We analyze quantum fluctuation effects at the onset of charge- or spin-density wave order with a $2k_F$ wave vector $\mathbf{Q}$ in two-dimensional metals, for the special case where $\mathbf{Q}$ connects a pair of hot spots situated at high symmetry points of the Fermi surface with a vanishing Fermi surface curvature. We compute the order parameter susceptibility and the fermion self-energy in one-loop approximation. The susceptibility has a pronounced peak at $\mathbf{Q}$, and the self-energy displays non-Fermi liquid behavior at the hot spots, with a linear frequency dependence of its imaginary part. The real part of the one-loop self-energy exhibits logarithmic divergencies with universal prefactors as a function of both frequency and momentum, which may be interpreted as perturbative signatures of power laws with universal anomalous dimensions. As a result, one obtains a non-Fermi liquid metal with a vanishing quasiparticle weight at the hot spots, and a renormalized dispersion relation with anomalous algebraic momentum dependencies near the hot spots.

The exchange interaction between carrier and localized spin in magnets generates intrinsic spin-orbit coupling induced by the magnetism. Recent studies for the exchange spin-orbit coupling (ESOC) in ferromagnets and antiferromagnets suggest the ESOC and the transport phenomena for spintronics. However, the study on the effects of ESOC on the magnetic phase transition has been still lacking especially for ferromagnets even though the ESOC should be one of the fundamental interactions to stabilize the magnetic phase. In this paper, we study the role of the ferromagnetic properties of the ESOC induced by the $p\text{−}d$ exchange interaction of the valence band in $n$-type ferromagnetic semiconductors. Based on the mean-field and $\mathit{k}·\mathit{p}$ methods, our theory explains the spin polarization of localized and carrier spin, and the Curie temperature $T_c$ for the iron concentration, electron density, and host semiconductor dependence. The theoretical $T_c$ fits experimental values well in low iron concentration and intermediate electron density regions, and the $p\text{−}d$ exchange interaction is essential for $T_c$. Furthermore, the $T_c$ is higher for narrower gap host semiconductors to provide insights into higher $T_c$. The theoretical framework of this paper is not limited to the specific materials and will find potential materials for spintronics and the fundamental aspects.

We investigated the electrical properties of a unipolar organic device by intentionally inserting traps at specific positions within the device. Our findings demonstrate that the trap position has a considerable impact on the charge distribution, electric field, and charge transport behavior of the device. In particular, when the traps are positioned closer to a charge-injection electrode, the organic layer containing the traps experiences more significant band bending, resulting in a higher charge injection barrier. We propose an electrical model that fully explains the observed changes in the electrical properties of the device as a function of trap position.

We propose that light-matter coupling can be used to realize synthetic lattices. In particular, we consider a one-dimensional chain of exciton-photon sites to create a comb lattice that exhibits a transition from a flat band to a finite mass dispersion by tuning site-dependent light-matter coupling. Moreover, in a non-Hermitian system with gain and loss, the flat-band phase is much more robust, and the transition is accompanied by the appearance of exceptional points in the complex energy spectrum. We demonstrate that by engineering the light-matter coupling in the synthetic lattice, one can explore various phases in the lasing regime. Our proposal paves the way for studying non-Hermitian systems in higher dimensions.

The surface electronic structure of Re(0001) has been investigated in a combined experimental and theoretical study. Spin- and angle-resolved photoemission was employed to unravel the spin-dependent $E\left({\mathbf{k}}_{\parallel }\right)$ dispersion of electronic states along the $\overline{\mathrm{\Gamma }}\overline{\mathrm{M}}$ and $\overline{\mathrm{\Gamma }}\overline{\mathrm{K}}$ directions. The results are compared with band-structure calculations based on density-functional theory. Additional calculations include transitions into final states by taking into account the corresponding matrix elements. Recently, Shockley- and Tamm-type surface states close to $E_{\mathrm{F}}$ were identified, which were found to be mixed by spin-orbit coupling. Here, we determined the Rashba parameter ${\alpha }_{\mathrm{R}}$ of the surface state around $\overline{\mathrm{\Gamma }}$ to 0.32 and $0.34\mathrm{eV}\mathring{\mathrm{A}}$ along $\overline{\mathrm{\Gamma }}\overline{\mathrm{M}}$ and $\overline{\mathrm{\Gamma }}\overline{\mathrm{K}}$, respectively. Furthermore, we extend our analysis to a wider $E\left({\mathbf{k}}_{\parallel }\right)$ range revealing a multitude of electronic states along both high-symmetry directions. In particular, Rashba-type spin splittings are observed around the high-symmetry $\overline{\mathrm{\Gamma }}$ and $\overline{\mathrm{M}}$ points. At variance with theoretical predictions for a perfect hcp(0001) surface, we do not find any out-of-plane spin polarization. This is caused by monatomic steps of a real Re(0001) surface with alternating terminations, leading on average to an effective sixfold surface symmetry and vanishing net out-of-plane spin polarization.

