Photoexcited metals can produce highly energetic hot carriers whose controlled generation and extraction is a promising avenue for technological applications. While hot-carrier dynamics in Au-group metals have been widely investigated, a microscopic description of the dynamics of photoexcited carriers in the mid-infrared and near-infrared Pt-group metals range is still scarce. Since these materials are widely used in catalysis and, more recently, in plasmonic catalysis, their microscopic carrier dynamics characterization is crucial. We employ ab initio many-body perturbation theory to investigate the hot-carrier generation, relaxation times, and mean free path in bulk Pd and Pt. We show that the direct optical transitions of photoexcited carriers in these metals are mainly generated in the near-infrared range. We also find that the electron-phonon mass enhancement parameter for Pt is $16\%$ higher than Pd, a result that helps explain several experimental results showing diverse trends. Moreover, we predict that Pd (Pt) hot electrons possess total relaxation times of up to 35 fs (24 fs), taking place at approximately 0.5 eV (1.0 eV) above the Fermi energy. Finally, an efficient hot electron generation and extraction can be achieved in nanofilms of Pd (110) and Pd (100) when subject to excitation energies ranging from 0.4 to 1.6 eV.

We report a comprehensive phase diagram of the antiferromagnetic Kondo-lattice compound ${\mathrm{CeRhAl}}_4{\mathrm{Si}}_2$ down to 0.3 K by investigating electrical resistivity, magnetoresistivity, and Hall resistivity under the magnetic field along the $c$ axis. At zero field, ${\mathrm{CeRhAl}}_4{\mathrm{Si}}_2$ undergoes two successive antiferromagnetic transitions at $T_{\mathrm{N}1}=14.2\mathrm{K}$ and $T_{\mathrm{N}2}=8.4\mathrm{K}$, respectively. Upon applying magnetic field, the first-order transition of $T_{\mathrm{N}2}$ is continuously suppressed at the critical field of 5.5 T. On the other hand, the smooth suppression of the second-order transition of $T_{\mathrm{N}1}$ terminates at a tricritical point of 6.6 T and 4.7 K, followed by a first-order transition line and an additional phase transition line, which are monotonically suppressed at 6.7 and 7.2 T, respectively, implying the field-induced multiple quantum phase transitions. Particularly, in the paramagnetic region before the Fermi liquid behavior is fully developed, a quantum critical phase with non-Fermi liquid behavior is revealed, indicating a potential spin liquid state that was predicted from the global quantum phase diagram. These results suggest that ${\mathrm{CeRhAl}}_4{\mathrm{Si}}_2$ is a promising candidate to study a quantum phase transition that occurs in electronic systems with high degree of frustration.

In addition to the well-known dependence of band gaps of compounds on crystal symmetry and the orbital character of the constituent atoms, $d$-electron perovskites manifest an additional dependence of gaps on the magnetic spin configuration, a dependence deciding both metal-insulator transitions and magnetic application. We find via electronic structure theory that the factors that $explicitly$ connect gapping magnitudes to magnetism depend on the nature of the band edge orbital character (BEOC) and scale with the number of antiferromagnetic contacts $z_i$ between neighboring transition metal ions. The dependence is weak when the BEOC is (d,d*)-like (“Mott insulators”), whereas this dependence is rather strong in (p,d*)-like BEOC (“charge-transfer” insulators). These rather unexpected rules are traced to the reduced magnitude of interorbital interactions as the number of antiferromagnetic contacts $z_i$ increases. In addition to these direct connections between magnetism and gaps, magnetic order also feeds back indirectly into the gapping problem by controlling the propensity of the lattice to develop lattice distortion. Such electronic instabilities toward Jahn-Teller distortions or bond disproportionation feed back into trends in band gap magnitudes. These results highlight the importance of establishing the appropriate magnetic order in the electronic structure theory for reliable rendering of gap-related phenomena such as doping, transport, metal-insulator transitions, and conductive interfaces.

Multiferroic van der Waals (vdW) heterostrucutres offer an exciting route toward high-performance nanoelectronics and spintronics device technology. Here we investigate the electronic and transport properties of multiferroic vdW heterostructures composed of a ferromagnetic ${\mathrm{FeCl}}_2$ monolayer and a ferroelectric ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ monolayer using first-principles density functional theory and quantum transport simulations. We show that ${\mathrm{FeCl}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ heterostructure can be reversibly switched from semiconducting to half-metallic behavior by electrically modulating the ferroelectric polarization states of ${\mathrm{Sc}}_2{\mathrm{CO}}_2$. Intriguingly, the half-metallic phase exhibits a type-III broken gap-band alignment, which can be beneficial for tunneling field-effect transistor applications. We perform a quantum transport simulation based on a proof-of-concept two-terminal nanodevice to demonstrate all-electric-controlled valving effects uniquely enabled by the nonvolatile ferroelectric switching of the heterostructure. These findings unravel the potential of ${\mathrm{FeCl}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ vdW heterostructures as a building block for designing the next generation of ultimately compact information processing, data storage, and spintronics devices.

