We performed room-temperature far-infrared ($40–650{\mathrm{cm}}^{-1}$) transmission measurements on undoped and bromine-doped powder samples of carbon nanofibers in stacked-cup cone geometry. The transmission spectra show enhanced transmittance after bromine doping and all spectra were fit to a Drude-Lorentz (DL) model. A decreased metallic conductivity along with a redshift of the lowest semiconducting gap was found with doped samples. A significant decrease in the scattering rate upon heavy doping has been qualitatively explained as partial ordering of intercalated dopant ions. Absorption spectra were derived from the transmission spectra under the assumption of nondispersive reflectance and subsequently compared with DL-model-derived spectra. The free-carrier density of the $n$-type powder and the electronic mean free path were estimated and compared with reported values for single-walled nanotubes and pyrolytic graphite.

The nitrogen-vacancy (NV) center in diamond has been found to be a powerful tool for various sensing applications. In particular, in ensemble-based sensors, the main “workhorse” so far has been optically detected electron resonance. Utilization of the nuclear spin has the potential to significantly improve sensitivity in rotation and magnetic field sensors. Ensemble-based sensors consume a substantial amount of power, leading to noticeable heating of the diamond and thus requiring an understanding of temperature-related shifts. In this paper, we provide a detailed study of the temperature shift of the hyperfine components of the NV center.

The effective electron $g$ factor, $g_{\text{eff}}$, is measured in a two-dimensional electron gas (2DEG) in modulation-doped ZnSe- and CdTe-based quantum wells by means of time-resolved pump-probe Kerr rotation. The measurements are performed in magnetic fields applied in the Voigt geometry, i.e., normal to the optical axis parallel to the quantum well plane, in the field range $0.05–6$ T at temperatures $1.8–50\mathrm{K}$. The $g_{\text{eff}}$ absolute value considerably increases with increasing electron density $n_e$. $g_{\text{eff}}$ changes in the ZnSe-based QWs from $+1.1$ to $+1.9$ in the $n_e$ range $3\times {10}^{10}-1.4\times {10}^{12}{\mathrm{cm}}^{-2}$ and in the CdTe-based QWs from $-1.55$ down to $-1.76$ in the $n_e$ range $5\times {10}^9-3\times {10}^{11}{\mathrm{cm}}^{-2}$. The modification of $g_{\text{eff}}$ reduces with increasing magnetic field, increasing temperature of lattice and 2DEG, the latter achieved by a higher photoexcitation density. A theoretical model is developed that considers the renormalization of the spin-orbit coupling constant of the two-dimensional electrons by the electron-electron interaction and takes into account corrections to the electron-electron interaction in the Hubbard form. The model results are in good agreement with experimental data.

We consider a screened Coulomb interaction between electrons in graphene and determine their dynamic response functions, such as a longitudinal and a transverse electric conductivity and a polarization function and compare them to the corresponding quantities in the short-range interaction model. The calculations are performed to all orders for short-range interaction by taking into account the self-energy renormalization of the electron velocity and using a ladder approximation to account for the vertex corrections, ensuring that the Ward identity (charge conservation law) is satisfied. Our findings predict a resonant response of interacting electron-hole pairs at a particular frequency below the threshold $qv=\omega$ and further predict an instability for sufficiently strong interactions.

We study the transport properties of an Anderson-Holstein model with orbital degeneracies and a tunneling phase that allows for the formation of dark states. The resulting destructive interference yields a characteristic pattern of positive and negative differential conductance features with enhanced shot noise, without further asymmetry requirements in the coupling to the leads. The transport characteristics are strongly influenced by the Lamb-shift renormalization of the system Hamiltonian. Thus, the electron-vibron coupling cannot be extracted by a simple fit of the current steps to a Poisson distribution. For strong vibronic relaxation, a simpler effective model with analytical solution allows for a better understanding and moreover demonstrates the robustness of the described effects.

