To investigate the normal-state magnetic properties of ${\mathrm{UTe}}_2$ under pressure, we perform ${}^{125}\mathrm{Te}$ nuclear magnetic resonance (NMR) measurements up to 2 GPa. Below 1.2 GPa, the $b$-axis NMR Knight shift shows a broad maximum at the so-called $T_{{\chi }_{\max }}$ on cooling, which is consistent with the magnetization measurement under pressure. $T_{{\chi }_{\max }}$ decreases with increasing pressure and disappears at the critical pressure $P_{\mathrm{c}}$ = 1.7 GPa, above which superconductivity is destroyed. This tendency is also observed in the temperature dependence of the nuclear spin-lattice relaxation rate $1/T_1$. At low pressures, $1/T_1$ shows a conventional Fermi-liquid behavior ($1/T_{1}T=\mathrm{const}$) at low temperatures, indicating the formation of the heavy-fermion state. Above $P_{\mathrm{c}}$, $1/T_{1}T$ follows a $1/T$ behavior without any crossover to the heavy-fermion state down to the lowest temperature ($\sim 3$ K). In addition, the NMR signals disappear below 3 K, due to the influence of the magnetically ordered moments. From the pressure dependence of the $T_{{\chi }_{\max }}$ and Knight shift, it was found that the Fermi surface character is abruptly changed at $P_{\mathrm{c}}$, and that superconductivity is observed only in the heavy-fermion state.

Metallic materials with kagome lattice structure are interesting because their electronic structures can host flat bands, Dirac cones, and van Hove singularities, resulting in strong electron correlations, nontrivial band topology, charge density wave (CDW), and unconventional superconductivity. Recently, kagome lattice compounds $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, Cs) are found to have intertwined CDW order and superconductivity. The origin of the CDW has been suggested to arise from Fermi-surface instabilities of van Hove singularity (saddle point) near the $M$ points with weak electron-phonon coupling. Here we use neutron scattering experiments to demonstrate that the CDW order in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ is associated with static lattice distortion and a sudden hardening of the $B_{3u}$ longitudinal optical phonon mode at the Brillouin zone boundary, thus establishing that the wave vector dependent electron-phonon coupling must also play an important role in the CDW order of $A{\mathrm{V}}_3{\mathrm{Sb}}_5$.

We deduce the symmetry of the pseudogap phase in the single- and double-layer bismuth-based cuprate superconductors, where charge and pseudogap order onset concurrently, by measuring and analyzing their circular and linear photogalvanic responses, which are related linearly to the chirality and inversion breaking, respectively, of the order parameter. After separating out the trivial contribution arising from the surface where inversion symmetry is already broken, we show that both responses start below the pseudogap temperature $T^{*}$ and grow below it to a sizable magnitude, revealing the broken symmetries in the bulk of the crystal. Through a detailed analysis of the dependence of the signals on the angle of incidence, the polarization of the light, and the orientation of the crystal, we are able to discover that the point group symmetry below $T^{*}$ is limited to $mm2$ or $mm2\underline{1}$ groups. Taking into account formation of domains and previous measurements, our results narrow down the possible symmetries of the microscopic origin of the phase transition(s) at $T^{*}$, which leads to the conclusion that while charge order may affect the observed symmetry it is not the main cause of the pseudogap order parameter.

