Understanding the physical properties of unconventional superconductors as well as of other correlated materials presents a formidable challenge. Their unusual evolution with doping, frequency, and temperature has frequently led to non-Fermi-liquid (non-FL) interpretations. Optical conductivity is a major challenge in this context. Here, the optical spectra of two archetypal cuprates, underdoped ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ and optimally doped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$, are interpreted based on the standard Fermi-liquid (FL) paradigm. At both dopings, perfect frequency-temperature FL scaling is found to be modified by the presence of a second, gapped electronic subsystem. This non-FL component emerges as a well-defined mid-infrared spectral feature after the FL contribution, determined independently by transport, is subtracted. Temperature, frequency, and doping evolution of the MIR feature identify a gapped rather than dissipative response. In contrast, the dissipative response is found to be relevant for pnictides and ruthenates. Such an unbiased FL/non-FL separation is extended across the cuprate phase diagram, capturing all the key features of the normal state and providing a natural explanation why the superfluid density is attenuated on the overdoped side. Thus, we obtain a unified interpretation of optical responses and transport measurements in all analyzed physical regimes and all analyzed compounds.

We introduce an exact framework to compute the positive frequency moments $M^{\left(\alpha \right)}\left(\mathbf{q}\right)=\langle {\omega }^{\alpha }\rangle$ of different dynamic properties from imaginary-time quantum Monte Carlo data. As a practical example, we obtain the first five moments of the dynamic structure factor $S(\mathbf{q},\omega )$ of the uniform electron gas at the electronic Fermi temperature based on ab initio path integral Monte Carlo simulations. We find excellent agreement with known sum rules for $\alpha =1,3$, and present results for $\alpha =2,4,5$. Our idea can be straightforwardly generalized to other dynamic properties such as the single-particle spectral function $A(\mathbf{q},\omega )$, and will be useful for a number of applications, including the study of ultracold atoms, exotic warm dense matter, and condensed matter systems.

We introduce the generalization of the slave-spin mean-field method to broken-symmetry phases. Through a variational approach we derive the single-particle energy shift in the mean-field equations which generates the appropriate self-consistent field responsible for the stabilization of the broken symmetry. With this correction the different flavors of the slave-spin mean field are actually the same method and they give identical results to Kotliar-Ruckenstein slave bosons and to the Gutzwiller approximation. We apply our formalism to the Néel antiferromagnetic state and study it in multiorbital models as a function of the number of orbitals and Hund's coupling strength, providing phase diagrams in the interaction-doping plane. We show that the doped antiferromagnet in proximity of half filling is typically unstable toward insulator-metal and magnetic-nonmagnetic phase separation. Hund's coupling extends the range of this antiferromagnet, and favors its phase separation.

The semi-Dirac point of type-II semi-Dirac (SD) materials is a merging of triple Dirac points, distinguishing from the conventional type-I SD with double-Dirac point merging, and exhibits unique topological property. Here, we investigate its longitudinal and transverse optical conductivities. By controlling the evolution of the SD point with a perturbation parameter $\mathrm{\Delta }$, we present large longitudinal optical conductivity at the Van Hove singularity, not only in linear but also in parabolic directions. Furthermore, we find the nonzero dynamical Hall conductivity, which is sensitive to the Fermi energy and Dirac mass. Through introducing a momentum-dependent mass term, e.g., irradiating with circularly polarized light, the dynamical Hall conductivity exhibits more featured structures, depending on the parameter $\mathrm{\Delta }$, due to opening new channels of interband transitions. It is found that the frequency-dependent Kerr/Faraday angle can present all features of the dynamical Hall conductivity at characteristic frequencies. By detecting the Kerr or Faraday spectra, it is helpful to understand the physics of evolution of the SD to Dirac regime and further to extract the systemic parameters of SD materials from characteristic frequencies.

