The recently observed unconventional ferroelectricity in AB bilayer graphene sandwiched by hexagonal boron nitride (hBN) presents a new platform to manipulate correlated phases in multilayered van der Waals heterostructures [Zheng et al., Nature 588, 71 (2020).]. We present a low-energy continuum model for AB bilayer graphene encapsulated by the top and bottom layers of either hBN or graphene, with two independent twist angles. For the graphene/hBN heterostructures, we show that twist angle asymmetry leads to a layer polarization of the valence and conduction bands. We also show that an out-of-plane displacement field not only tunes the layer polarization but also flattens the low-energy bands. We extend the model to show that the electronic structures of quadrilayer graphene heterostructure consisting of AB bilayer graphene encapsulated by the top and bottom graphene layers can similarly be tuned by an external electric field.

Detection of single, itinerant microwave photons is an important functionality for emerging quantum technology applications as well as of fundamental interest in quantum thermodynamics experiments on heat transport. In a recent experiment [W. Khan et al., Nat. Commun. 12, 5130 (2021)], it was demonstrated that a double quantum dot (DQD) coupled to a microwave resonator can act as an efficient and continuous photodetector by converting an incoming stream of photons to an electrical photocurrent. In the experiment, average photon and electron flows were analyzed. Here we theoretically investigate, in the same system, the fluctuations of the photocurrent through the DQD for a coherent microwave drive of the resonator. We consider both the zero-frequency full counting statistics as well as the finite-frequency noise (FFN) of the photocurrent. Numerical results and analytical expressions in limiting cases are complemented by a mean-field approach neglecting dot-resonator correlations, providing a compelling and physically transparent picture of the photocurrent statistics. We find that for ideal, unity efficiency detection, the fluctuations of the charge current reproduce the Poisson statistics of the incoming photons, while the statistics for nonideal detection is sub-Poissonian. Moreover, the FFN provides information of the system parameter dependence of detector short-time properties. Our results give insight into microwave photon-electron interactions in hybrid dot-resonator systems and provide guidance for further experiments on continuous detection of single microwave photons.

A multifold Hopf semimetal is a topological point node semimetal possessing an anisotropy in the internal electronic structure, e.g., the dipole structure of the Berry curvature. In this paper, the unique features of threefold Hopf semimetals in terms of the optical conductivity are theoretically investigated with a minimal theoretical model by using linear response theory for a linearly polarized photon. The frequency spectrum of the optical conductivity shows an anisotropic dependence on the polarization angle of the incident photon even if the electronic band structure is completely isotropic. The longitudinal optical conductivity linearly depends on the photon frequency and possesses steplike changes in the frequency spectrum. The anisotropic electronic structure has a varying number of steps with the orientation of the photon polarization axis. We reveal that the anisotropy is attributed to symmetries preserving the point node in threefold Hopf semimetals and that the threefold fermion in the Hopf semimetal is qualitatively different from so-called spin-1 fermion. The linearly polarized photon also induces a Hall current, but it vanishes with the photon polarization axis parallel to the Berry dipole axis. The numerical calculations show that these characteristic features can be observed even with a nonzero temperature and disorder.

${\mathrm{Ga}}_2{\mathrm{O}}_3$ is emerging as a promising wide band-gap semiconductor for high-power electronics and deep ultraviolet optoelectronics. It is highly desirable to dope it with controllable carrier concentrations for different device applications. This work reports a combined photoemission spectroscopy and theoretical calculation study on the electronic structure of Si doped ${\mathrm{Ga}}_2{\mathrm{O}}_3$ films with carrier concentration varying from $4.6\times {10}^{18}\mathrm{c}{\mathrm{m}}^{-3}$ to $2.6\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$. Hard x-ray photoelectron spectroscopy was used to directly measure the widening of the band gap as a result of occupation of conduction band and band-gap renormalization associated with many-body interactions. A large band-gap renormalization of 0.3 eV was directly observed in heavily doped ${\mathrm{Ga}}_2{\mathrm{O}}_3$. Supplemented with hybrid density functional theory calculations, we demonstrated that the band-gap renormalization results from the decrease in energy of the conduction band edge driven by the mutual electrostatic interaction between added electrons. Moreover, our work reveals that Si is a superior dopant over Ge and Sn, because $\mathrm{Si}3s$ forms a resonant donor state above the conduction band minimum, leaving the host conduction band mostly unperturbed and a high mobility is maintained though the doping level is high. Insights of the present work have significant implications in doping optimization of ${\mathrm{Ga}}_2{\mathrm{O}}_3$ and realization of optoelectronic devices.