We find transport and spectroscopic signatures that are consistent with a magnetic field induced transition from a vortex liquid to Bose metal in a two-dimensional amorphous superconductor, $a$-MoGe, using a combination of magnetotransport and scanning tunneling spectroscopy (STS). Below the superconducting transition, $T_c\sim 1.36\mathrm{K}$, the magnetoresistance isotherms cross at a nearly temperature independent magnetic field, $H_c^{*} \sim 36\mathrm{kOe}$. Above this field, the temperature coefficient of resistance is weakly negative, but the resistance remains finite as $T\to 0$, as expected in a bad metal. From STS conductance maps and transport measurements at 450 mK we observe a very disordered vortex lattice at very low fields that melts into a vortex liquid above 3 kOe. Up to $H_c^{*}$ the tunneling spectra display a superconducting gap and coherence peak over a broad background caused by electron-electron interactions, as expected in a vortex liquid. However, above $H_c^{*}$ the tunneling spectra continue to display the gap but the coherence peak gets completely suppressed, suggesting that Cooper pairs lose their phase coherence. We conclude that $H_c^{*}$ demarcates a transition from a vortex liquid to Bose metal, that eventually transforms to a regular metal at a higher field $H^{*}$ where the gap vanishes in the electronic spectrum.

Spin-orbit interaction in semiconductor structures with broken space inversion symmetry leads to spin splitting of electron and hole states even in the absence of magnetic field. We discover that, beyond the Rashba and Dresselhaus contributions, there is an additional type of the zero-field spin splitting which is caused by the interplay of the cubic shape of crystal unit cell and macroscopic structure asymmetry. In quantum wells grown along low-symmetry crystallographic axes, this type of spin-orbit interaction couples the out-of-plane component of carrier's spin with the in-plane momentum whereas the coupling strength is controlled by structure inversion asymmetry. We carry out numerical calculations and develop an analytical theory, which demonstrate that this interaction dominates $\mathit{k}$-linear spin splitting of heavy-hole subbands in a wide range of parameters.

We study acoustic wave propagation in a waveguide loaded with two resonant side-branch channels. In the low-frequency regime, one-dimensional models are derived in which the effect of the channels are reduced to jump conditions across the junction. When the separation distance is on the scale of the wavelength, which is the case that is usually considered, the jump conditions involve a single channel and acoustically induced transparency occurs due to out-of-phase interferences between the two junctions. In contrast, when the separation distance is subwavelength, a single junction has to be considered and the jump conditions account for the evanescent field coupling the two channels. Such channel pairs can scatter as a dipole, resulting in perfect transmission due to Autler-Townes splitting. We show that combining the two mechanisms offers additional degrees of freedom to control the transmission spectra.

It is well known that the temperature dependence of the effective magnetocrystalline anisotropy energy obeys the $l(l+1)/2$ power law of magnetization in the Callen-Callen theory. Therefore, according to the Callen-Callen theory, the magnetocrystalline anisotropy energy is assumed to be zero at the critical temperature where the magnetization is approximately zero. This study estimates the temperature dependence of the magnetocrystalline anisotropy energy by integrating the magnetization versus magnetic field ($M-H$) curves, and found that the magnetocrystalline anisotropy is still finite even above the Curie temperature in the uniaxial anisotropy, whereas this does not appear in the cubic anisotropy case. The origin is the fast reduction of the anisotropy field, which is the magnetic field required to saturate the magnetization along the hard axis, in the case of cubic anisotropy. Therefore, the magnetization anisotropy and anisotropic magnetic susceptibility, which are the key factors of magnetic anisotropy, could not be established in the case of cubic anisotropy. In addition, the effect of magnetocrystalline anisotropy on magnetocaloric properties, as the difference between the entropy change curves of ${\mathrm{AlFe}}_2{\mathrm{B}}_2$ appears above the Curie temperature, is in good agreement with a previous experimental study. This is proof of magnetic anisotropy at slightly above Curie temperature.

