade dominated tunneling under weak coupling to the effectively noninteracting resonant tunneling under strong coupling. Both regimes are characterized by single-flux and -charge oscillations in conductance persisting up to temperatures over 2 orders of magnitude higher than previous reports in other material systems. Such graphene quantum Hall antidots may serve as a promising platform for building and studying quantum circuits for quantum simulation and computation.

We theoretically investigate the dynamical conductivity in the multiply degenerate point-nodal semimetal CoSi. The dynamical conductivity is calculated as a function of photon energy by using the first-principles band calculation and the linear response theory. In the nodal semimetal, the band structure holds point nodes at the $\mathrm{\Gamma }$ and R points in the Brillouin zone and more than three bands touch at the nodes. Around the nodes, electronic states are predicted to be described as the multifold chiral fermion, a class of fermion in condensed matter. We show that the dynamical conductivity exhibits a characteristic spectrum corresponding to the band structure and the chiral fermionic states. The chirality leads to the prohibition of transition between the lower and upper bands of threefold chiral fermion and thus the transition between the middle and lower bands is relevant to the dynamical conductivity. This transition property is different from the Dirac and Weyl semimetals, the other point-nodal semimetals, where the excitation between the upper and lower bands is relevant to the dynamical conductivity. We show the prohibition causes the reduction of dynamical conductivity in the low-photon energy region.

Landau levels of AB-stacked four- to six-layer graphene were studied experimentally in the presence of a perpendicular electric field. The Landau levels at a low magnetic field showed a characteristic structure that originated from the mini-Dirac cones created by the perpendicular electric fields. Sixfold or 12-fold degenerate Landau levels arising from the mini-Dirac cones were observed in the vicinity of the charge neutrality point as a radial structure in a plot of their Landau fan diagrams. The structure of four-layer graphene had approximate electron-hole symmetry near the charge neutrality point, while the five- and six-layer graphene showed asymmetry. Numerical calculations of the dispersion relation and Landau level spectra indicated that trigonal warping played an essential role in forming the experimentally observed Landau level structure in low magnetic fields. In high magnetic fields, trigonal warping becomes less significant and the Landau level spectra become simpler, approaching those that were calculated without considering trigonal warping. The appearance of mini-Dirac cones in a perpendicular electric field would be a common feature of AB-stacked multilayer graphene.

Understanding carrier creation and evolution in materials initiated by pulsed optical excitation is central to developing ultrafast optoelectronics. We demonstrate herein that the dynamic response of a system can be drastically modified when its physical dimension is reduced to the atomic scale, the ultimate limit of device miniaturization. A comparative study of single-layer (SL) tungsten diselenide $\left(\mathrm{WS}{\mathrm{e}}_2\right)$ relative to bulk $\mathrm{WS}{\mathrm{e}}_2$ shows substantial differences in the transient response as measured by time- and angle-resolved photoemission spectroscopy (TRARPES). The conduction-band minimum in bulk $\mathrm{WS}{\mathrm{e}}_2$, populated by optical pumping, decays promptly. The corresponding decay for SL $\mathrm{WS}{\mathrm{e}}_2$ is much slower and exhibits two time constants. The results indicate the presence of two distinct decay channels in the SL that are correlated with the breaking of space inversion symmetry in the two-dimensional limit. This symmetry breaking lifts the spin degeneracy of the bands, which in turn causes the blockage of decay for one spin channel. The stark contrast between the single layer and the bulk illustrates the basic carrier scattering processes operating at different timescales that can be substantially modified by dimensional and symmetry-reduction effects.

We study the spin reorientation in ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ by examining the anisotropic magnetoresistivity in detail, exploiting the dependence of the resistivity on the direction between magnetization and applied current. We determine the distribution of the magnetic domains as a function of temperature between 360 K and 2 K and find the reorientation transition to peak at 120 K. We discover that both out-of-plane and in-plane phases coexist at temperatures around the spin reorientation, indicative of a first-order phase transition. Although the volume of the magnetic domains in the different phases sharply changes at the spin reorientation transition, no appreciable changes in the electronic structure for a specific magnetization is detected by anisotropic magnetoresistivity at the spin reorientation. In contrast, we observe an electronic transition around 40 K, hitherto unreported, and reflected in both the zero-field resistivity and anisotropic resistivity.

Tungsten tetraboride is outstanding among transition-metal light-element compounds for its easy synthesis and superior mechanical properties. Its crystal structure, however, has eluded scientists for over half a century, impeding fundamental understanding and rational property optimization. Recent x-ray and neutron diffraction studies suggest rare boron trimers occupying vacant metal sites in the highest boride of tungsten ${\mathrm{WB}}_{4.2}$, but a viable crystal structure and key mechanical properties remain unresolved. Here we identify a ${\mathrm{WB}}_{4.2}$ phase in orthorhombic symmetry with a very large 104-atom unit cell using a tailored search algorithm treating boron trimer as a coherent unit. First-principles studies establish phase stability and unveil the mechanism for strength enhancement by newly identified bonding features. These findings solve a challenging crystal structure and elucidate its benchmark mechanical behaviors. This work offers powerful insights for exploring novel structures and properties of materials containing intricate multiatomic constituent structural units.