We observe strong spin-orbit interactions in a two-dimensional electron system formed on $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ films grown epitaxially on p-Si(001). High-resolution transmission electron microscopy and related analytical techniques reveal a sharp interface between Si and $\mathrm{SrTi}{\mathrm{O}}_3$. Strain mapping analysis shows in-plane strain in $\mathrm{SrTi}{\mathrm{O}}_3$ very close to the interface. Analysis of the low temperature magnetoconductance measurements reveals that both quantum interference and electron-electron interactions are important. The contributions of these two quantum phenomena to the temperature and magnetic field dependence of the conductance and Hall effect are separated. The electron system has carrier concentrations larger than ${10}^{14}\mathrm{c}{\mathrm{m}}^{-2}$, appears to be confined within ∼4 nm, and has an estimated spin-splitting energy ∼12 meV.

Higher-order topological phases are characterized by protected states localized at the corners or hinges of the system. By applying time-periodic quenches to a two-dimensional lattice with balanced gain and loss, we obtain a rich variety of non-Hermitian Floquet second-order topological insulating phases. Each of the phases is characterized by a pair of integer topological invariants, which predict the numbers of non-Hermitian Floquet corner modes at zero and $\pi$ quasienergies. We establish the topological phase diagram of the model and find a series of non-Hermiticity-induced transitions between different Floquet second-order topological phases. We further generalize the mean chiral displacement to two-dimensional non-Hermitian systems and use it to extract the topological invariants of our model dynamically. This paper thus extends the study of higher-order topological matter to more generic nonequilibrium settings, in which the interplay between Floquet engineering and non-Hermiticity yields fascinating phases.

We analyze the out-of-time-order correlation functions of a solvable model of a large number $N$ of $M$-component quantum rotors coupled by Gaussian-distributed random, infinite-range exchange interactions. We focus on the growth of commutators of operators at a temperature $T$ above the zero temperature quantum critical point separating the spin-glass and paramagnetic phases. In the large $N,M$ limit, the squared commutators of the rotor fields do not display any exponential growth of commutators, in spite of the absence of any sharp quasiparticlelike excitations in the disorder-averaged theory. We show that in this limit, the problem is integrable and points out interesting connections to random-matrix theory. At leading order in $1/M$, there are no modifications to the critical behavior but an irrelevant term in the fixed-point action leads to a small exponential growth of the squared commutator. We also introduce and comment on a generalized model involving $p$-pair rotor interactions.

We report on the structural and spectroscopic characterization of the multiferroic ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$. Synchrotron x-ray and neutron diffraction as well as thermal expansion measurements reveal a lattice anomaly at $T_{\mathrm{N}}\simeq 60\mathrm{K}$ but do not show any symmetry lowering in the magnetically ordered state. The lattice parameter $c$ exhibits a nonmonotonic behavior with a pronounced minimum around 200 K, which is also reflected in an anomalous behavior of some of the observed infrared-active optical excitations and parallels the onset of short-range magnetic order. The infrared reflectivity spectra measured between 5 and 300 K in the frequency range of $100\text{–}8000{\mathrm{cm}}^{-1}$ reveal most of the expected phonon modes in comparison with the eigenfrequencies obtained by density-functional calculations. The $A_1$ phonons show an overall hardening upon cooling, whereas a nonmonotonic behavior is observed for some of the $E_1$ modes. These modes also show a strongly increased phonon lifetime below $T_{\mathrm{N}}$, which we associate with the quenched direction of the orbital moment in the magnetically ordered state. A similar increase is observed in the lifetime of the higher-lying $d\text{−}d$ excitations of the tetrahedral ${\mathrm{Fe}}^{2+}$ site that become clearly visible below $T_{\mathrm{N}}$ only.

Electric field modulations of magnetism in ferromagnetic metals are in general ascribed to the electric field effects on the orbitals. However, a direct observation of the electric-field-induced changes in the orbital states of a ferromagnetic metal is lacking. In this Rapid Communication, we demonstrate that an electric field applied to an ultrathin Co film can change the electron occupations according to the orbitals, combining x-ray absorption and x-ray magnetic circularly dichroism spectroscopies with the sum rules. Under a positive electric field, the $d_{xz\left(yz\right)}$-occupied states decrease mainly via the modification of orbital hybridizations, while the $d_{z^2}$-occupied state increases. The changes in the $d_{xz}$ and $d_{yz}$ states dominantly alter the Curie temperature.