By using spectroscopy ellipsometry, we investigated the optical absorption spectra of monolayer and bilayer graphene in the deep-ultraviolet region up to 6.42 eV as a function of temperature. The optical absorption spectra of monolayer and bilayer graphene can be fitted by two Breit-Wigner-Fano (BWF) spectral functions, in which each BWF is originated by the interference of discrete excitonic spectra and continuous surface plasmon spectra. Each BWF peak shows narrower linewidth and larger peak intensity with increasing temperature. The unusual temperature dependence of the BWF peaks is understood by a shorter lifetime of the surface plasmon at a higher temperature, which gives a smaller BWF asymmetric parameter. The optical absorption by the surface plasmon is usually observed only at the surface of the three-dimensional crystal or two-dimensional material. The contribution of the surface plasmon becomes relatively larger in the case of monolayer or bilayer graphene than that of single-crystal graphite because of the two-dimensional nature of electrons. The observed optical absorption spectra are confirmed by calculating the complex dielectric function using the first-principles calculations.

When a static electrical field is applied to a two-dimensional (2D) Dirac material, Landau-Zener transition (LZT) and Bloch-Zener oscillations can occur. Employing $\alpha \text{−}{\mathcal{T}}_3$ lattices as a paradigm for a broad class of 2D Dirac materials, we uncover two phenomena. First, due to the arbitrarily small energy gaps near a Dirac point that make it more likely for LZTs to occur than in other regions of the Brillouin zone, the distribution of differential LZT probability in the momentum space can form a complicated morphological pattern. Second, a change in the LZT morphology as induced by a mutual switching of the two distinct Dirac points can lead to irregular Bloch-Zener oscillations characterized by a nonsmooth behavior in the time evolution of the electrical current density associated with the oscillation. These phenomena are due to mixed interference of quantum states in multiple bands modulated by the geometric and dynamic phases. We demonstrate that the adiabatic-impulse model describing Landau-Zener-Stückelberg interferometry can be exploited to calculate the phases, due to the equivalence between the $\alpha \text{−}{\mathcal{T}}_3$ lattice subject to a constant electrical field and strongly periodically driven two- or three-level systems. The degree of irregularity of Bloch-Zener oscillations can be harnessed by selecting the morphology pattern, which is potentially experimentally realizable.

Managing light-matter interactions on timescales faster than the loss of electronic coherence is key for achieving full quantum control of the final products in solid-solid transformations. In this Letter, we demonstrate coherent optical control of the orbital occupation that determines the insulator-to-metal transition in the prototypical Mott insulator ${\mathrm{V}}_2{\mathrm{O}}_3$. Selective excitation of a specific interband transition with two phase-locked light pulses manipulates the occupation of the correlated bands in a way that depends on the coherent evolution of the photoinduced superposition of states. A comparison between experimental results and numerical solutions of the optical Bloch equations provides an electronic coherence time on the order of 5 fs. Temperature-dependent experiments suggest that the electronic coherence time is enhanced in the vicinity of the insulator-to-metal transition critical temperature, thus highlighting the role of fluctuations in determining the electronic coherence. These results open different routes to selectively switch the functionalities of quantum materials and coherently control solid-solid electronic transformations.