The recent discovery of topologically nontrivial behavior in ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ stimulated a notable interest in this itinerant ferromagnet ($T_{\mathrm{C}}=174$ K). The exact magnetic state remains ambiguous, with several reports indicating the existence of a second transition in the range 125–130 K, with antiferromagnetic and glassy phases proposed to coexist with the ferromagnetic phase. Using detailed angle-dependent dc and ac magnetization measurements on large, high-quality single crystals we reveal a highly anisotropic behavior of both the static and dynamic response of ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$. It is established that many observations related to sharp magnetization changes when $B\parallel c$ are influenced by the demagnetization factor of a sample. On the other hand, a genuine transition has been found at $T_{\mathrm{P}}=128$ K, with the magnetic response being strictly perpendicular to the $c$ axis and several orders of magnitude smaller than for $B\parallel c$. Calculations using density-functional theory indicate that the ground state magnetic structure consist of magnetic moments canted away from the $c$ axis by a small angle ($\sim 1.5^{\circ }$). We argue that the second transition originates from a small additional canting of moments within the kagome plane, with two equivalent orientations for each spin.

The anomalous Hall effect (AHE) is a fundamental spin-dependent transport property that is widely used in spintronics. It is generally expected that currents carrying net spin polarization are required to drive the AHE. Here, we demonstrate that, in contrast to this common expectation, a spin-neutral tunneling AHE (TAHE), i.e., a TAHE driven by spin-neutral currents, can be realized in an antiferromagnetic (AFM) tunnel junction where an AFM electrode with a non-spin-degenerate Fermi surface and a normal metal electrode are separated by a nonmagnetic barrier with strong spin-orbit coupling (SOC). The symmetry mismatch between the AFM electrode and the SOC barrier results in an asymmetric spin-dependent momentum filtering of the spin-neutral longitudinal current generating the transverse Hall current in each electrode. We predict a sizable spin-neutral TAHE in an AFM tunnel junction with a ${\mathrm{RuO}}_2$-type AFM electrode and a SnTe-type SOC barrier and show that the Hall currents are reversible by the Néel vector switching. With the Hall angle being comparable with that in conventional AHE bulk materials, the predicted spin-neutral TAHE can be used for the Néel vector detection in AFM spintronics.

Full or empty narrow bands near the Fermi level are known to enhance superconductivity by promoting scattering processes and spin fluctuations. Here, we demonstrate that doublon-holon fluctuations in systems with half-filled narrow bands can similarly boost the superconducting $T_c$. We study the half-filled attractive bilayer Hubbard model on the square lattice using dynamical mean-field theory. The band structure of the noninteracting system contains a wide band formed by bonding orbitals and a narrow band formed by antibonding orbitals, with bandwidths tunable by the interlayer hopping. The shrinking of the narrow band can lead to a substantial increase in the superconducting order parameter and phase stiffness in the wide band. At the same time, the coupling to the wide band allows the narrow band to remain superconducting—and to reach the largest order parameter—in the flat band limit. We develop an anomalous worm sampling method to study superconductivity in the limit of vanishing effective hopping. By analyzing the histogram of the local eigenstates, we clarify how the interplay between different interaction terms in the bonding/antibonding basis promotes pair fluctuations and superconductivity.

We demonstrate high-precision dispersion tailoring in a metamaterial waveguide for wave compression and spatially tunable signal amplification. Spatial control over refractive index is achieved via digital controllers connected to piezoelectric unit cells. The digital shunt circuits allow numerically optimized circuit parameters to be uploaded directly to the experimental platform, enabling arbitrary spatial programming of the refractive index of the waveguide. Thus, the system acts as a reconfigurable piezoelectric array, altering the wavelength and group velocity of a given wave according to user specifications. In this work, optimizations are performed to implement gradual wave compression, realizing an effect analogous to that of an acoustic black hole. At the operating frequency of the design, the waveguide exhibits increased sensitivity and amplification of piezoelectric voltage output due to the compressed wavelength, shunt circuit resonance, and gradual variation in unit cell properties, avoiding internal reflections. Numerical and experimental results show that voltage amplification can be achieved in a desired position and frequency in the metamaterial waveguide.