The spin dynamics of the spin-1/2 kagome-lattice antiferromagnet ${\mathrm{Cs}}_2{\mathrm{Cu}}_3\mathrm{Sn}{\mathrm{F}}_{12}$ is studied using high-resolution, time-of-flight inelastic neutron scattering. The flat mode, a characteristic of the frustrated kagome antiferromagnet, and the low-energy dispersive mode, which is dominated by magnons, can be well described by the linear spin-wave theory. However, the theory fails to describe three weakly dispersive modes between 9 and 14 meV. These modes could be attributed to two-spinon bound states, which decay into free spinons away from the zone center and at a high temperature, giving rise to continuum scattering.

The coexistence of multiple quasidegenerate orders is the hallmark of the strongly correlated materials. Experiments often reveal several spatially modulated orders in the underdoped cuprates. This has come to the forefront with the possible detection of the pair density wave states. However, microscopic calculations often struggle to stabilize such spatially modulating orders as the ground state in the strong correlation limit. This work uses the $t-t^{\prime }-J$ model with an additional nearest-neighbor repulsion to stabilize spatially oscillating charge, bond, and pairing orders in the underdoped regime. We employ the standard Gutzwiller approach while treating the inhomogeneity for the spatial orders using the self-consistent Hartree-Fock-Bogoliubov methodology. Our calculations reveal that unidirectional bond density states coexisting with charge and pairing modulations can have lower energy than the uniform superconducting state over an extensive doping range. These modulating states vanish monotonically as the modulation wave vector becomes shorter with increased dopings. The finite momentum orders give way to a vestigial nematic phase on increasing doping which only breaks the rotational symmetry of the system. The nematic order vanishes on further increasing doping, and only uniform superconductivity survives. The spatial features of the ground state at each doping reveal multiple wave vectors, which potentially drive the incommensuration of charge orders. Interestingly, the spatially modulating states are absent when the strong correlations criteria are relaxed, suggesting that the removal of double occupancy aids the stabilization of density wave orders.

Atomic scale simulations at finite temperature are an ideal approach to study the thermodynamic properties of magnetic transition metals. However, the development of interatomic potentials explicitly taking into account magnetic variables is a delicate task. In this context, we present a tight-binding model for magnetic transition metals in the Stoner approximation. This potential is integrated into a Monte Carlo structural relaxations code where trials of atomic displacements as well as fluctuations of local magnetic moments are performed to determine the thermodynamic equilibrium state of the considered systems. As an example, the Curie temperature of cobalt is investigated while showing the important role of atomic relaxations. Furthermore, our model is generalized to other transition metals highlighting a local magnetic moment distribution that varies with the gradual filling of the $d$ states. Consequently, the successful validation of the potential for different magnetic configurations indicates its great transferability and makes it a good choice for atomistic simulations sampling a large configuration space.

Unlike conventional semiconductors, Dirac fermions in surface states of topological insulators (TIs) exhibit many fascinating electronic properties relevant for several optoelectronic and spintronic applications. We investigate the relaxation dynamics of photoexcited carriers in bismuth telluride (${\mathrm{Bi}}_2{\mathrm{Te}}_3$) nanocrystals—a prominent candidate among the three-dimensional (3D) topological insulators, using ultrafast time resolved terahertz (THz) spectroscopy. Upon photoexcitation using an 800 nm pump pulse, we observe a decrease in the THz transmission due to the absorption of THz radiation by the photoexcited carriers. The time evolution of the real part of pump induced conductivity [$\mathrm{\Delta }{\sigma }_R\left({\tau }_{\text{pp}}\right)={\sigma }_{\text{pump}}{}_{\text{on}}\left({\tau }_{\text{pp}}\right)-{\sigma }_{\text{pump}}{}_{\text{off}}$] with respect to the pump-probe delay time (${\tau }_{\text{pp}}$), exhibits a sign change from positive to negative as carriers relax back to their equilibrium distribution, which is also evident from the spectral signatures of $\mathrm{\Delta }\sigma \left(\omega \right)$, measured at various ${\tau }_{\text{pp}}$. While the positive sign of $\mathrm{\Delta }{\sigma }_R\left({\tau }_{\text{pp}}\right)$ is attributed to the contribution from the semiconducting bulk states, the negative sign of $\mathrm{\Delta }{\sigma }_R\left({\tau }_{\text{pp}}\right)$ at longer delay times is shown to arise from the Dirac surface states of TIs due to increased scattering rate of photoexcited charge carriers. We model the differential conductivity spectrum [$\mathrm{\Delta }\sigma \left(\omega \right)$] and the transient dynamics [$\mathrm{\Delta }{\sigma }_R\left({\tau }_{\text{pp}}\right)$] quantitatively using the Boltzmann transport equation applied to the surface and bulk states. This includes the energy dependence of scattering rates along with the dynamics of hot electron temperature ($T_e$) evaluated in a self-consistent manner. Such an approach enables us to understand various scattering mechanisms associated with the photoexcited carriers both in Dirac surface as well as in bulk electronic states.