We employ polarization-resolved electronic Raman spectroscopy and density functional theory to study the primary and secondary order parameters, as well as their interplay, in the charge density wave (CDW) state of the kagome metal $A{\mathrm{V}}_3{\mathrm{Sb}}_5$. Previous x-ray diffraction data at 15 K established that the CDW order in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ comprises of a $2\times 2\times 4$ structure: one layer of inverse-star-of-David and three consecutive layers of star-of-David pattern. We analyze the lattice distortions based on the $2\times 2\times 4$ structure at 15 K, and find that the $U_1$ lattice distortion is the primarylike (leading) order parameter while $M_1^{+}$ and $L_2^{-}$ distortions are secondarylike order parameters for vanadium displacements. This conclusion is confirmed by the calculation of bare susceptibility ${\chi }_0^{\prime }\left(q\right)$ that shows a broad peak at around $q_z=0.25$ along the hexagonal Brillouin zone face central line ($U$ line). We also identify several phonon modes emerging in the CDW state, which are lattice vibration modes related to V and Sb atoms as well as alkali-metal atoms. The detailed temperature evolution of these modes' frequencies, half-width at half-maximums, and integrated intensities support a phase diagram with two successive structural phase transitions in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$: the first one with a primarylike order parameter appearing at $T_S=94\mathrm{K}$ and the second isostructural one appearing at around $T^{*}=70\mathrm{K}$. Furthermore, the $T$ dependence of the integrated intensity for these modes shows two types of behavior below $T_S$: the low-energy modes show a plateaulike behavior below $T^{*}$ while the high-energy modes monotonically increase below $T_S$. These two behaviors are captured by the Landau free-energy model incorporating the interplay between the primarylike and the secondarylike order parameters via trilinear coupling. Especially, the sign of the trilinear term that couples order parameters with different wave vectors determines whether the primarylike and secondarylike order parameters cooperate or compete with each other, thus determining the shape of the $T$ dependence of the intensities of Bragg peak in x-ray data and the amplitude modes in Raman data below $T_S$. These results provide an accurate basis for studying the interplay between multiple CDW order parameters in kagome metal systems.

Impurity-vacancy complexes in diamond are an attractive family of spin defects since $\mathrm{N}{V}^{–}, \mathrm{Si}{V}^{–}, \mathrm{Ge}{V}^{–}$, and $\mathrm{Sn}{V}^{–}$ have emerged as promising platforms for quantum applications. Although boron is most easily incorporated into diamond, a boron-vacancy complex in the negative charge state $\left(\mathrm{B}{V}^{–}\right)$ has eluded experimental observation. This center was theoretically predicted as another promising spin qubit. In this work, we experimentally observed an electron paramagnetic resonance (EPR) spectrum identified as $\mathrm{B}{V}^{–}$ in synthetic diamonds via a Fermi-level tuning. Fingerprints of $\mathrm{B}{V}^{–}$ such as the spin multiplicity of $S=1, C_{3\mathrm{v}}$ symmetry, and the zero-field splitting ($D=2913$ MHz), in addition to ${}^{10}\mathrm{B}$ and ${}^{11}\mathrm{B}$ hyperfine (HF) interactions, have been confirmed. Moreover, optically pumped spin polarization has been observed with 3.0–3.6 eV excitation. However, unlike the $\mathrm{N}{V}^{–}$ center, the photoluminescence as well as optically detected magnetic resonance from $\mathrm{B}{V}^{–}$ have not been confirmed even at low temperatures. We speculate that the Jahn-Teller instability in the triplet excited states of the $\mathrm{N}{V}^{–}$ and $\mathrm{B}{V}^{–}$ centers results in different optical properties.

We study the superconducting gap function of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. By solving the linearized Eliashberg equation with a correlated pairing vertex extracted from a dynamical mean-field calculation we identify the dominant pairing channels. An analysis of the candidate gap functions in orbital and quasiparticle band basis reveals that an interorbital singlet pairing of even parity is in agreement with experimental observations. It reconciles in particular the occurrence of a two-component order parameter with the presence of line nodes of quasiparticles along the $c$ axis in the superconducting phase. The strong angular dependence of the gap along the Fermi surface is in stark contrast to its quasilocality when expressed in the orbital basis. We identify local interorbital spin correlations as the driving force for the pairing and thus reveal the continuation of Hund's physics into the superconducting phase.