We present an organic compound exhibiting a spin-Peierls (SP) transition to an effective spin-1 antiferromagnetic uniform chain, that is, the Haldane chain. The clear disappearance of magnetization, accompanied by a structural phase transition, is well explained by the deformation to an effective spin-1 Haldane chain. The flexibility of the molecular orbitals in the organic radical compound allows the transformation of the exchange interactions into the Haldane state with different topologies. The SP transition in the present compound demonstrates a mechanism different from that of the conventional systems, paving another path for research in quantum phenomena originating from spin-lattice couplings.

The Wiedemann-Franz law and Mott relation are textbook paradigms on the ratios of the thermal and thermoelectric conductivities to electrical conductivity, respectively. Deviations from them usually reveal insights for intriguing phases of matter. The recent topological kagome magnets $\mathrm{Tb}{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ and ${\mathrm{Mn}}_3\mathrm{Ge}$ show confusingly opposite derivations in the Hall measurement. We calculate the topological and disorder corrections to the Wiedemann-Franz law and Mott relation for the Hall responses in topological kagome magnets and Dirac materials. The calculation indicates the dominance of the topological correction in the experiments. More importantly, we derive analytic correction formulas, which can universally capture the two opposite experiments with the chemical potential as the only parameter and will be a powerful guidance for future explorations on the magnetic topological matter and Dirac materials.

The accurate equation of state (EOS) of MgO can be used as a pressure scale, but the EOS currently used has too much uncertainty to serve as a pressure scale, especially at high-pressure and high-temperature conditions. Here we present a complete EOS of solid MgO up to 345 GPa, 8500 K, and its derived pressure-volume-temperature (P-V-T) EOS form, which can be used as a pressure scale. In this EOS, the free energy is expressed as a three-parameter function based on a quasi-Debye thermodynamic model, which establishes a relatively accurate phonon thermodynamic description by determining the phonon dispersion relation and phonon density of states from the volume-dependent sound velocity. The three parameters are determined by a global optimization method, which minimizes the sum of the weighted variances between the physical quantities calculated by the thermodynamic model and the relevant experimental values within the unified EOS framework. To improve the accuracy of the EOS, the parameters are constrained by room-temperature sound velocities and shock compression data, including P-V-T data and sound velocities up to 265 GPa along the principal Hugoniot of MgO measured by us, the P-V-T data at 174–203 GPa, and partial P-V-T data of MgO preheated to 1850 K by others. The derived EOS of MgO can well reproduce not only the above data used to define the parameters, but also the shock compression data and the measured isobaric specific heat not used in the parameter optimization.

Shallow negatively charged silicon-vacancy centers have applications in magnetic quantum sensing and other quantum applications. Vacancy centers near the surface (within 100 nm) have different spin relaxation rates and optical spin polarization, affecting the optically detected magnetic resonance (ODMR) signal. This makes it essential to characterize these centers. Here we present the relevant spin properties of such centers. ODMR with a contrast of up to 6%, which is better than the state of the art, allowed us to determine the zero-field splitting, which is relevant for most sensing applications. We also present intensity-correlation data to verify that the signal originates from a single center and to extract transition rates between different electronic states.

We study the properties of the impurity band in heavily doped nonmagnetic semiconductors using the Jacobi-Davidson algorithm and the supervised deep learning method. The disorder averaged inverse participation ratio and Thouless number calculation show us the rich structure inside the impurity band. A convolutional neural network model, which is trained to distinguish the extended/localized phase of the Anderson model with high accuracy, shows us the results in good agreement with the conventional approach. Together, we find that there are three mobility edges in the impurity band for a specific on-site impurity potential, which means the presence of the extended states while filling the impurity band.

The wetting transition in the transverse-field spin-$\frac{1}{2}$ XY model with opposite boundary fields $h_L^x{h}_R^x<0$ is studied analytically and numerically. We find that the phase diagram is complex and that the wetting transition is of three types: first, second, and fourth order. The energy gap is obtained analytically, and the magnetization profile, correlation functions, and wetting layer thickness are obtained numerically. For $|h_L^x|,|h_R^x|<h_w$, a first-order phase transition occurs at $h_L^x=-h_R^x$, where $h_w$ is the continuous wetting transition point. For $|h_R^x|$ larger than $h_w$, the continuous wetting transition occurs at $h_L^x=h_w$, and vice versa. For $g\ne 1-{\gamma }^2$, the wetting transition is second order, and commensurate and incommensurate phases occur for $g<1-{\gamma }^2$ and $g>1-{\gamma }^2$, respectively. For $g=1-{\gamma }^2$, the wetting transition is fourth order. For this fourth-order phase transition, the third derivative of the surface magnetization oscillates and diverges near the transition point. The correlation length exponent is $\nu =2$, and the dynamic exponent is $z=2$. Thus, this fourth-order transition belongs to a new universality class. The wetting behavior is induced by asymmetric boundary fields $h_L^x=-h_R^x$ and its finite-size scaling is discussed.