Most of the monolayer transition metal dichalcogenides (TMDCs) are chemically stable and have a combination of electronic, linear, and nonlinear optical (NLO) characteristics not generally found in bulk solids. This combination of traits has driven intense interest in their application in a variety of optoelectronic and (photo)electrocatalytic contexts. To realize these applications, it is often necessary to bring the TMDC monolayer into contact with a metal surface. However, while the interaction between TMDCs and dielectric substrates has been intensely studied, much less has been reported on the interaction between TMDCs and metals. Here we use azimuthal-dependent sum frequency generation (SFG) spectroscopy to study the interaction of monolayer ${\mathrm{MoS}}_2$ and gold by comparing the second-order NLO response of ${\mathrm{MoS}}_2$ on $\mathrm{Si}/{\mathrm{SiO}}_2$ as a function of azimuthal angle. In contrast to the well-known sixfold symmetric pattern of ${\mathrm{MoS}}_2$ on $\mathrm{Si}/{\mathrm{SiO}}_2$ in all polarization combinations, both the symmetric pattern and relative intensities of the azimuthal-dependent SFG response of ${\mathrm{MoS}}_2$ on Au depend strongly on polarization. Analysis of the components and magnitudes of the second-order nonlinear susceptibility (${\chi }^{\left(2\right)}$) of the ${\mathrm{MoS}}_2$/${\mathrm{SiO}}_2$, ${\mathrm{MoS}}_2$/Au, and gold substrate reveals a strong interaction between monolayer ${\mathrm{MoS}}_2$ and gold substrate broadly consistent with prior theoretical studies. Our measurement of the photon energy dependent nonlinear optical signal symmetry on ${\mathrm{MoS}}_2/\mathrm{Au}$ is strong evidence that we observe a mode-specific electronic effect that depends on the exciton resonance. In addition our approach allows the quantification of the ${\chi }^{\left(2\right)}$ of monolayer ${\mathrm{MoS}}_2$ on ${\mathrm{SiO}}_2$ and Au, $(7.9\pm 1.6)\times {10}^{-20}$, and $(1.4\pm 0.3)\times {10}^{-19}{\mathrm{m}}^2$/V, respectively. The current study demonstrates that the interaction between Au and a model TMDC monolayer substantially alters both the magnitude and symmetry of the NLO response. Such effects are important in application of these materials in optoelectronic devices and offer a contact-free probe of substrate-induced changes in the monolayer's electronic structure.

Chiral multifold fermions are quasiparticles that appear only in chiral crystals such as transition metal silicides in the cubic B20 structure (i.e., the CoSi family), and they may show exotic physical properties. Here we study the injection and shift photoconductivities and also the related geometrical quantities for several types of chiral multifold fermions, including spin-1/2 as well as pseudospin-1 and -3/2 fermions, dubbed as Kramers Weyl, triple-point, and Rarita-Schwinger-Weyl (RSW) fermions, respectively. We utilize the minimal symmorphic model to describe the triple-point fermions (TPF). We also consider the more realistic model Hamiltonian for the CoSi family including both linear and quadratic terms. We find that injection currents due to circularly polarized light are quantized as a result of the Chern numbers carried by the multifold fermions within the linear models. Surprisingly, we discover that in the TPF model, the linear shift conductivities, responsible for the shift current generation by linearly polarized light, are proportional to the pseudo spin-orbit coupling and independent of photon frequency. In contrast, for the RSW and Kramer Weyl fermions, the linear shift conductivity is linearly proportional to photon frequency. The numerical results agree with the power-counting analysis for quadratic Hamiltonians. The frequency independence of the linear shift conductivity could be attributed to the strong resonant symplectic Christoffel symbols of the flat bands. Moreover, the calculated symplectic Christoffel symbols show significant peaks at the nodes, suggesting that the shift currents are due to the strong geometrical response near the topological nodes.