We study the XXZ Heisenberg model in a staggered magnetic field using the higher-order tensor renormalization group (HOTRG) method. Built into the tensor representation of the XXZ model is the U(1) symmetry, which is systematically maintained at each renormalization step. We determine the phase diagram of the model from the low-lying spectrum, and from the finite-size dependence of the spectrum we extract scaling dimensions, which are compared to predictions of low-energy field theory.

Calculating the observables of a Hamiltonian requires taking matrix elements of operators in the eigenstate basis. Since eigenstates are only defined up to arbitrary phases that depend on Hamiltonian parameters, analytical expressions for observables are often difficult to simplify. In this paper, we show how for small Hilbert space dimension $N$ all observables can be expressed in terms of the Hamiltonian and its eigenvalues using the properties of the $\mathrm{SU}\left(N\right)$ algebra, and we derive explicit expressions for $N=2,3,4$. Then we present multiple applications specializing the case of Bloch electrons in crystals, including the computation of Berry curvature, quantum metric, and orbital moment, as well as a more complex observable in nonlinear response, the linear photogalvanic effect (LPGE). As a physical example, we consider multiband Hamiltonians with nodal degeneracies to show first how constraints between these observables are relaxed when going from two- to three-band models, and second how quadratic dispersion can lead to constant LPGE at small frequencies.

Optical spectroscopy, x-ray diffraction measurements, density functional theory (DFT), density functional theory + embedded dynamical mean-field theory ($\mathrm{DFT}+eDMFT$), and crystal-field calculations have been used to characterize structural and electronic properties of hexagonal $M_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ $\left(M=\mathrm{Fe},\mathrm{Mn}\right)$ polar magnets. Our experimental data are consistent with the room-temperature structure belonging to the space group $P{6}_{3}mc$ for both compounds. The experimental structural and electronic properties at room temperature are well reproduced within $\mathrm{DFT}+eDMFT$ method, thus establishing its predictive power in the paramagnetic phase. With decreasing temperature, both compounds undergo a magnetic phase transition, and we argue that this transition is concurrent with a structural phase transition (symmetry change from $P{6}_{3}mc$ to $P{6}_3$) in the Fe compound and an isostructural transition (no symmetry change from $P{6}_{3}mc$) in the Mn compound.

A clear signature of Zeeman split states crossing is observed in a Landau fan diagram of strained germanium two-dimensional hole gas. The underlying mechanisms are discussed based on a perturbative model yielding a closed formula for the critical magnetic fields. These fields depend strongly on the energy difference between the topmost and neighboring valence bands and are sensitive to the quantum well thickness, strain, and spin-orbit interaction. The latter is a necessary feature for the crossing to occur. This framework enables a straightforward quantification of the hole-state parameters from simple measurements, thus paving the way for its use in design and modeling of hole-based quantum devices.

We compute binding energies, Stark shifts, electric-field-induced dissociation rates, and the Franz-Keldysh effect for excitons in phosphorene in various dielectric surroundings. All three effects show a pronounced dependence on the direction of the in-plane electric field with the dissociation rates, in particular, decreasing by several orders of magnitude upon rotating the electric field from the armchair to the zigzag axis. To better understand the numerical dissociation rates, we derive an analytical approximation to the anisotropic rates induced by weak electric fields thereby generalizing the previously obtained result for isotropic two-dimensional semiconductors. This approximation is shown to be valid in the weak-field limit by comparing it to the exact rates. The anisotropy is also apparent in the large difference between armchair and zigzag components of the exciton polarizability tensor, which we compute for the five lowest-lying states. As expected, we also find much more pronounced Stark shifts in either the armchair or the zigzag direction, depending on the symmetry of the state in question. Finally, an isotropic interaction potential is shown to be an excellent approximation to a more accurate anisotropic interaction derived from the Poisson equation, confirming that the anisotropy of phosphorene is largely due to the direction dependence of the effective masses.

