We report high field de Haas–van Alphen (dHvA) effect measurements in ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$ both below and above the first-order 26 T metamagnetic transition from an antiferromagnetic to a polarized paramagnetic state. The dHvA frequencies observed above the transition are much higher than those observed below, implying a drastic change of the Fermi-surface size. The dHvA frequencies above the transition and their angular dependence are in good agreement with band-structure calculations for ${\mathrm{LaRh}}_2{\mathrm{Si}}_2$, which correspond to ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$ with localized $f$ electrons. Given that the $f$ electrons are also localized at low fields in ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$, the Fermi-surface reconstruction is due to the suppression of antiferromagnetism and the restoration of the crystallographic Brillouin zone rather than the delocalization of the $f$ electrons. This example suggests that the intuitive notation of “small” and “large” Fermi surfaces commonly used for localized and itinerant $f$ electrons, respectively, requires careful consideration, taking into account the modification of the Brillouin zone in the antiferromagnetic state, when interpreting experimental results.

Ultrafast photocurrent experiments are performed on semiconducting, single-walled carbon nanotubes under a resonant optical excitation of their subbands. The photogenerated excitons are dissociated at large electric fields and the resulting transport of the charge carriers turns out to be ballistic. Thermionic emission processes to the contacts dominate the photocurrent amplitude. The charge current without laser excitation is well described by a Fowler-Nordheim tunneling. The time-averaged photocurrent changes polarity as soon as sufficient charge carriers are injected from the contacts, which can be explained by an effective population inversion in the optically pumped subbands.

The low-temperature electronic properties of rare-earth intermetallics are substantially influenced by the symmetry and magnitude of the crystal electric field. The direct spectroscopic analysis of crystal-field splitting can be challenging, especially in low-dimensional systems, because it requires both high spectral resolution and pronounced sensitivity. We demonstrate the eligibility of electronic Raman spectroscopy for this purpose by the direct determination of the $4f$ level splitting in ultrathin ordered ${\mathrm{CePt}}_5$ films down to $\approx 1.5$ nm thickness on Pt(111). Crystal-field excitations of Ce $4f$ electrons give rise to Raman peaks at energy losses up to $\approx 25$ meV. Three distinct peaks occur which we attribute to inequivalent Ce sites, located (i) at the interface to the substrate, (ii) next to the Pt-terminated film surface, and (iii) in the ${\mathrm{CePt}}_5$ layers in between. The well-resolved Raman signatures allow us to identify a reduced crystal-field splitting at the interface and an enhancement at the surface, highlighting its strong dependence on the local atomic environment.

Defect-free ${\mathrm{SrTiO}}_3$ (STO) is a band insulator but angle resolved photoemission spectroscopy (ARPES) experiments have demonstrated the existence of a nanometer thin two-dimensional electron liquid (2DEG) at the (001) oriented surface of this compound. The bulk is a trivial insulator, but our theoretical study reveals that the parity of electronic wave functions in this 2DEG is inverted in the vicinity of special points in reciprocal space where the low-energy dispersion consists of four gapped Dirac cones with a tilted and anisotropic shape. This gives rise to linearly dispersing topological edge states at the one-dimensional boundary. We propose to probe these modes by measuring the Josephson radiation from gapless bound Andreev states in STO based junctions, as it is predicted that they display distinctive signatures of topology.

Semiconductor nanoparticles of cadmium chalcogenides are known to exhibit pronounced thickness-dependent $E_0$ series of exciton transitions at the Γ point of the Brillouin zone (BZ). In this work we report an experimental evidence for high-energy series of exciton transitions, which originates from BZ points different from the Γ point, in the family of cadmium chalcogenide quasi-two-dimensional (2D) nanoplatelets (NPLs). Intensive UV absorption bands demonstrating a pronounced size effect are observed for CdTe, CdSe, and CdS NPLs in addition to the $E_0$ exciton bands in the visible region. These additional bands are attributed to transitions analogous to the $E_1,E_1+{\mathrm{\Delta }}_1$, and $E_2$ series observed in bulk crystals. First-principles DFT calculations of the electronic structure and absorption spectra support this explanation and show that the main contribution to these optical transitions comes from $X$ and $M$ points of the 2D BZ, which originate from $L$ and $X$ points of the 3D BZ. At the same time, the $E_0$ series of transitions at the Γ point is well described by the multiband effective-mass model. The observation of the UV exciton bands reveals tunable optical properties of cadmium chalcogenide NPLs in UV spectral region, which may be interesting for practical applications.