Second-harmonic generation (SHG) is a well-known nonlinear spectroscopy method to probe electronic structure, specifically, in transition metal dichalcogenide (TMDC) monolayers. This work investigates the nonlinear dynamics of a strongly excited TMDC monolayer by solving the time evolution equations for the density matrix. It is shown that the presence of excitons qualitatively changes the nonlinear dynamics leading, in particular, to a huge enhancement of the nonlinear signal as a function of the dielectric environment. It is also shown that the SHG polarization angular diagram and its dependence on the driving strength are very sensitive to the type of exciton state. This sensitivity suggests that SHG spectroscopy is a convenient tool for analyzing the fine structure of excitonic states.

We investigate superconducting pairing tendencies of a two-dimensional electron fluid with both valley and spin degrees of freedom, both without and in the presence of an in-plane magnetic field. We present suggestive theoretical arguments that spontaneous valley polarization can lead to exotic singlet superconducting tendencies, including pair density wave order at zero field and reentrant superconductivity at high field. We also find a reduced magnetic response in the polarized valley, which allows a finite violation of the Pauli limit. These results are obtained by a mean-field approach to a generalized $t\text{−}J$ model on a triangular lattice and in the dilute limit. Phenomenological similarities to results of recent magic-angle twisted trilayer graphene experiments are noted.

Macroscopic continuous-medium descriptions fail to capture the electromagnetic response of mesoscopic-scale objects due to the quantum nature of electrons. Such deviations from classical treatments can be alleviated by introducing surface parameters known as the Feibelman $d$ parameters in the boundary conditions of electric and magnetic fields, which has been experimentally demonstrated. In this Research Letter, the Feibelman $d$ parameters are incorporated into transformation optics, and an analytical method is established to handle mesoscopic-scale objects with complex shapes and boundaries. Then, the loss spectra and quantum corrections to the plasmonic resonance can be obtained analytically and are consistent with fully numerical calculations. The theoretical method has been further applied to charged mesoscopic objects and various metals. Transformation optics converting complex structures into simple ones not only makes the analytical method accurate and efficient but also provides a general routine to investigate the light-matter interaction in mesoscopic-scale complex plasmonic systems.

We present a computational approach for electronically correlated metallic surfaces and interfaces, which combines density functional and dynamical mean-field theory using a multiorbital perturbative solver for the many-body problem. Our implementation is designed to describe ferromagnetic metallic thin films on a substrate. The performances are assessed in detail for a Fe monolayer on a W(110) substrate, a prototypical nanoscale magnetic system. Comparing our results to photoemission data, we find qualitative and quantitative improvements in the calculated spectral function with respect to the results of density functional theory within the local spin density approximation. In particular, the spin splitting of the $d$ states is drastically reduced and, at the same time, their spectral width becomes narrower. The method is, therefore, able to account for the main correlation effects in the system.

Strong gate control of proximity-induced spin-orbit coupling was recently predicted in bilayer graphene/transition metal dichalcogenide (BLG/TMDC) heterostructures, as charge carriers can easily be shifted between the two graphene layers, and only one of them is in close contact to the TMDC. The presence of spin-orbit coupling can be probed by weak antilocalization (WAL) in low-field magnetotransport measurements. When the spin-orbit splitting in such a heterostructure increases with the out-of-plane electric displacement field $\overline{D}$, one intuitively expects a concomitant increase of WAL visibility. Our experiments show that this is not the case. Instead, we observe a maximum of WAL visibility around $\overline{D}=0$. This counterintuitive behavior originates in the intricate dependence of WAL in graphene on symmetric and antisymmetric spin lifetimes, caused by the valley-Zeeman and Rashba terms, respectively. Our observations are confirmed by calculating spin precession and spin lifetimes from an $8\times 8$ model Hamiltonian of BLG/TMDC.

In this work, we explore experimentally and theoretically the spectrum of magnetic excitations of the ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Yb}}^{3+}$ ions in ytterbium iron garnet $\left({\mathrm{Yb}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}\right)$. We present a complete description of the crystal-field splitting of the $4\mathit{f}$ states of ${\mathrm{Yb}}^{3+}$, including the effect of the exchange field generated by the magnetically ordered Fe subsystem. We also consider a further effect of the Fe-Yb exchange interaction, which is to hybridize the Yb crystal field excitations with the Fe spin-wave modes at positions in the Brillouin zone where the two types of excitations cross. We present detailed measurements of these hybridized excitations, and we propose a framework that can be used in the quantitative analysis of the coupled spectra in terms of the anisotropic $4\mathit{f}\text{−}3d$ exchange interaction.

