We investigate single CoFeB nanotubes with hexagonal cross section using dynamic cantilever magnetometry (DCM). We develop both an analytical model based on the Stoner-Wohlfarth approximation and a broadly applicable numerical framework for analyzing DCM measurements of magnetic nanostructures. Magnetometry data taken at 4.2 K show the presence of a uniformly magnetized configuration at high external fields with ${\mu }_0{M}_s=1.3\pm 0.1\mathrm{T}$ and nonuniform configurations at low fields. In this low-field regime, comparison between numerical simulations and DCM measurements supports the existence of flux-closure configurations. Crucially, evidence of such configurations is only apparent because of the sensitivity of DCM to single nanotubes, whereas conventional measurements of ensembles are often obscured by sample-to-sample inhomogeneities in size, shape, and orientation.

${}^{71}\mathrm{Ga}$ NQR, magnetization, and specific-heat measurements have been performed on polycrystalline Ge-doped ${\mathrm{FeGa}}_3$ samples. A crossover from an insulator to a correlated local moment metal in the low-doping regime and the evolution of itinerant ferromagnet upon further doping is found. For the nearly critical concentration at the threshold of ferromagnetic order, $x_C=0.15, {}^{71}(1/T_{1}T)$ exhibits a pronounced $T^{-4/3}$ power law over two orders of magnitude in temperature, which indicates three-dimensional quantum critical ferromagnetic fluctuations. Furthermore, for the ordered $x=0.2$ sample ($T_C\approx 6$ K), ${}^{71}(1/T_{1}T)$ could be fitted well in the frame of Moriya's self-consistent renormalization theory for weakly ferromagnetic systems with $1/T_{1}T\sim \chi$. In contrast to this, the low-doping regime nicely displays local moment behavior where $1/T_{1}T\sim {\chi }^2$ is valid. For $T\to 0$, the Sommerfeld ratio $\gamma =(C/T)$ is enhanced $(70\mathrm{mJ}/\mathrm{mole}{\mathrm{K}}^2$ for $x=0.1)$, which indicates the formation of heavy $3d$ electrons.

Angle-resolved photoemission spectroscopy is used to study the band structure of superconducting electrochemically intercalated ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$. We find that in these samples, the band gap at the $\mathrm{\Gamma }$ point is much larger than in pristine ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Comparison to the results of band structure calculations indicates that the origin of this large gap is internal stress caused by disorder created by the Cu intercalation. We suggest that the internal pressure may be necessary for superconductivity in ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$.

Finding novel atomically thin heterostructures and understanding their characteristic properties are critical for developing better nanoscale optoelectronic devices. In this study, we investigate the electronic and optical properties of a GaS-Ca(OH)${}_2$ heterostructure using first-principle calculations. The band gap of the GaS-Ca(OH)${}_2$ heterostructure is significantly reduced when compared to those of the isolated constituent layers. Our calculations show that the GaS-Ca(OH)${}_2$ heterostructure is a type-II heterojunction which can be used to separate photoinduced charge carriers where electrons are localized in GaS and holes in the Ca(OH)${}_2$ layer. This leads to spatially indirect excitons which are important for solar energy and optoelectronic applications due to their long lifetime. By solving the Bethe-Salpeter equation on top of a single shot GW calculation $\left(G_0{W}_0\right)$, the dielectric function and optical oscillator strength of the constituent monolayers and the heterostructure are obtained. The oscillator strength of the optical transition for the GaS monolayer is an order of magnitude larger than the Ca(OH)${}_2$ monolayer. We also found that the calculated optical spectra of different stacking types of the heterostructure show dissimilarities, although their electronic structures are rather similar. This prediction can be used to determine the stacking type of ultrathin heterostructures.