We present a detailed theoretical study of effective spin-orbit coupling (SOC) Hamiltonians for graphene-based systems, covering global effects such as proximity to substrates and local SOC effects resulting, for example, from dilute adsorbate functionalization. Our approach combines group theory and tight-binding descriptions. We consider structures with global point group symmetries $D_{6h}, D_{3d}, D_{3h}, C_{6v}$, and $C_{3v}$ that represent, for example, pristine graphene, graphene miniripple, planar boron nitride, graphene on a substrate, and free standing graphone, respectively. The presence of certain spin-orbit coupling parameters is correlated with the absence of the specific point group symmetries. Especially in the case of $C_{6v}$—graphene on a substrate, or transverse electric field—we point out the presence of a third SOC parameter, besides the conventional intrinsic and Rashba contributions, thus far neglected in literature. For all global structures we provide effective SOC Hamiltonians both in the local atomic and Bloch forms. Dilute adsorbate coverage results in the local point group symmetries $C_{6v}, C_{3v}$, and $C_{2v}$, which represent the stable adsorption at hollow, top and bridge positions, respectively. For each configuration we provide effective SOC Hamiltonians in the atomic orbital basis that respect local symmetries. In addition to giving specific analytic expressions for model SOC Hamiltonians, we also present general (no-go) arguments about the absence of certain SOC terms.

We use the coupled cluster method to study the zero-temperature quantum phase diagram of the spin-$\frac{1}{2}{J}_1\text{−}{J}_2\text{−}{J}_1^{\perp }$ model on the honeycomb bilayer lattice. In each layer, we include both nearest-neighbor and frustrating next-nearest-neighbor antiferromagnetic exchange couplings, of strength $J_1>0$ and $J_2\equiv \kappa J_1>0$, respectively. The two layers are coupled by an interlayer nearest-neighbor exchange, with coupling constant $J_1^{\perp }\equiv \delta J_1>0$. We calculate directly in the infinite-lattice limit both the ground-state energy per spin and the Néel magnetic order parameter, as well as the triplet spin gap. By implementing the method to very high orders of approximation we obtain an accurate estimate for the full boundary of the Néel phase in the $\kappa \delta$ plane. For each value $\delta <{\delta }_c^{>}\left(0\right)\approx 1.70\left(5\right)$, we find an upper critical value ${\kappa }_c\left(\delta \right)$, such that Néel order is present for $\kappa <{\kappa }_c\left(\delta \right)$. Conversely, for each value $\kappa <{\kappa }_c\left(0\right)\approx 0.19\left(1\right)$, we find an upper critical value ${\delta }_c^{>}\left(\kappa \right)$, such that Néel order persists for $0<\delta <{\delta }_c^{>}\left(\kappa \right)$. Most interestingly, for values of $\kappa$ in the range ${\kappa }_c\left(0\right)<\kappa <{\kappa }^{>}\approx 0.215\left(2\right)$, we find a reentrant behavior such that Néel order exists only in the range ${\delta }_c^{<}\left(\kappa \right)<\delta <{\delta }_c^{>}\left(\kappa \right)$, with ${\delta }_c^{<}\left(\kappa \right)>0$. These latter upper and lower critical values coalesce when $\kappa ={\kappa }^{>}$, such that ${\delta }_c^{<}\left({\kappa }^{>}\right)={\delta }_c^{>}\left({\kappa }^{>}\right)\approx 0.25\left(5\right)$.

We use a continuum method informed by transport coefficients computed using self-consistent mean field theory to model vacancy-mediated diffusion of substitutional Si solutes in FCC Ni near an $\frac{a}{2}\left[1\overline{1}0\right]\left(111\right)$ edge dislocation. We perform two sequential simulations: first under equilibrium boundary conditions and then under irradiation. The strain field around the dislocation induces heterogeneity and anisotropy in the defect transport properties and determines the steady-state vacancy and Si distributions. At equilibrium both vacancies and Si solutes diffuse to form Cottrell atmospheres with vacancies accumulating in the compressive region above the dislocation core while Si segregates to the tensile region below the core. Irradiation raises the bulk vacancy concentration, driving vacancies to flow into the dislocation core. The out-of-equilibrium vacancy fluxes drag Si atoms towards the core, causing segregation to the compressive region, despite Si being an oversized solute in Ni.