Although it is generally accepted that structural parameters like width, shape, and edge structure crucially affect the electronic characteristics of graphene nanoribbons (GNRs), the exact relationship between geometry and charge transport remains largely unexplored. In this paper, we present in situ through-transport measurements of various topological GNRs and GNR heterostructures by lifting the ribbon with the tip of a scanning tunneling microscope. At the same time, we develop a comprehensive transport model that enables us to understand various features, such as obscuring of localized states in through transport, the effect of topology on transport, as well as negative differential conductance in heterostructures with localized electronic modes. The combined experimental and theoretical efforts described in this paper serve to elucidate general charge transport phenomena in GNRs and GNR heterostructures.

Chiral anomaly or Adler-Bell-Jackiw anomaly in Weyl semimetals (WSMs) has a significant impact on the electron transport behaviors, leading to remarkable longitudinal or planar electrical and thermoelectric transport phenomena in the presence of electromagnetic gauge fields. These phenomena are consequences of the imbalanced chiral charge and energy induced by chiral anomaly in the presence of nonorthogonal external fields, namely $\mathit{E}·\mathit{B}\ne 0$ or $\mathit{B}·\nabla T\ne 0$ ($\mathit{E},\mathit{B}$, and $\mathbf{\nabla }T$ are the electric field, magnetic field, and thermal gradient, respectively). We here propose another two fascinating transport properties, namely, the nonlinear planar Nernst effect and nonlinear planar thermal Hall effect induced by chiral anomaly in the presence of $\mathit{B}·\nabla T\ne 0$ in WSMs. Using the semiclassical Boltzmann transport theory, we derive the analytical expressions for the chiral anomaly induced nonlinear Nernst and thermal Hall transport coefficients and also evaluate the fundamental mathematical relations among them in the nonlinear regime. The formulas we find in this current work are consistent with that predicted for the nonlinear anomalous electrical and thermoelectric effects induced by Berry curvature dipole recently. Additionally, in contrast to the recent work, by utilizing the lattice Weyl Hamiltonian with intrinsic chiral chemical potential, we find that the chiral anomaly induced nonlinear planar effects can exist even for a pair of oppositely tilted or nontilted Weyl cones in both time reversal and inversion broken WSMs. The chiral anomaly induced nonlinear planar effects predicted here along with the related parameter dependencies are hence possible to be realized in realistic WSMs in experiment.

We consider an electronic Hong-Ou-Mandel interferometer in the integer quantum Hall regime, where the colliding electronic states are generated by applying voltage pulses (creating for instance levitons) to ohmic contacts. The aim of this work is to investigate possible mechanisms leading to a reduced visibility of the Pauli dip, i.e., the noise suppression expected for synchronized sources. It is known that electron-electron interactions cannot account for this effect and always lead to a full suppression of the Hong-Ou-Mandel noise. Focusing on the case of filling factor $\nu =2$, we show instead that a reduced visibility of the Pauli dip can result from mixing of the copropagating edge channels, arising from tunneling events between them.

We present high-pressure resistance measurements on the newly discovered V-based superconductors $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A$ = Rb and K), which have an ideal kagome lattice of vanadium. Two superconducting domes under pressure are observed in both compounds, as previously observed in their sister compound ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. For ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$, the $T_{\mathrm{c}}$ increases from 0.93 K at ambient pressure to a maximum of 4.15 K at 0.38 GPa in the first dome. The second superconducting dome has the highest $T_{\mathrm{c}}$ of 1.57 K at 28.8 GPa. ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ displays a similar double-dome phase diagram, however, its two maximum $T_{\mathrm{c}}\mathrm{s}$ are lower, and the $T_{\mathrm{c}}$ drops faster in the second dome than ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$. An integrated temperature-pressure phase diagram of $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A$ = Cs, Rb, and K) is constructed, showing that the ionic radius of the intercalated alkali-metal atoms has a significant effect. Our work demonstrates that double-dome superconductivity under pressure is a common feature of these V-based kagome metals.