Considering two-dimensional electron gases under a perpendicular magnetic field, we pinpoint a specific kind of long-range bipartite entanglement of the electronic motions. This entanglement is achieved through the introduction of bicomplex spinorial eigenfunctions admitting a polar decomposition in terms of a real modulus and three real phases. Within this bicomplex geometry the cyclotron motions of two electrons are intrinsically tied, so that the highlighted eigenstates of the kinetic energy operator actually describe the free motion of a genuine electron pair. Most remarkably, these states embody phase singularities in the four-dimensional (4D) space, with singular points corresponding to the simultaneous undetermination of the three phases. Because the entanglement between the two electrons forming a pair, as well as the winding and parity quantum numbers characterizing the 4D phase singularity, are topological in nature, we expect them to manifest some robustness in the presence of a smooth disorder potential and an electron-electron interaction potential. The relevance of this effective approach in terms of 4D vortices of electron pairs is discussed in the context of the fractional quantum Hall effect.

In disordered Weyl semimetals, mechanisms of topological origin lead to novel mechanisms of transport, which manifest themselves in unconventional types of electromagnetic response. Prominent examples of transport phenomena particular to the Weyl context include the anomalous Hall effect, the chiral magnetic effect, and the formation of totally field-dominated regimes of transport in which the longitudinal conductance is proportional to an external magnetic field. In this paper, we discuss the manifestations of these phenomena at large length scales including the cases of strong disorder and/or magnetic field which are beyond the scope of diagrammatic perturbation theory. Our perhaps most striking finding is the identification of a regime of drift-diffusion transport where diffusion at short scales gives way to effectively ballistic dynamics at large scales before a reentrance to diffusion takes place at yet larger scales. We show that this regime plays a key role in understanding the interplay of the various types of magnetoresponse of the system. Our results are obtained by describing the strongly disordered system in terms of an effective field theory of Chern-Simons type. The paper contains a self-contained derivation of this theory and a discussion of both equilibrium and nonequilibrium (noise) transport phenomena following from it.

The existence of long-lived nonequilibrium states without showing thermalization, which has previously been demonstrated in time evolution of ultracold atoms, suggests the possibility of their spatial analog in transport behavior of interacting electrons in solid-state systems. Here we report long-lived nonequilibrium states in one-dimensional edge channels in the integer quantum Hall regime. An indirect heating scheme in a counterpropagating configuration is employed to generate a nontrivial binary spectrum consisting of high- and low-temperature components. This unusual spectrum is sustained even after traveling 5–$10\mu \mathrm{m}$, much longer than the length for electronic relaxation (about 0.$1\mu \mathrm{m})$, without showing significant thermalization. This observation is consistent with the integrable model of Tomonaga-Luttinger liquid. The long-lived spectrum implies that the system is well described by noninteracting plasmons, which are attractive for carrying information for a long distance.

We analyze the Casimir interaction of doped graphene. To this end we derive a simple expression for the finite-temperature polarization tensor with a chemical potential. It is found that doping leads to a strong enhancement of the Casimir force, reaching almost $60\%$ in quite realistic situations. This result should be important for planning and interpreting Casimir measurements, especially taking into account that the Casimir interaction of undoped graphene is rather weak.

We study the electronic spin relaxation effect in the hole-doped monolayer and bilayer transition-metal dichalcogenide in the presence of the crystal defects. We consider realistic models of the lattice vacancy and actually estimate the spin relaxation rate using the multi-orbital tight-binding model. In the monolayer, the spin-relaxation time is found to be extremely long compared to the momentum relaxation time, and this is attributed to the fact that the spin hybridization in the band structure is suppressed by the mirror reflection symmetry. The bilayer TMD has a much shorter spin relaxation time in contrast, and this is attributed to stronger spin hybridization due to the absence of the mirror symmetry.

Strong electron correlations and interference effects are discussed in parallel-coupled single-level or orbitally doubly degenerate quantum dots. The finite-$U$ mean-field slave boson approach is used to study many-body effects. The analysis is carried out in a wide range of parameter space including both atomic-like and molecular-like Kondo regimes and taking into account various perturbations, like interdot tunneling, interdot interaction, mixing of the electrode channels, and exchange interaction. We also discuss the influence of singularities of electronic structure and the impact of polarization of electrodes. Special attention is paid to potential spintronic applications of these systems showing how current polarization can be controlled by adjusting interference conditions and correlations by gate voltage. Simple proposals of double dot spin valve and bipolar electrically tunable spin filter are presented.