Recent experiments showed that the reduced multilayer rare-earth (RE) nickel oxides of form $\mathrm{R}{\mathrm{E}}_{p+1}{\mathrm{Ni}}_p{\mathrm{O}}_{2p+2}$ may belong to the novel family of superconducting lanthanide nickelates. Here, the correlated electronic structure of ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ and ${\mathrm{Nd}}_6{\mathrm{Ni}}_5{\mathrm{O}}_{12}$ is studied by means of an advanced realistic many-body framework. It is revealed that the low-energy physics of both systems is dominated by an interplay of Ni-$d_{x^2-y^2}$ and Ni-$d_{z^2}$ degrees of freedom. While the Ni-$d_{x^2-y^2}$ orbitals are always highly correlated near a (orbital-selective) Mott-insulating regime, the Ni-$d_{z^2}$ orbitals give rise to intriguing nondispersive features. At low temperature, the Pr compound still displays quasiparticle-like Ni-$d_{x^2-y^2}$-derived states at the Fermi level, but the interacting fermiology of the Nd compound is outshined by an emergent Ni-$d_{z^2}$-controlling flat band. These findings translate well to the previous characterization of doped infinite-layer nickelates, and hence further make the case for a mechanism of unconventional superconductivity, which is distinct from the one in high-$T_{\mathrm{c}}$ cuprates.

We propose a model to estimate the energy barrier responsible for the hysteresis of the thermally driven Mott phase transition. The fitting results are in good agreement with experimental data, where the associated barrier values for ${\mathrm{VO}}_2$ films with various hysteretic behavior are determined. For heating to a fixed temperature in the hysteresis, the electric pulse $(\sim 100\mathrm{V}/\mathrm{cm})$ induced responses exhibit steps of the resistance in ${\mathrm{VO}}_2$ thin films, forming multiple nonvolatile states, whereas no remarkable changes occur for the cooling case. We propose that the memory ability can be attributed to the barrier in the hysteresis regime and the multilevel resistances induced electrically are associated with the configuration of the metal and insulator domains.

We explore the physics of the quantum Hall effect using the Haldane mapping of dimerized $\mathrm{SU}(N+M)$ spin chains, the large $N$ expansion, and the density matrix renormalization group technique. We show that while the transition is first order for $N+M>2$, the system at zero temperature nevertheless displays a continuously diverging length scale $\xi$ (correlation length). The numerical results for $(N,M)=(3,1),(2,2),(5,1)$, and $(7,1)$ indicate that $\xi$ is a directly observable physical quantity, namely the spatial width of the edge states. We relate the physical observables of the quantum spin chain to those of the quantum Hall system (and, hence, the $\vartheta$ vacuum concept in quantum field theory). Our numerical investigations provide strong evidence for the conjecture of superuniversality which says that the dimerized spin chain quite generally displays all the basic features of the quantum Hall effect, independent of the specific values of $N$ and $M$. For the cases at hand we show that the singularity structure of the quantum Hall plateau transitions involves a universal function with two scale parameters that may in general depend on $N$ and $M$. This includes not only the Hall conductance but also the ground state energy as well as the correlation length $\xi$ with varying values of $\vartheta \sim \pi$.

The foundational Franz-Keldysh effect and Einstein model are applied in this work to characterize semiconductor band-edge absorption—and its departures from ideality. We unify the Franz-Keldysh and Einstein models to fully characterize the field-induced tunneling of photoexcited electrons from degenerate valence bands into the conduction band, with encroachment into the band gap arising as an Urbach tail. Our unified model is implemented for semi-insulating indium phosphide (SI-InP) with strong agreement seen between the theoretical and experimental results for varied photon energies and electric fields.

Manipulation of coherent light-matter interactions at the nanoscale with the suppression of the incoherent damping process is of great importance for both fundamental research and future applications of quantum information devices. Here, we propose a low-loss dielectric nanoparticle-on-mirror (NPoM) system for flexible control of the coherent interaction between magnetic Mie resonances and the excitonic mode in a monolayer of $\mathrm{W}{\mathrm{Se}}_2$. We demonstrate that the dielectric NPoM system simultaneously enables a large field enhancement and low absorption in the dielectric nanoresonator, thus greatly facilitating the coherent magnetic Mie-exciton coupling. Importantly, the efficiency of radiating the magnetic mode to the far field can be readily manipulated by changing the thickness of the dielectric spacer and the particle size, therefore offering flexible tuning of the interaction from the intermediate to strong coupling regime. Moreover, the highly directional emission at the Mie resonance dramatically suppresses the incoherent damping process between the two individual systems via the continuum reservoir. Such a NPoM-based hybrid system is expected to offer not only the possibility to manipulate light-matter interactions but also new avenues for low-loss, high-efficiency coherent light control at the deep nanoscale.