The resistance across a step on ultrathin films of three different topological insulators, ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, and ${\left({\mathrm{Bi}}_{1-x}{\mathrm{Pb}}_x\right)}_2{\mathrm{Te}}_3$, was measured through anisotropy in two-dimensional resistivity by using the in situ square four-point probe method in ultrahigh vacuum. The step resistance was much larger for ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ than for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ in the range of 1–10 quintuple-layer thickness, due to the smaller critical thickness for isolation of topological surface states in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. The transmission probability of carriers across a step is much higher for the bulk-insulating ${\left({\mathrm{Bi}}_{1-x}{\mathrm{Pb}}_x\right)}_2{\mathrm{Te}}_3$ than bulk-metallic ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, due to prevention of scattering of surface-state carriers into the bulk states. We were able to deduce microscopic information concerning the transmission probability at individual steps from the resistance data obtained macroscopically.

Due to recent advances in scanning-probe technology, the electronic structure of individual molecules can now also be investigated if they are immobilized by adsorption on nonconductive substrates. As a consequence, different molecular charge states are now experimentally accessible. Thus motivated, we investigate as an experimentally relevant example the electronic and structural properties of a NaCl(001) surface with and without pentacene adsorbed (neutral and charged) by employing density-functional theory. We estimate the polaronic reorganization energy to be $E_{\mathrm{reog}}\simeq 0.8-1.0$ eV, consistent with experimental results obtained for molecules of similar size. To account for environmental effects on this estimate, different models for charge screening are compared. Finally, we calculate the density profile of one of the frontier orbitals for different occupations and confirm the experimentally observed localization of the charge density upon charging and relaxation of molecule-insulator interface from ab initio calculations.

We present a way to partly reincorporate the effects of the localized bonding electrons on the dynamics of their itinerant counterparts in Hubbard-like Hamiltonians. This is done by relaxing the constraint that the former should be entirely frozen in the chemical bonds between the underlying lattice sites through the employment of a Born-Oppenheimer-like ansatz for the wave function of the whole electronic system. Accordingly, the latter includes itinerant as well as bonding electron coordinates. Going beyond the adiabatic approximation, we show that the net effect of virtual transitions of bonding electrons between their ground and excited states is to furnish the itinerant electrons with an effective interelectronic momentum-momentum interaction. Although we have applied these ideas to the specific case of rings, our assumptions can be generalized to higher-dimensional systems sharing the required properties of which we have made use herein.

By means of the density functional theory in combination with the dynamical mean-field theory, we tried to investigate the electronic structure of hexagonal FeGe, in which the Fe atoms form a quasi-two-dimensional layered kagome lattice. We predict that it is a representative kagome metal characterized by orbital selective Dirac fermions and extremely flat bands. Furthermore, Fe's $3d$ electrons are strongly correlated. They exhibit quite apparent signatures of electronic correlation induced by Hund's rule coupling, such as spin-orbital separation with non-Fermi-liquid behavior and spin freezing at intermediate temperature. Thus, hexagonal FeGe can be regarded as an ideal platform to study the interplay of kagome physics and Hundness.

We propose a quantum heat engine based on the ultrafast dynamical control of the magnetic properties of a nanoscale working body. The working principle relies on nonlinear phononics, an example for dynamical materials design. We describe the general recipe for identifying candidate materials, and also propose ${\mathrm{Cr}}_2{\mathrm{O}}_3$ as a promising working body for a quantum Otto cycle. Using a spin Hamiltonian as a model for ${\mathrm{Cr}}_2{\mathrm{O}}_3$, we investigate the performance in terms of efficiency, output power, and quantum friction. To assess the assumptions underlying our effective spin Hamiltonian, we also consider a working substance composed of several unit cells. We show that even without an implementation of transitionless driving, the quantum friction is very low compared to the total produced work and the energy cost of counterdiabatic driving is negligible. This is an advantage of the working substance, as experimentally hard-to-implement shortcuts to adiabaticity are not needed. Moreover, we discuss some remarkable thermodynamic features due to the quantumness of the proposed system such as a nonmonotonic dependence of the efficiency on the temperature of the hot bath. Finally, we explore the dependence of the performance on the system parameters for a generic model of this type of quantum heat engine and identify properties of the energy spectrum required for a well-performing quantum heat engine.