We perform a sudden quench on the Haldane model with long range interactions, more specifically generalizing to the next to next nearest neighbor hopping, referred to as the $N3$ model in our work. Such a model possesses both isotropic and multiple anisotropic (satellite) Dirac points which lead to a rich topological phase diagram consisting of phases with higher Chern number $\left(C\right)$. Quenches between the topological and the nontopological phases of such an infinite system probe the effect of the presence of the anisotropic Dirac points on the nonequilibrium response of the topological system. Interestingly, the Chern number remains the same before and after the quench for both the quenching protocols, even when the quench of the system is carried out between two different topological phases. However, for a finite system, we establish that the initial edge current asymptotically decays to zero when the system is quenched to the nontopological phase although the Chern number for the corresponding periodically wrapped system remains unaltered; what is remarkable is that when the Hamiltonian is quenched from $\left|C\right|=2$ phase to the nontopological phase the edge current associated with the inner channel decays at a faster rate than the outer channel resembling a situation in which the system passes through the phase with $\left|C\right|=1$ before ending up in the phase $C=0$.

We study the magnetic field effects on the diluted spin-ice materials using the replica-exchange Monte Carlo simulation. We observe five plateaus in the magnetization curve of the diluted nearest-neighbor spin-ice model on the pyrochlore lattice when a magnetic field is applied in the [111] direction. This is in contrast to the case of the pure model with two plateaus. The origin of five plateaus is investigated from the spin configuration of two corner-sharing tetrahedra in the case of the diluted model.

The Nagaoka $\left(U=\infty \right)$ limit of the Hubbard model on a square lattice is mapped onto the itinerant-localized Kondo model at infinitely strong coupling. Such a model is well suited to perform quantum Monte Carlo (QMC) simulations to compute spin correlation functions. For periodic boundary conditions, this model is shown to exhibit no short-range ferromagnetic (FM) spin correlations at any doping $\delta \ge 0.01$ and at finite temperature $T=0.1t.$ Our simulations give no indication that there is a tendency towards ferromagnetic ordering in the ground state, with more than one hole. Employing on the other hand the open boundary conditions (or mixed boundary conditions) may result in the qualitatively different results for the thermodynamic limit depending on the way one chooses to approach this limit. These observations imply that the relevant thermodynamic limit remains unclear.

At interfaces between oxide materials, lattice and electronic reconstructions always play important roles in exotic phenomena. In this study, the density functional theory and maximally localized Wannier functions are employed to investigate the (${\mathrm{LaTiO}}_3$)${}_n/\left({\mathrm{LaVO}}_3\right){}_n$ magnetic superlattices. The electron transfer from ${\mathrm{Ti}}^{3+}$ to ${\mathrm{V}}^{3+}$ is predicted, which violates the intuitive band alignment based on the electronic structures of ${\mathrm{LaTiO}}_3$ and ${\mathrm{LaVO}}_3$. Such unconventional charge transfer quenches the magnetism of ${\mathrm{LaTiO}}_3$ layer mostly and leads to metal-insulator transition in the $n=1$ superlattice when the stacking orientation is altered. In addition, the compatibility among the polar structure, ferrimagnetism, and metallicity is predicted in the $n=2$ superlattice.

The Kapitza, or thermal boundary, resistance (TBR), which limits heat dissipation from a thin film to its substrate, is a major factor in the thermal management of ultrathin nanoelectronic devices and is widely assumed to be a property of only the interface. However, data from experiments and molecular dynamics simulations suggest that the TBR between a multilayer two-dimensional (2D) crystal and its substrate decreases with increasing film thickness. To explain this thickness dependence, we generalize the recent theory for single-layer 2D crystals by Z.-Y. Ong et al. [Phys. Rev. B 94, 165427 (2016)], which is derived from the theory by B. N. J. Persson et al. [J. Phys.: Condens. Matter 23, 045009 (2011)], and use it to evaluate the TBR between bare $N$-layer graphene and ${\mathrm{SiO}}_2$. Our calculations reproduce quantitatively the TBR thickness dependence seen in experiments and simulations as well as its asymptotic convergence and predict that the low-temperature TBR scales as $T^{-4}$ in few-layer graphene. Analysis of the interfacial transmission coefficient spectrum shows that the TBR reduction in few-layer graphene is due to the additional contribution from higher flexural phonon branches. Our theory sheds light on the role of flexural phonons in substrate-directed heat dissipation and provides the framework for optimizing the thermal management of multilayered 2D devices.