Topological orders have been intrinsically identified in a class of systems such as fractional quantum Hall states and spin liquids. Accessing such states often requires extreme conditions such as low temperatures, high magnetic fields, pure samples, etc. Another approach would be to engineer the topological orders in systems with more accessible ingredients. In this work, we present networks of Majorana bound states, which are currently accessible in semiconductor nanowires proximitized to conventional superconductors, and show that the effective low-energy theory is topologically ordered. We first demonstrate the main principles in a lattice made of Kitaev superconducting chains comprising both spin species. The lattice is coupled to free magnetic moments through the Kondo interaction. We then show that at the weak-coupling limit, effective ring spin interactions are induced between magnetic moments with a topological order enjoying a local ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ gauge symmetry. We then show that the same topological order and also the ${\mathbb{Z}}_2$ one can be engineered in architecture patterns of semiconductor nanowires hosting Majorana bound states. The basic blocks of patterns are the time-reversal Majorana Cooper boxes coupled to each other by metallic leads, and the Majorana states are allowed to tunnel to quantum dots sitting on the vertices of the lattices. In the limit of strong onsite Coulomb interactions, where the charge fluctuations are suppressed, the magnetic moments of dots on the square and honeycomb lattices are described by topologically ordered spin models with underlying ${\mathbb{Z}}_2$ and ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ gauge symmetries, respectively. Finally, we show that the latter topological order can also be realized in a network of purely Majorana zero modes in the absence of coupling to quantum dots.

Divacancy in its neutral charge state (${\mathrm{V}}_{\text{C}}{\mathrm{V}}_{\text{Si}}^0$) in 4H silicon carbide (SiC) is a leading quantum bit (qubit) contender. Owing to the lattice structure of 4H SiC, four different ${\mathrm{V}}_{\text{C}}{\mathrm{V}}_{\text{Si}}$ configurations can be formed. The ground and the optically accessible excited states of ${\mathrm{V}}_{\text{C}}{\mathrm{V}}_{\text{Si}}^0$ configurations exhibit a high-spin state, and the corresponding optical transition energies are around $\approx 1.1\mathrm{eV}$ falling in the near-infrared wavelength region. Recently, photoluminescence (PL) quenching has been experimentally observed for all ${\mathrm{V}}_{\text{C}}{\mathrm{V}}_{\text{Si}}$ configurations in 4H SiC at cryogenic temperatures. It has been shown that ${\mathrm{V}}_{\text{C}}{\mathrm{V}}_{\text{Si}}^0$ is converted to ${\mathrm{V}}_{\text{C}}{\mathrm{V}}_{\text{Si}}^{-}$ and it remains in this shelving dark state at cryogenic temperatures until photoexcitation with the threshold energies or above is applied to convert ${\mathrm{V}}_{\text{C}}{\mathrm{V}}_{\text{Si}}^{-}$ back to ${\mathrm{V}}_{\text{C}}{\mathrm{V}}_{\text{Si}}^0$. In this study, we demonstrate both in experiments and theory that the threshold energy for reionization is temperature dependent. We carry out density functional theory (DFT) calculations in order to investigate the temperature dependent reionization spectrum, i.e., the spectrum of the ${\mathrm{V}}_{\text{C}}{\mathrm{V}}_{\text{Si}}^{-}\to {\mathrm{V}}_{\text{C}}{\mathrm{V}}_{\text{Si}}^0$ process. We find that simultaneous optical reionization and qubit manipulation can be carried out at room temperature with photoexcitation at the typical excitation wavelength used for readout of the divacancy qubits in 4H SiC, in agreement with our experimental data. We also provide the analysis of the PL spectrum of ${\mathrm{V}}_{\text{C}}{\mathrm{V}}_{\text{Si}}^0$, characteristic for each ${\mathrm{V}}_{\text{C}}{\mathrm{V}}_{\text{Si}}^0$ configuration in 4H SiC, using the Huang-Rhys theory, and find that one configuration in 4H SiC stands out in terms of the strength of coherent emission among the four configurations.