Charge and heat transport of monolayer borophene are investigated by means of Boltzmann's transport equation, which is solved self-consistently and analyzed with the relaxation time approximation (RTA). To simulate a realistic experimental condition, a borophene monolayer is first considered to be affected by two types of randomly distributed impurity. Charge transport near the Dirac neutrality point and highly-doped regime changes according to the orientation of measurement, which reflects the anisotropy of the Dirac cone. Characteristic of electron at large doping follows the typical free electron gas but then decays onto interacting like ensemble at low doping signalled by an increase of resistivity around zero doping and a violation of Wiedemann-Franz's law. When acoustic phonon is accounted for, charge transport is found to be sensitive to the magnitude of electron-phonon interaction yielding further enhancement of resistivity at low and large doping. A resistivity consists of $T^4$ power law at low temperature, which evolves onto linear $T$ at high temperature. A similar trend is also observed when borophene is deposited on top of polar substrate, which manifests itself in the resistivity and mobility. Depending on the dielectric constant of the substrate, remote phonon scattering is shown to substantially affect transport at room temperature in a way that ultrahigh mobility at the order of ${10}^6{\mathrm{cm}}^2/\mathrm{Vs}$ falls down to the order of ${10}^4{\mathrm{cm}}^2/\mathrm{Vs}$. In contrast to charge transport, heat quantities such as thermopower, figure of merit do not provide any signature of anisotropy of low-energy dispersion and deviate from the Mott's formula. The impact of impurity or perturbation by lattice vibration could, however, be recognized upon varying temperature and charge density carrier. An interesting signature of electron-phonon interaction is shown by phonon drag effect, which not only varies with temperature according to $T^3$ but also depends on the orientation. It is argued that this unique trait might be a useful tool for the search of phonon drag effect in borophene, which has been proven to be difficult in other two-dimensional systems. The analysis is further elaborated with a survey on the electron cooling mechanism involving acoustic and optical phonon. At low temperature, power loss indicates that energy exchange between hot carrier and its environment depends on the carrier density, tilted velocity and dielectric constant. The use of polar substrate is shown to provide additional channel for relaxation of energy and momentum of hot electrons facilitating a faster cooling at high temperature.

Infinite-layer nickelates $\left(R{\mathrm{NiO}}_2\right)$ exhibit some distinct differences as compared to cuprate superconductors, leading to a debate concerning the role of rare-earth ions $(R=\mathrm{La},\mathrm{Pr},\mathrm{Nd})$ in the low-energy many-body physics. Although rare-earth $4f$ orbitals are typically treated as inert “core” electrons in studies, this approximation has been questioned. An active participation of $4f$ states is most likely for ${\mathrm{PrNiO}}_2$ based on an analogy to cuprates where Pr cuprates differ significantly from other cuprates. Here, we adopt density functional plus dynamical mean-field theory to investigate the role of Pr $4f$ orbitals and more generally the correlated electronic structure of ${\mathrm{PrNiO}}_2$ and its hole-doped variant. We find that the Pr $4f$ states are insulating and show no evidence for either a Kondo resonance or Zhang-Rice singlet formation as they do not have any hybridization channels near the Fermi energy. The biggest effects of hole doping are to shift the Pr $5d$ and $4f$ states further away from the Fermi energy whereas enhancing the Ni $3d\text{−}\mathrm{O}$ $2p$ hybridization, thus, reducing correlation effects as the O $2p$ states get closer to the Fermi energy. We again find no evidence for either Kondo or Zhang-Rice physics for the $4f$ states upon hole doping. We conclude by commenting on implications for other reduced valence nickelates.

The Fermi surface topology plays a crucial role in the study of high-temperature superconductivity cuprates. The conventional method for determining the Fermi surface is the maximum spectral intensity method, which involves numerical analytical continuation of Matsubara data for the one-body Green's function. However, the numerical analytical continuation is sensitive to the noise or the precision of the Matsubara data, and hence is not always reliable. With this in mind, we propose a simple and specific notion as a reference for the Fermi surface. It is the derivative of the momentum distribution function $n_{\mathit{k}}$ with respect to the chemical potential $\mu$. Our analysis of the noninteracting system shows that the momentum, at which $d{n}_{\mathit{k}}/d\mu$ takes the maximum value, constitutes a surface, which coincides with the Fermi surface. The relationship between $d{n}_{\mathit{k}}/d\mu$ and the Fermi surface in general cases is also analyzed. In order to numerically demonstrate this relationship, we calculate and compare on the two-dimensional Hubbard model within the HGW method. The results show that, at least in the weak and intermediate coupling regimes, using the notion of $d{n}_{\mathit{k}}/d\mu$ to determine the Fermi surface is reliable. We further studied the cases with model parameters standing for realistic materials, and found the surface determined by $d{n}_{\mathit{k}}/d\mu$ exhibits topological transition with the charge filling, similar to the Fermi surface in the cuprates. We believe that using the derivative of the momentum distribution function with respect to the chemical potential to determine the Fermi surface is a reasonable, efficient, and potentially valuable approach.

