Metals can be defined as materials with a Fermi surface or as materials exhibiting metallic conduction (i.e., $d\rho /dT>0$). Usually, these definitions both hold at low temperatures, such as liquid-helium temperatures, as the Fermi energy is sufficiently larger than the thermal energy. However, they may not both hold in correlated electron systems where the Fermi energy is reduced by renormalization. In this paper, we demonstrate that although the resistivity of ${\mathrm{CeFe}}_2{\mathrm{Al}}_{10}$ increases with decreasing temperature below $\sim 20$ K, ${\mathrm{CeFe}}_2{\mathrm{Al}}_{10}$ is a metal with a Fermi surface. This assertion is based on the observation of Shubnikov–de Haas oscillations and a Hall resistivity that changes sign with the magnetic field, which requires the coexistence of electron and hole carriers. Our analysis of Shubnikov–de Haas and magnetotransport data indicates that the Fermi energies are as small as $\sim 30$ K and that, despite the increasing carrier mobility with decreasing temperature as in conventional metals, the loss of thermally excited carriers leads to nonmetallic conduction below $\sim 20$ K. Furthermore, we investigate how this anomalous metal transforms to a more conventional metal with metallic conduction with the application of high pressure and a high magnetic field.

We report on the pressure evolution of the electronic and crystal structures of the noncentrosymmetric antiferromagnet ${\mathrm{EuCoGe}}_3$. Using a diamond anvil cell, we performed high pressure fluorescence detected near-edge x-ray absorption spectroscopy at the Eu $L_3$, Co $K$, and Ge $K$ edges and synchrotron powder x-ray diffraction. In the Eu $L_3$ spectrum, both divalent and trivalent Eu peaks are observed from the lowest pressure measurement ($\sim 2$ GPa). By increasing pressure, the relative intensity of the trivalent Eu peak increases, and an average Eu valence continuously increases from 2.2 at 2 GPa to 2.31 at $\sim 50$ GPa. On the other hand, no discernible changes are observed in the Co $K$ and Ge $K$ spectra as a function of pressure. With the increase in pressure, lattice parameters continuously decrease without changing $I4mm$ symmetry. Our study revealed a robust divalent Eu state and an unchanged crystal symmetry of ${\mathrm{EuCoGe}}_3$ against pressure.

We study the effects of strong Coulomb disorder on the transverse magnetoresistance in Weyl semimetals at low temperatures. Using the diagrammatic technique and the Keldysh model to sum up the leading terms in the diagrammatic expansion, we find that the linear magnetoresistance exhibits a strong renormalization due to the long-range nature of the Coulomb interaction ${\rho }_{xx}\propto Hln(eH\hslash v^2/c{T}_{\mathrm{imp}}^2),\mathrm{\Omega }{\alpha }^{-1/6}\ll T_{\mathrm{imp}}\ll \mathrm{\Omega }/{\alpha }^{-3/4}$, where $\mathrm{\Omega }=v\sqrt{2eH\hslash /c}$ is the distance between the zeroth and the first Landau levels, $T_{\mathrm{imp}}=\hslash v{n}_{\mathrm{imp}}^{1/3}$ measures the strength of the impurity potential in terms of the impurity concentration $n$ and the Fermi velocity $v$, and $\alpha =e^2/\hslash v$ is the effective fine structure constant of the material. As disorder becomes even stronger (but still in the parametric range, where the Coulomb interaction can be treated as a long-range one), we find that the magnetoresistivity becomes quadratic in the magnetic field ${\rho }_{xx}\propto H^2$.

By developing a cluster sampling of the stochastic series expansion quantum Monte Carlo method, we investigate a spin-$\frac{1}{2}$ model on a bilayer square lattice with intralayer ferromagnetic (FM) Ising coupling and interlayer antiferromagnetic Heisenberg interaction. The continuous quantum phase transition which occurs at $g_c=3.045\left(2\right)$ between the FM Ising phase and the dimerized phase is studied via large-scale simulations. From analysis of the critical exponents we show that this phase transition belongs to the ($2+1$)-dimensional Ising universality class. In addition, the quantum entanglement is strong between the two layers, especially in the dimerized phase. The effective Hamiltonian of a single layer seems like a transverse-field Ising model. However, we found that the quantum entanglement Hamiltonian is a pure classical Ising model without any quantum fluctuations. Furthermore, we give a more general explanation about how a classical entanglement Hamiltonian emerges.