In frustrated spin ladders, the interplay of frustration and correlations leads to the familiar Haldane (H) and rung-singlet (RS) phases. The nature of the transition between these two phases is still under debate. In this paper we tackle this issue using tools of quantum information theory. We consider frustrated mixed-spin-(1, 1/2) ladders with antiferromagnetic leg, rung and diagonal couplings, and calculate various quantities, such as the entanglement entropy (EE), the Schmidt gap, and the level degeneracy of the entanglement spectrum (ES). We use two numerical techniques, the infinite time-evolving block decimation (iTEBD) and the density matrix renormalization group (DMRG). We demonstrate that there exists an intermediate phase in which the ES levels do not exhibit the characteristic degeneracies of the H and RS phases. To understand the underlying physics in this phase, we investigate short-range spin correlations along legs, rungs and diagonals and show that in this intermediate phase long-wavelength modulations occur, akin to bond-order waves.

We study the opto-electric response in metallic wire structures. The aim is to understand the origin of helicity-dependent photoconductivity. For nonmagnetic metals this effect is generally believed to probe spin polarization. Using magnetic wires we show that this method enables background free imaging of spin textures. Analyzing the physical origin we find that the circular dichroism slightly modulates the absorption. The corresponding thermal modulation explains the measured electrical signals. We apply this result to examine the spin Hall effect induced spin accumulation in heavy metals. Here, we show that previously reported results in nonmagnetic wires are well reproducible, but not related to the spin polarization.

We report crystal structure, magnetization, and specific heat measurements on single crystals of the hexagonal polar magnet ${\mathrm{Co}}_{2\text{−}x}{\mathrm{Zn}}_x{\mathrm{Mo}}_3{\mathrm{O}}_8$ magnetically diluted by replacing Co by Zn. In contrast to the transformation from the antiferromagnetic to a ferrimagnetic state observed in the isostructural ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ upon small Zn doping, a robust antiferromagnetic behavior is preserved in Zn-doped ${\mathrm{Co}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ up to $x=0.55$. The Néel temperature decreases from $T_{\mathrm{N}}=40$ K at $x=0$ to 23 K at $x=0.55$, thus extrapolating to $x=1.27$ (36% filling) as the percolation threshold typical for a three-dimensional, highly coordinated network. This indicates strong magnetic couplings beyond the honeycomb planes in ${\mathrm{Co}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$. A sharp peak in the specific heat and a clear cusp in the susceptibility associated with the onset of magnetic order is observed up to $x=0.55$, whereas at $x=0.66$ these features are broadened due to increased disorder. Interestingly, the in-plane lattice parameter, the Curie-Weiss temperature, and the magnetic entropy vary with $x$ in a concerted but nonmonotonic manner. These observations can be traced back to the observed site-selective Zn substitution. We found that in the low-doping regime ($x<0.2$) ${\mathrm{Zn}}^{2+}$ ions primarily occupy the octahedrally coordinated sites, although they have a clear preference for occupying the tetrahedrally coordinated sites at higher doping levels. Due to the multiple interlayer exchange paths, dependent on the coordination of the ${\mathrm{Co}}^{2+}$ ions, this behavior is reflected in the nonmonotonic variation of the Curie-Weiss temperature and magnetic entropy with substitution.

We consider a disordered one-dimensional tight-binding model with power-law decaying hopping amplitudes to disclose wave-function maximum distributions related to the Anderson localization phenomenon. Deeply in the regime of extended states, the wave-function intensities follow the Porter-Thomas distribution while their maxima assume the Gumbel distribution. At the critical point, distinct scaling laws govern the regimes of small and large wave-function intensities with a multifractal singularity spectrum. The distribution of maxima deviates from the usual Gumbel form and some characteristic finite-size scaling exponents are reported. Well within the localization regime, the wave-function intensity distribution is shown to develop a sequence of pre-power-law, power-law, exponential, and anomalous localized regimes. Their values are strongly correlated, which significantly affects the emerging extreme values distribution.

Diamond, a material that exhibits ultrahigh thermal conductivity with a sensitive thermal response to stress, is an ideal heat-sink material in the embedded cooling paradigm. This study uses first-principles calculations with the phonon Boltzmann transport equation to predict the variation rules of the thermal conductivity of diamond along the $\langle 100\rangle$ crystal direction under strains at three orders of magnitude: 0.1%, 1%, and 10.5%. Density functional theory is used to predict the stress–strain dependence of diamond and the temperature-dependent thermal conductivity of unstrained diamond. The predictions are in good agreement with the experimental results. The calculated uniaxial strain–thermal conductivity dependence results reveal that the thermal conductivity of diamond abnormally increases by approximately 15% under small-scale uniaxial strain because of the weakened anharmonic interatomic force constants. Under large-scale strain, the thermal conductivity considerably decreases because of reduced phonon group velocities and increased numbers of phonon scattering channels. The findings in this study will guide analyses of the dependence of thermal conductivity on strain in other diamondlike structures, such as Group IV element-based materials. The abnormal thermal response at small strain is expected to lead to the realization of an artificial thermal conduction channel.