We have studied the dependence of the rotation angle and ellipticity on the sample orientation and incident polarization from metallic nanohole arrays. The arrays have fourfold symmetry and thus do not possess any intrinsic chirality. We elucidate the role of surface plasmon polaritons (SPPs) in determining the extrinsic chirality, and we verify the results by using finite-difference time-domain (FDTD) simulation. Our results have indicated the outgoing reflection arises from the interference between the nonresonant background, which preserves the input polarization, and the SPP radiation damping, which is linearly polarized but carries a different polarization defined by the vectorial field of SPPs. More importantly, the interference manifests various polarization states ranging from linear to elliptical across the SPP resonance. We analytically formulate the outgoing waves based on temporal coupled mode theory (CMT), and the calculations agree with the FDTD results. From CMT, we find the polarization conversion depends on the interplay between the absorption and radiative decay rates of SPPs and the sample orientation.

The main flaw of the well-known Brinkman-Rice description, obtained through the Gutzwiller approximation, of the paramagnetic Mott transition in the Hubbard model is in neglecting high-energy virtual processes that generate, for instance, the antiferromagnetic exchange $J\sim t^2/U$. Here, we propose a way to capture those processes by combining the Brinkman-Rice approach with a variational Schrieffer-Wolff transformation, and apply this method to study the single-band metal-to-insulator transition in a Bethe lattice with infinite coordination number, where the Gutzwiller approximation becomes exact. We indeed find for the Mott transition a description very close to the real one provided by the dynamical mean-field theory, an encouraging result in view of possible applications to more involved models.

The high room temperature mobility and the electron effective mass in ${\mathrm{BaSnO}}_3$ are investigated in depth by evaluation of the free carrier absorption observed in infrared spectra for epitaxial films with free electron concentrations from $8.3\times {10}^{18}$ to $7.6\times {10}^{20}{\mathrm{cm}}^{-3}$. Both the optical band gap widening by conduction band filling and the carrier scattering mechanisms in the low- and high-doping regimes are consistently described employing parameters solely based on the intrinsic physical properties of ${\mathrm{BaSnO}}_3$. The results explain the current mobility limits in epitaxial films and demonstrate the potential of ${\mathrm{BaSnO}}_3$ to outperform established wide-band gap semiconductors also in the moderate doping regime.

We investigate quantum Hall stripes under an in-plane magnetic field $B_{\parallel }$ in a variable-density two-dimensional electron gas. At filling factor $\nu =9/2$, we observe one, two, and zero $B_{\parallel }$-induced reorientations at low, intermediate, and high densities, respectively. The appearance of these distinct regimes is due to a strong density dependence of the $B_{\parallel }$-induced orienting mechanism which triggers the second reorientation, rendering stripes parallel to $B_{\parallel }$. In contrast, the mechanism which reorients stripes perpendicular to $B_{\parallel }$ showed no noticeable dependence on density. Measurements at $\nu =9/2$ and $11/2$ at the same, tilted magnetic field allow us to rule out the density dependence of the native symmetry-breaking field as a dominant factor. Our findings further suggest that screening might play an important role in determining stripe orientation, providing guidance in developing theories aimed at identifying and describing native and $B_{\parallel }$-induced symmetry-breaking fields.

Topological states of matter are characterized by global topological invariants which change their value across a topological quantum phase transition. It is commonly assumed that the transition between topologically distinct noninteracting gapped phases of fermions is necessarily accompanied by the closing of the band gap as long as the symmetries of the system are maintained. We show that such a quantum phase transition is possible without closing the gap in the case of a three-dimensional topological band insulator. We demonstrate this by calculating the free energy of the minimal model for a topological insulator, the Bernevig-Hughes-Zhang model, and show that as the band curvature continuously varies, a jump between the band-gap minima corresponding to the topologically trivial and nontrivial insulators occurs. Therefore, this first-order phase transition is a generic feature of three-dimensional topological band insulators. For a certain parameter range we predict a reentrant topological phase transition. We discuss our findings in connection with the recent experimental observation of a discontinuous topological phase transition in a family of topological crystalline insulators.