Nuclear spins show exceptionally long coherence times but the underlying good isolation from their environment is a challenge when it comes to controlling nuclear spin qubits. A particular difficulty, not only for nuclear spin qubits, is the realization of two-qubit gates between distant qubits. Recently, strong coupling between an electron spin and microwave resonator photons as well as a microwave resonator mediated coupling between two electron spins both in the resonant and the dispersive regime have been reported and, thus, a microwave resonator mediated electron spin two-qubit gate seems to be in reach. Inspired by these findings, we theoretically investigate the interaction of a microwave resonator with a hybrid quantum dot-donor (QDD) system consisting of a gate-defined Si QD and a laterally displaced ${}^{31}\mathrm{P}$ phosphorous donor atom implanted in the Si host material. We find that driving the QDD system allows to compensate the frequency mismatch between the donor nuclear spin splitting in the MHz regime and typical superconducting resonator frequencies in the GHz regime, and also enables an effective nuclear spin-photon coupling. While we expect this coupling to be weak, we predict that coupling the nuclear spins of two distant QDD systems dispersively to the microwave resonator allows the implementation of a resonator-mediated nuclear spin two-qubit $\sqrt{i\mathrm{SWAP}}$ gate with a gate fidelity approaching $90\%$.

Quantum states of a few-particle system capacitively coupled to a metal gate can be discriminated by measuring the quantum capacitance, which can be identified with the second derivative of the system energy with respect to the gate voltage. This approach is here generalized to the multivoltage case, through the introduction of the quantum capacitance matrix. The matrix formalism allows us to determine the dependence of the quantum capacitance on the direction of the voltage oscillations in the parameter space and to identify the optimal combination of gate voltages. As a representative example, this approach is applied to the case of a quantum dot array, described in terms of a Hubbard model. Here, we first identify the potentially relevant regions in the multidimensional voltage space with the boundaries between charge stability regions, determined within a semiclassical approach. Then, we quantitatively characterize such boundaries by means of the quantum capacitance matrix. Altogether, this provides a procedure for optimizing the discrimination between states with different particle numbers and/or total spins.

Quasisymmetry is an approximated symmetry in the sense that it is not the crystalline symmetry but commutes with the Hamiltonian at a lower order. Unlike exact symmetry, quasisymmetry emerges in the context of low-energy effective theory. One of the most desirable properties added by quasisymmetry is the large Berry curvatures due to the perturbatively small band gaps, which are robust against chemical and physical perturbations. Ising spin-orbit coupling (SOC) is a special type of SOC with electron spins polarizing to out-of-plane direction and can protect the spin coherence against the relaxation. For Ising-type superconductors, Ising SOC significantly affects the superconducting properties such as Pauli limit. It is commonly believed that the out-of-plane mirror symmetry $M_z$ is necessary for Ising SOC, which puts a constraint on the allowed materials. Here we demonstrate that the strong Ising SOC exists beyond the system with $M_z$ symmetry and is protected by the quasisymmetry. Based on the group-theoretical analysis and density-functional theory calculations, we show that the lower-order SOC Hamiltonian for certain representation is Ising type, which is protected by the quasisymmetry. Several promising systems with strong Ising SOC of several hundreds of meV near the Fermi energy are identified. In addition, we propose the general design principles to search for the realistic materials. Our results advance the fundamental understanding of Ising spin-orbit physics and greatly broaden the range of the materials hosting the Ising SOC.

The terahertz (THz) electrodynamics of few-layer $\mathrm{W}{\mathrm{Te}}_2$ is dominated by the plasmon response. However, THz surface plasmons (SPs) with long wavelengths in two-dimensional exfoliated crystals are typically confined by the lateral geometry. Direct visualization of the plasmonic standing wave patterns is challenging due to the spatial confinement and low quality factor of the SP, especially for samples that are only a few monolayers thick. Here, we resolve subtle real-space features of the plasmonic response of $\mathrm{W}{\mathrm{Te}}_2$ by augmenting more common scattering amplitude experiments with the phase contrast accomplished within the time-domain version of THz nanoimaging. Amplitude and phase images allow us to quantitatively evaluate the evolution of the plasmonic response at cryogenic temperatures in samples with variable thickness from 3 to 12 monolayers. The proposed imaging modality is universally applicable to the THz near-field nanoscopy of low-dimensional materials.