We theoretically propose a mechanism to understand the violation of the Pauli-Clogston limit for the upper critical field ${\mathrm{H}}_{\mathrm{c}2}$ observed in the Ce bearing heavy Fermion material ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ from the view point of spin singlet pairing. It is based on a duality concept, the dual simultaneous aspects of an electron: The itinerant part and localized part of quasiparticles (QPs) originated from the 4f electrons of the Ce atoms. While the itinerant QPs directly participate in forming the Cooper pairs, the localized QPs exert the internal field so as to oppose the applied field through the antiferromagnetic exchange interaction between them. This is inherent in the dense Kondo lattice system in general. We argue that this mechanism can be applied not only to the locally noncentrosymmetric material ${\mathrm{CeRh}}_2{\mathrm{As}}_2$, but also to globally inversion symmetry broken Ce-based materials such as ${\mathrm{CePt}}_3\mathrm{Si}$. Moreover, we point out that it also works for strongly Pauli-limit-violated spin triplet pairing systems, such as ${\mathrm{UTe}}_2$.

Recent experimental papers on hole-doped overdoped cuprates have argued that a series of observations showing unexpected behavior in the superconducting state imply the breakdown of the quasiparticle-based Landau–BCS paradigm in that doping range. In contrast, some of the present authors have argued that a phenomenological “dirty $d$-wave” theoretical analysis explains essentially all aspects of thermodynamic and transport properties in the superconducting state, provided the unusual effects of weak, out-of-plane dopant impurities are properly accounted for. Here we attempt to place this theory on a more quantitative basis by performing ab initio calculations of dopant impurity potentials for LSCO and Tl-2201. These potentials are more complex than the pointlike impurity models considered previously, and require calculation of forward scattering corrections to transport properties. Including realistic, ARPES-derived band structures, Fermi liquid renormalizations, and vertex corrections, we show that the theory can explain semiquantitatively the unusual superfluid density measurements of the two most studied overdoped materials.

In its ground state, ${\mathrm{RuO}}_2$ was long thought to be an ordinary metallic paramagnet. Recent neutron and x-ray diffraction revealed that bulk ${\mathrm{RuO}}_2$ is an antiferromagnet with $T_N$ above 300 K. Furthermore, epitaxial strain induces superconductivity in thin films of ${\mathrm{RuO}}_2$ below 2 K. Here, we present a resonant elastic x-ray scattering study at the $\mathrm{Ru}{L}_2$ edge of the strained ${\mathrm{RuO}}_2$ films exhibiting the strain-induced superconductivity. We observe an azimuthal modulation of the 100 Bragg peak consistent with bulk. Most notably, in the strained films displaying superconductivity, we observe a $\sim 1\mathrm{eV}$ shift of the $\mathrm{Ru}{e}_g$ orbitals to a higher energy. The energy shift is smaller in thicker, relaxed films and films with a different strain direction. Our results provide further evidence of the utility of epitaxial strain as a tuning parameter in complex oxides.

Candidate materials for the Kitaev spin liquid phase have been intensively studied recently because of their potential applications in fault-tolerant quantum computing. Although most of the studies on Kitaev spin liquid have been done in $4d$- and $5d$-based transition-metal compounds, recently there has been a growing research interest in Co-based quasi-two-dimensional honeycomb magnets, such as ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$ because of formation of spin-orbit-entangled $J_{\mathrm{eff}}=1/2$ pseudospin moments at ${\mathrm{Co}}^{2+}$ sites and potential realizations of Kitaev-like magnetism therein. Here, we obtain high-accuracy crystal and electronic structure of ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$ by employing combined density-functional and dynamical mean-field theory calculations, which correctly capture the Mott-insulating nature of the target system. We show that ${\mathrm{Co}}^{2+}$ ions form a high-spin configuration, $S=3/2$, with an active $L_{\mathrm{eff}}=1$ orbital degree of freedom, in the absence of spin-orbit coupling. The size of trigonal distortion within ${\mathrm{CoO}}_6$ octahedra is found to be not strong enough to completely quench the orbital degree of freedom, so that the presence of spin-orbit coupling can give rise to the formation of spin-orbit-entangled moments and the Kitaev exchange interaction. Our finding supports recent studies on potential Kitaev magnetism in this compound and other Co-based layered honeycomb systems.