We describe the implementation and analysis of a first-principles theory, derived in an earlier work, for the leading terms in an expansion of a Gibbs free energy of a multicomponent alloy in terms of order parameters that characterize potential, compositional phases. The theory includes the effects of rearranging charge and other electronics from changing atomic occupancies on lattice sites. As well as the rigorous description of atomic short-range order in the homogeneously disordered phase, pairwise interaction parameters suited for atomistic modeling in a multicomponent setting can be calculated. From our study of an indicative series of the Cantor-Wu alloys, NiCo, NiCoCr, NiCoFeCr, and NiCoFeMnCr, we find that the interactions are not approximated well either as pseudobinary or restricted to nearest-neighbor range. Our computed order-disorder transition temperatures are low, consistent with experimental observations, and the nature of the ordering is dominated by correlations between Ni, Co, and Cr, while Fe and Mn interact weakly. Further atomistic modeling suggests that there is no true single-phase low-temperature ground state for these multicomponent systems. Instead the single-phase solid solution is kept stable to low temperatures by the large configurational entropy and the Fe and Mn dilution effects. The computational cost-effectiveness of our method makes it a good candidate for further exploration of the space of multicomponent alloys.

Drumhead surface states that link together loops of nodal lines arise in Dirac nodal-line semimetals as a consequence of the topologically nontrivial band crossings. We used low-temperature scanning tunneling microscopy and Fourier-transformed scanning tunneling spectroscopy to investigate the quasiparticle interference (QPI) properties of ZrSiTe. Our results show two scattering signals across portions of the drumhead state resolving the energy-momentum relationship through the occupied and unoccupied energy ranges it is predicted to span. Observation of this drumhead state is in contrast to previous studies on ZrSiS and ZrSiSe, where the QPI was dominated by topologically trivial bulk bands and surface states. Furthermore, we observe a near $\mathbf{k}\to -\mathbf{k}$ scattering process across the $\mathrm{\Gamma }$ point, enabled by scattering between the spin-split drumhead bands in this material, showing the persistence of the drumhead state even in the presence of spin-orbit coupling.

We analyze in detail the superconductivity that arises in an extended Hubbard model describing a multiband system with repulsive interactions. We show that virtual interband processes induce an effective attractive interaction for small momentum transfers, a situation not found in most models of superconductivity from repulsion. This attraction can be traced back, in real space, to the presence of correlated hopping terms induced by interband polarization. We reveal this physics with both strong-coupling expansion and many-body perturbation theory, supplemented by numerical calculations. Finally, we point out interesting similarities with the problem of interacting electrons in twisted bilayer graphene, suggesting the importance of the interband contribution to superconductivity.

We give a complete classification of fully symmetric as well as chiral ${\mathbb{Z}}_2$ quantum spin liquids on the pyrochlore lattice using a projective symmetry group analysis of Schwinger boson mean-field states. We find 50 independent Ansätze, including the 12 fully symmetric nearest-neighbor ${\mathbb{Z}}_2$ spin liquids that have been classified by Liu et al. [Phys. Rev. B 100, 075125 (2019)]. For each class, we specify the most general symmetry-allowed mean-field Hamiltonian. Additionally, we test the properties of a subset of the spin-liquid Ansätze by solving the mean-field equations for the spin-$1/2$ XXZ model near the antiferromagnetic Heisenberg point. We find four chiral spin liquids that break the screw symmetry of the lattice modulo time-reversal symmetry. These states have a different symmetry from the previously studied monopole flux state, and their unique characteristic is a $\frac{\pi }{3}$ flux enclosed by every rhombus of the lattice.