Despite enormous experimental and theoretical efforts, obtaining generally accepted conclusions regarding the intrinsic magnetic and electronic properties of superconducting nickelates remains exceptionally challenging. Experiments show a significant degree of uncertainty, indicating hidden factors in the synthesized films, which call for further investigations. One of those hidden factors is the possibility of intercalating hydrogen during the chemical reduction process from $\mathrm{Nd}\left(\mathrm{La}\right){\mathrm{NiO}}_3$ to $\mathrm{Nd}\left(\mathrm{La}\right){\mathrm{NiO}}_2$ using ${\mathrm{CaH}}_2$. While hydrogen has been detected in experimental samples, not much is known about its distribution through the crystal and its influence on the electronic environment. Here, we show the tendency toward the formation of one-dimensional hydrogen chains in infinite-layers ${\mathrm{LaNiO}}_2$ superconductors using density-functional theory supplemented by dynamical mean-field theory. The formation of such hydrogen chains induces a coexistence of different oxidation states of Ni and competing magnetic phases, and possibly explains the recently observed charge order states in nickelate superconductors. Furthermore, it contributes to the difficulty of synthesizing homogeneous nickelates and determining their ground states. The smoking gun to detect excess hydrogen in nickelates is the flat phonon modes, which are infrared active and quite insensitive to the exact arrangement of the H atoms.

Braiding and fusion rules of topological excitations are indispensable topological invariants in topological quantum computation and topological orders. While excitations in two dimensions (2D) are always particlelike anyons, those in three dimensions (3D) incorporate not only particles but also loops, spatially nonlocal objects, making it novel and challenging to study topological invariants in higher dimensions. While 2D fusion rules have been well understood from bulk Chern-Simons field theory and edge conformal field theory, it is yet to be thoroughly explored for 3D fusion rules from higher-dimensional bulk topological field theory. Here, we perform a field-theoretical study on (i) how loops that carry Abelian gauge fluxes fuse and (ii) how loops are shrunk into particles in the path integral, which generates fusion rules, loop-shrinking rules, and descendent invariants, e.g., quantum dimensions. We first assign a gauge-invariant Wilson operator to each excitation and determine the number of distinct excitations through equivalence classes of Wilson operators. Then, we adiabatically shift two Wilson operators together to observe how they fuse and are split in the path integral; despite the Abelian nature of the gauge fluxes carried by loops, their fusions may be of non-Abelian nature. Meanwhile, we adiabatically deform world sheets of unknotted loops into world lines and examine the shrinking outcomes; we find that the resulting loop-shrinking rules are algebraically consistent to fusion rules. Interestingly, fusing a pair of loop and antiloop may generate multiple vacua, but fusing a pair of anyon and antianyon in 2D has one vacuum only. By establishing a field-theoretical ground for fusion and shrinking in 3D, this work leaves intriguing directions for future exploration, e.g., symmetry enrichment, quantum gates, and topological order of braided monoidal 2-category of 2-group.

We show that the analytic structure of the dynamical exchange-correlation (xc) kernels of semiconductors and insulators can be sensed in terms of its poles, which mark physically relevant frequencies of the system where the counterphase motion of discrete collective excitations occurs: If excited, the collective modes counterbalance each other, making the system exhibit none at all or extremely weak density response. This property can be employed to construct simple and practically relevant approximations of the dynamical xc kernel for time-dependent density functional theory (TDDFT). Such kernels have simple analytic structures, are able to reproduce dominant excitonic features of the absorption spectra of monolayer semiconductors and bulk solids, and promise high potential for future uses in efficient real-time calculations with TDDFT.