We use time-resolved photoluminescence (TRPL) spectroscopy to unequivocally clarify the microscopic origin of the nanosecond free exciton photoluminescence rise in GaAs at low temperatures. In crucial distinction from previous work, we examine the TRPL of the GaAs free exciton second LO-phonon replica. This enables us to simultaneously monitor the unambiguous time evolution of the total exciton population and the cooling dynamics of the initially hot free exciton ensemble. We demonstrate by a model based on the Saha equation and the experimentally determined cooling behavior that the long-debated slow photoluminescence rise is caused by time-dependent shifts in the thermodynamic quasiequilibrium between free excitons and the uncorrelated electron-hole plasma.

Transport by helical edge states of a quantum spin Hall insulator is experimentally characterized by a weakly temperature-dependent mean free path of a few microns and by reproducible conductance oscillations, challenging proposed theoretical explanations. We consider a model where edge electrons experience spatially random Rashba spin-orbit coupling and couple to a magnetic impurity with spin $S\ge 1/2$. In a finite bias steady state, we find for $S>1/2$ an impurity induced resistance with a temperature dependence in agreement with experiments. Since backscattering is elastic, interference between different scatterers possibly explains conductance fluctuations.

We achieve the enhancement of the circular photogalvanic effect arising from the photoinjection of spins in topological insulator thin films by tuning the Fermi level $\left(E_{\mathrm{F}}\right)$. A series of ${\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_3$ thin films were tailored so that the Fermi energy ranges above 0.34 eV to below 0.29 eV of the Dirac point, i.e., from the bulk conduction band bottom to the valence band top through the bulk in-gap surface-Dirac cone. The circular photogalvanic current, indicating a flow of spin-polarized surface-Dirac electrons, shows a pronounced peak when the $E_{\mathrm{F}}$ is set near the Dirac point and is also correlated with the carrier mobility. Our observation reveals that there are substantial scatterings between the surface-Dirac and bulk state electrons in the generation process of spin-polarized photocurrent, which can be avoided by designing the electronic structure in topological insulators.

Increasing the spin imbalance in superconductors can spatially modulate the gap by forming Cooper pairs with finite momentum. For large imbalances compared to the Fermi energy, the inhomogeneous FFLO superconductor ultimately becomes a normal metal. There is mounting experimental evidence for this scenario in two-dimensional (2D) organic superconductors in large in-plane magnetic fields; this is complemented by ongoing efforts to realize this scenario in coupled tubes of atomic Fermi gases with spin imbalance. Yet, a theory for the phase transition from a metal to an FFLO superconductor has not been developed so far and the universality class has remained unknown. Here we propose and analyze a spin imbalance driven quantum critical point between a 2D metal and an FFLO phase in anisotropic electron systems. We derive the effective action for electrons and bosonic FFLO pairs at this quantum phase transition. Using this action, we predict non-Fermi-liquid behavior and the absence of quasiparticles at a discrete set of hot spots on the Fermi surfaces. This results in strange power laws in thermodynamics and response functions, which are testable with existing experimental setups on 2D organic superconductors and may also serve as signatures of the elusive FFLO phase itself. The proposed universality class is distinct from previously known quantum critical metals and, because its critical fluctuations appear already in the pairing channel, a promising candidate for naked metallic quantum criticality over extended temperature ranges.

Electronic structures associated with the flat (polar) Se/Te- or Tl-terminated surfaces of ${\mathrm{TlBiSe}}_2$ and ${\mathrm{TlBiTe}}_2$ are predicted to harbor not only Dirac cone states, but also trivial dangling bond states near the Fermi energy. However, the latter, trivial states have never been observed in photoemission measurements. In order to address this discrepancy, we have carried out ab initio calculations for various surfaces of ${\mathrm{TlBiSe}}_2$ and ${\mathrm{TlBiTe}}_2$. A rough nonpolar surface with an equal number of Se/Te and Tl atoms in the surface atomic layer is found to destroy the trivial dangling bond states, leaving only the Dirac cone states in the bulk energy gap. The resulting energy dispersions of the Dirac states are in good accord with the corresponding experimental dispersions in ${\mathrm{TlBiSe}}_2$ as well as ${\mathrm{TlBiTe}}_2$. We also show that in the case of flat, Se terminated, high-index (221) and (112) surfaces of ${\mathrm{TlBiSe}}_2$, the trivial surface states shift energetically below the Dirac node and become well separated from the Dirac cone states.