A single crystal of NbReSi which crystallizes in the hexagonal structure with the noncentrosymmetric space group $P\overline{6}2m$ (No. 189) has been grown by the Czochralski method using a tetra-arc furnace. The anisotropic physical properties have been studied by measuring the magnetic susceptibility and electrical transport. The superconducting transition temperature $\left(T_{\mathrm{c}}\right)$ is around 6.1 K, and the measured upper critical field $\left(H_{\mathrm{c}2}\right)$ values are 8.39 T and 11.65 T for applied field parallel to [0001] and $\left[11\overline{2}0\right]$, respectively. The anisotropy ratio $\mathrm{\Gamma }=H_{\mathrm{c}2}^a/H_{\mathrm{c}2}^c$ is close to 1.38. The GL parameters ${\kappa }_c$ and ${\kappa }_a$ are much larger than $1/\sqrt{2}$, indicating that NbReSi is a strong type-II superconductor. The lower critical field $\left(H_{\mathrm{c}1}\right)$ values are very small (1.65 mT for $H\left|\right|\left[0001\right]$ and 1.22 mT for $H\left|\right|\left[11\overline{2}0\right]$) compared to $H_{\mathrm{c}2}$. From the magnetization hysteresis loops, calculated critical current at low field is of the order ${10}^4$ $\mathrm{A}/{\mathrm{cm}}^2$ which indicates the presence of strong pinning in the sample.

We design, simulate, and experimentally characterize an acoustic metasurface comprising of a one-dimensional (1D) array of open, sound-hard cavities modulated with beyond-nearest-neighbor (BNN) couplings in the form of additional connecting cavities embedded beneath the surface. The hidden complex structure is realized readily with additive manufacturing techniques [three-dimensional (3D) printing]. The dispersive properties of the supported localized acoustic surface waves are influenced by competing power-flow channels provided by the BNN couplings, that generate extrema in the dispersion spectra within the first Brillouin zone. The structure supports negatively dispersing “backwards” waves that we experimentally verify. Such structures thereby provide a route to enhanced acoustic sensing by acoustic metasurfaces.

Recent experimental advances have been marked by the observations of ballistic electron transport in moiré superlattices in highly aligned heterostructures of graphene and hexagonal boron nitride (hBN). Here, we predict that a high-quality graphene bilayer aligned with an hBN substrate features $T^2$-dependent resistivity caused by umklapp electron-electron (Uee) scattering from the moiré superlattice, that is, a momentum kick by Bragg scattering experienced by a pair of electrons. Substantial Uee scattering appears upon $p$ doping of the bilayer above a characteristic threshold, and its contribution towards the resistivity grows rapidly with hole density, until it reaches a peak value, then falling off by an order of magnitude. This rapid, nonmonotonic dependence of resistivity, in the density range where the system is otherwise highly conductive, suggests the possibility of a nonconventional field-effect transistor operation. We also analyze the influence of an electrostatically induced interlayer asymmetry (and the associated band gap) in the bilayer and trigonal warping on the electron-electron umklapp scattering.

We show that a magnetic line defect on the surface of a topological insulator generically supports two distinct branches of spin-polarized and current-carrying one-dimensional bound states. We identify the components of magnetic scattering that lead to the bound states. The velocity, and hence spin texture, of each of those branches can be independently tuned by a magnetic field rotated in the plane of the surface. We compute the local net and spin-resolved density of states as well as spin accumulation and charge currents. The net spin polarization and current due to both bound and scattering states vary stepwise as a function of the electrostatic and magnetic components of the scattering potential, and can be tuned by an applied field. We discuss stability of the bound states with respect to impurity scattering.