In condensed matter physics, materials with kagome lattice display a range of exotic quantum states, including charge density wave (CDW), superconductivity, and magnetism. Recently, the intermetallic kagome metal ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$ was discovered to undergo a first-order structural phase transition with the formation of a $\sqrt{3}\times \sqrt{3}\times 3$ CDW at around 92 K. The bulk electronic band properties are crucial to understanding the origin of the structural phase transition. Here, we conducted an optical spectroscopy study in combination with band structure calculations across the structural transition. Our findings showed abrupt changes in the optical reflectivity/conductivity spectra as a result of the structural transition, without any observable gap formation behavior. The optical measurements and band calculations actually reveal a sudden change of the band structure after transition. It is important to note that this phase transition is of the first-order type, which distinguishes it from conventional density-wave type condensations. Our results provide an insight into the origin of the structural phase transition in this new and unique kagome lattice intermetallic material.

Higher-order topology in non-Hermitian (NH) systems has recently become one of the most promising and rapidly developing fields in condensed-matter physics. Many distinct phases that were not present in the Hermitian equivalents are revealed in these systems. In this work, we examine how higher-order Weyl semimetals are impacted by NH perturbation. We identify a new type of topological semimetal, i.e., non-Hermitian higher-order Weyl semimetal (NHHOWS) with surface diabolic points. We demonstrate that in such an NHHOWS, new exceptional points inside the bulk can be created and annihilated, therefore allowing us to manipulate their number. At the boundary, these exceptional points are connected through unique surface states with diabolic points and hinge states. For specific system parameters, the surface of NHHOWS behaves as a Dirac phase with linear dispersion or a Luttinger phase with a quadratic dispersion, thus paving a way for Dirac-Luttinger switching. Finally, we employ the biorthogonal technique to reinstate the standard bulk-boundary correspondence for NH systems and compute the topological invariants. The obtained quantized biorthogonal Chern number and quadruple moment topologically protect the unique surface and hinge states, respectively.

Graphene is a two-dimensional Dirac semimetal showing interesting properties as a result of its dispersion relation with both quasiparticles and quasiholes or matter and antimatter. We introduce a topological nodal ring semimetal in graphene with a robust one-dimensional chiral edge mode similar to the quantum Hall effect and a quadratic Fermi-liquid spectrum for the quasiparticles and quasiholes in the bulk. We describe its realization through circularly polarized light. The bulk band degeneracy at the Fermi energy is protected through a ${\mathbb{Z}}_2$ symmetry related to the two spin polarizations of an electron and a “double-orthogonality” structure in the sublattice and spin quantum numbers of the two crossing eigenstates. The system may have applications in nanoelectronics and in quantum mechanical entanglement applied to band theory.

We show that ultrathin metasurfaces with a specific multiresonant response can enable simultaneously arbitrarily strong and arbitrarily broadband dispersion compensation, pulse (de)chirping, and compression or broadening. This breakthrough overcomes the fundamental limitations of both conventional nonresonant approaches (bulky) and modern singly resonant metasurfaces (narrowband) for quadratic phase manipulations of electromagnetic signals. The required nonuniform trains of resonances in the electric and magnetic sheet conductivities that completely control phase delay, group delay, and chirp are rigorously derived and the limitations imposed by fundamental physical constraints are thoroughly discussed. Subsequently, a practical, truncated approximation by finite sequences of physically realizable linear resonances is constructed and the associated error is quantified. By appropriate spectral ordering of the resonances, operation can be achieved either in transmission or reflection mode, enabling full space coverage. The proposed concept is not limited to dispersion compensation, but introduces a generic and powerful ultrathin platform for the spatiotemporal control of broadband real-world signals with a myriad of applications in modern optics, microwave photonics, radar, and communication systems.

The interaction between magnons and mechanical vibrations dynamically modifies the properties of the mechanical oscillator, such as its frequency and decay rate. Known as dynamical backaction, this effect is the basis for many theoretical protocols, such as entanglement generation or mechanical ground-state cooling. However, dynamical backaction is also detrimental for specific applications. Here, we demonstrate the implementation of a triple-resonance cavity magnomechanical measurement that fully evades dynamical backaction effects. Through careful engineering, the magnomechanical scattering rate into the hybrid magnon-photon modes can be precisely matched, eliminating dynamical backaction damping. Backaction evasion is confirmed via the measurement of a drive-power-independent mechanical linewidth.