We perform, as a function of uniaxial stress, a temperature-dependent optical-reflectivity investigation of the parent Fe-arsenide compound ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ over a broad spectral range, from the far infrared up to the ultraviolet, across the coincident structural tetragonal-to-orthorhombic and spin-density-wave (SDW) phase transitions at $T_{s,N}=135$ K. Our results provide knowledge to the complete electrodynamic response of the title compound over a wide energy range as a function of both tunable variables. For temperatures below $T_{s,N}$, varying the uniaxial stress in situ affects the twin domain population and yields hysteretic behavior of the optical properties as the stress is first increased and then decreased, whereas for temperatures above $T_{s,N}$ the stress-induced optical anisotropy is reversible, as anticipated. In particular, by analyzing the low-frequency infrared response, we obtain detailed insight to the effects determining the intrinsic anisotropy of the (metallic) charge dynamics in the orthorhombic state, and similarly the induced one due to applied uniaxial stress at higher temperatures in the tetragonal phase. The low-frequency optical conductivity thus allows establishing a link to the $dc$ transport properties and reveals that they are determined almost exclusively by changes in the Drude weight, therefore by the anisotropy in the Fermi surface parameters. Finally, we show that the spectral weight distribution in the SDW state occurs for energies below approximately 1 eV, and therefore points towards a correlation mechanism due to Hund's coupling rather than on-site Coulomb interactions.

The presence of stacking fault tetrahedra (SFTs) in face-centered-cubic metals substantially modifies the material response to external loading. These defects are extremely stable with increasing energetic stability as they grow in size. At the sizes visible within a transmission electron microscope, they appear nearly immobile. We have recently shown that these defects might indeed migrate, with defective SFTs exhibiting particularly high mobilities. In this paper, using molecular dynamics, we show how mobile SFTs interact with various types of extended defects, including free surfaces, dislocations, and interfaces in Cu and Cu-Nb systems. We observe a direct relation between the energetics of a single vacancy interacting with each external defect and the propensity for the SFT to be absorbed. Finally, using mesoscale modeling, we show how the fact that SFTs can migrate influences the system evolution and potentially important observables of interest such as the void denuded zones around defect sinks.

${\text{Eu}}_{1-x}{\text{Ho}}_x{\text{MnO}}_3$ with $x<0.5$ has an orthorhombic perovskite structure, where the partial substitution of europium by holmium increases the orthorhombic distortion and changes the low-temperature magnetic structure from canted antiferromagnetic to the multiferroic ground state with cycloidal spin arrangement for $x>0.2$. We employ polarized, temperature-dependent Raman spectroscopy to study the lattice dynamics and spin-phonon coupling (SPC) effects for the two representative compositions $x=0.1$ and $x=0.3$. We observe an enhancement of phonon frequencies by ${\text{Ho}}^{3+}$ substitution and we use the $A_g\left(4\right)$ mode as a measure to evaluate the orthorhombic distortion angle $\theta$. SPC manifests itself as mode-specific softening for $T<100$ K, induced by the magnetic ordering of ${\text{Mn}}^{3+}$ spins. Furthermore, we present evidence that the hitherto elusive Raman peak at $\approx 650{\text{cm}}^{-1}$ is a phonon of $B_{3g}$ symmetry and ascribe it to the $B_{3g}\left(1\right)$ out-of-phase ${\text{MnO}}_6$-breathing mode. We show that this mode has the strongest SPC effect of all observed phonon modes, a fact which we explain by the participation of the apical O(1) ions, modulating the Mn-O(1)-Mn bond whose SPC contribution was hitherto not accessible. Finally, we present a quantitative evaluation of the SPC contribution by the antiferromagnetic plane-to-plane coupling through the Mn-O(1)-Mn bonds and discriminate if from the in-plane ferromagnetic Mn-O(2)-Mn part.