Nonreciprocal transport phenomena, such as current direction dependent resistance, have recently attracted substantial research interest due to the underlying physics and potential applications for high-performance rectifiers. Here we report the observation of nonreciprocal spin transport in Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ nanoplates under nonlocal configuration. Nonlocal spin voltage amplitude exhibits an anomalous nonreciprocity with respect to the direction of local bias current. It is found that the nonreciprocity is prominent in the region near current leads and becomes more significant upon increasing the bias current amplitude and tuning the Fermi level toward the Dirac point. This unusual nonreciprocity can be well understood by combining the charge current spreading and thermally induced spin polarization of topological surface states. Our results reveal the influence of thermal effect on the spin transport of topological surface states in nonlocal configuration, which should be valuable for thermal spintronics based on topological materials.

As a crucial signature of hydrodynamic phonon transport, second sound is currently raising research attention in the field of non-Fourier heat conduction. In this work, through molecular dynamics simulations, the fundamental characteristics of second sound are explored from a transient heat conduction modeling in the single-layer and multilayer graphene and graphite. Our simulation results demonstrate that second sound can carry more heat energy and maintain for a longer lifetime than that of the ballistic pulse. The underlying mechanisms are carefully discussed from the modal energy analysis. In addition, we also unveil that the thickness in multilayer graphene and graphite significantly suppresses the propagation of second sound due to the enhanced weight of umklapp scattering. The effects of excitation temperature and ambient temperature on the second sound are also studied. Our simulation results reveal that the second sound in single-layer graphene can persist up to 110 K. This work provides valuable insight into the basic characteristics of second sound, which should be critical in the understanding of hydrodynamic phonon transport and thermal physics in graphene and its derivatives.

The niobium surface is almost always covered by a native oxide layer which greatly influences the performance of superconducting devices. Here we investigate the highly stable niobium oxide overlayer of Nb(110), which is characterized by its distinctive nanocrystal structure as observed by scanning tunneling microscopy (STM). Our ab initio density functional theory (DFT) calculations show that a subtle reconstruction in the surface niobium atoms gives rise to rows of fourfold coordinated oxygen separated by regions of threefold coordinated oxygen. The fourfold oxygen rows are determined to be the source of the nanocrystal pattern observed in STM and the two chemical states of oxygen observed in core-level x-ray photoelectron spectroscopy (XPS) are ascribed to the threefold and fourfold oxygens. Furthermore, we find excellent agreement between the DFT calculated electronic structure with scanning tunneling spectroscopy and valence XPS measurements.

Quantum dot molecules (QDMs) are one of the few quantum light sources that promise deterministic generation of one- and two-dimensional photonic graph states. The proposed protocols rely on coherent excitation of the tunnel-coupled and spatially indirect exciton states. Here, we demonstrate power-dependent Rabi oscillations of direct excitons, spatially indirect excitons, and excitons with a hybridized electron wave function. An off-resonant detection technique based on phonon-mediated state transfer allows for spectrally filtered detection under resonant excitation. Applying a gate voltage to the QDM device enables a continuous transition between direct and indirect excitons and, thereby, control of the overlap of the electron and hole wave function. This does not only vary the Rabi frequency of the investigated transition by a factor of $\approx 3$, but also allows to optimize graph state generation in terms of optical pulse power and reduction of radiative lifetimes.

In this paper, we develop the basic formalism to study trions in semiconductor layered materials using the Faddeev equations in momentum space for three different particles lying in two dimensions. We solve the trion Faddeev coupled integral equations for both short-range one-term separable Yamaguchi potential and Rytova-Keldysh (RK) interaction applied to the $\mathrm{Mo}{\mathrm{S}}_2$ layer. We devise two distinct regularization methods to overcome the challenge posed by the repulsive electron-electron RK potential in the numerical solution of the Faddeev equations in momentum space. The first method regulates the repulsive interaction in the infrared region, while the second regulates it in the ultraviolet region. By extrapolating the trion energy to the situation without screening, the two methods gave consistent results for the $\mathrm{Mo}{\mathrm{S}}_2$ layer with a trion binding energy of $-49.5\left(1\right)$ meV for the exciton energy of $-753.3$ meV. We analyzed the trion structure for the RK and Yamaguchi potentials in detail, showing their overall similarities and the dominant cluster structure, where the strongly bound exciton is weakly bound to an electron. We found that this property is manifested in the dominance of two of the Faddeev components over the one where the hole is a spectator of the interacting electron pair.