$1/f$ electronic noise is a conductance fluctuation, expressed in terms of a mobility “$\alpha$-noise” by Hooge and Kleinpenning. Understanding this noise in graphene is key for high-performance electronics. Early investigations pointed out a deviation from the standard Hooge formula, with the free-carrier density substituted by a constant density $n_{\mathrm{\Delta }}\sim {10}^{12}{\mathrm{cm}}^{-2}$. Here we investigate hBN-encapsulated graphene transistors where high mobility gives access to the velocity-saturation regime. We show that $\alpha$-noise is still accounted for by the Hooge formula on substituting conductance by differential conductance $G$, resulting in a bell-shaped dependence of flicker noise with bias voltage. The same analysis holds in the Zener regime at even larger bias, with two main differences. The first one is a strong enhancement of the Hooge parameter reflecting the hundred-times larger coupling of interband excitations to the hyperbolic phonon-polariton (HPhP) modes of the midinfrared Reststrahlen (RS) bands of hBN, which is supported by microwave noise thermometry measurements. The second is an exponential suppression of this coupling at large fields, which we attribute to decoherence effects. The phenomenology of $1/f$ noise in graphene supports a quantum-coherent bremsstrahlung interpretation of $\alpha$-noise.

An interferometric device is proposed in order to analyze the quartet mode in biased three-terminal Josephson junctions (TTJs), and to provide experimental evidence for the emergence of a single stationary phase, the so-called quartet phase. In such a quartet superconducting quantum interference device (quartet SQUID), the flux sensitivity exhibits period $hc/4e$, which is the fingerprint of a transient intermediate state involving two entangled Cooper pairs. The quartet SQUID provides two pieces of information: an amplitude that measures a total “quartet critical current,” and a phase lapse coming from the superposition of the following two current components: the quartet supercurrent which is odd in the quartet phase, and the phase-sensitive multiple Andreev reflection (phase MAR) quasiparticle current, which is even in the quartet phase. This makes a TTJ a generically “$\theta$ junction.” Evidence for phase MARs plays against conservative scenarios involving synchronization of AC Josephson currents, based on “adiabatic” phase dynamics and resistively shunted junction–like models.

The selective confinement of light holes (LHs) is demonstrated by introducing a low-dimensional system consisting of a highly tensile-strained Ge quantum well enabling the design of an ultrafast gate-defined spin qubit under the electric dipole spin resonance. The qubit size-dependent $g$ factor and dipole moment are mapped, and the parameters inducing their modulation are discussed. It is found that the LH qubit dipole moment is two to three orders of magnitude higher than that of the canonical heavy-hole qubit. This behavior originates from the significant spin splitting resulting from the combined action of large cubic and linear Rashba spin-orbit interactions that are peculiar to LHs. The qubit relaxation rate is also affected by the strong spin-orbit interaction and follows typically a $B^7$ behavior. The proposed all-group IV, direct band-gap LH qubit provides an effective platform for a scalable qubit-optical photon interface sought after for long-range entanglement distribution and quantum networks.

Vanadium dioxide ($\mathrm{V}{\mathrm{O}}_2$) has drawn significant attention for its near room temperature insulator-to-metal transition and associated structural phase transition. The underlying Physics behind the temperature induced insulator-to-metal and concomitant structural phase transition in $\mathrm{V}{\mathrm{O}}_2$ is yet to be fully understood. We have investigated the kinetics of the phase transitions of $\mathrm{V}{\mathrm{O}}_2$ with the help of resistivity measurements and Raman spectroscopy. Resistance thermal hysteresis scaling and relaxation measurements across the temperature induced insulator-to-metal transition reveal an unusual behavior of this first-order phase transition, whereas relaxation phenomena investigated by Raman spectroscopy show that the temperature induced monoclinic to rutile phase transition in $\mathrm{V}{\mathrm{O}}_2$ follows usual behavior and is consistent with mean field prediction. Insulator-to-metal and structural phase transitions have been found to decouple with an increased temperature sweep rate. The observed unusual thermal hysteresis scaling behavior with temperature sweep rate during insulator-to-metal transition may be the consequences of independent diffusion of charge and heat due to unconventional quasiparticle dynamics in $\mathrm{V}{\mathrm{O}}_2$. Unconventional quasi particle dynamics, i.e., significantly lowered electronic thermal conductivity across insulator-to-metal transition in our sample is verified by ultrafast optical pump-probe time domain thermoreflectance measurements.