The photonic spin Hall effect (PSHE) plays an important role in both fundamental science and precision metrology. In this paper, we theoretically propose a polarization-dependent bound state in the continuum (BIC) in a compound grating waveguide structure based on the selectable guided resonance at near-infrared wavelengths. Empowered by the unique resonant property and polarization-dependent property of the quasi-BIC, the transverse shift of the PSHE can be intensively enhanced to the order of hundreds of micrometers. Besides, the enhancement of the transverse shift of the PSHE is robust against the geometric parameters. Our work not only provides an all-dielectric platform to achieve giant PSHE, but also offers a viable approach to design high-performance PSHE-based optical devices.

We study the ground-state properties of the quantum spin liquid (QSL) phases of the spin-$1/2$ antiferromagnetic Heisenberg model on the triangular lattice with nearest- $\left(J_1\right)$, next-nearest- $\left(J_2\right)$, and third-neighbor $\left(J_3\right)$ interactions by using the density-matrix renormalization group (DMRG) method. By combining parallel DMRG with $\mathrm{SU}\left(2\right)$ spin rotational symmetry, we are able to obtain accurate results on large cylinders with length up to $L_x=48$ and circumference $L_y=6–12$. Our results suggest that the QSL phase of the $J_1\text{−}{J}_2$ Heisenberg model is gapped, which is characterized by the absence of a gapless mode and by short-range spin-spin and dimer-dimer correlations. In the presence of $J_3$ interaction, we find that a critical QSL with a single gapless mode emerges. While both spin-spin and scalar chiral-chiral correlations are short ranged, dimer-dimer correlations are quasi long ranged and decay as a power law at long distances.

We report continuous measurements of quasiparticles trapping and clearing from Andreev bound states in aluminum nanobridge Josephson junctions integrated into a superconducting-qubit-like device. We find that trapping is well modeled by independent spontaneous emission events. Above 80 mK the clearing process is well described by the absorption of thermal phonons, but other temperature-independent mechanisms dominate at low temperature. We find a complex structure in the dependence of the low-temperature clearing rate on the Andreev bound state energy. Our results shed light on quasiparticle behavior in qubitlike circuits.

We address the outstanding problem of electron pairing in the presence of strong Coulomb repulsion at small to moderate values of the Coulomb parameter, $r_s\lesssim 2$, and demonstrate that the pseudopotential framework is fundamentally biased and uncontrolled. Instead, one has to break the net result into two distinctively different effects: the Fermi liquid renormalization factor and the change in the effective low-energy coupling. The latter quantity is shown to behave nonmonotonically with an extremum at $r_s\approx 0.75$. Within the random-phase approximation, Coulomb interaction starts to enhance the effective pairing coupling at $r_s>2$, and the suppression of the critical temperature is entirely due to the renormalized Fermi liquid properties. Leading vertex corrections change this picture only quantitatively. Our results call for radical reconsideration of the widely accepted repulsive pseudopotential approach and show the need for precise microscopic treatment of Coulomb interactions in the problem of superconducting instability.

The two-dimensional (2D) to three-dimensional (3D) structural evolution of transition metal dichalcogenides (TMDCs) under high pressure is a significant subject since the change of dimensionality could induce a drastic change in various physical properties. A more compact 3D structure of TMDCs could result in metallization and even superconductivity. In this work, we report an unusual 3D state of an $\mathrm{Hf}{\mathrm{S}}_2$ crystal that can still maintain its semiconducting nature over a wide pressure range. X-ray diffraction results show that the layered $\mathrm{Hf}{\mathrm{S}}_2$ undergoes a reversible transformation to a 3D $Immm$ structure at $\sim 12$ GPa, accompanied by a semiconductor-to-semiconductor transition. The Raman and optical absorption measurements show that the phase transition may occur as early as $\sim 9.2$ GPa. The semiconducting state is stable up to $\sim 65$ GPa, and the metallization occurs at 68.6 GPa, the maximum pressure in this work. This work not only reports the 2D-to-3D structural phase transition in an $\mathrm{Hf}{\mathrm{S}}_2$ crystal driven by pressure, but also gives a clear physical picture that shows the intrinsic electrical transport properties and electronic behavior that enriches our understanding of the electronic behavior of TMDCs and sheds light on the future design of novel optoelectronic devices.