We investigate the thermodynamic properties and the lattice stability of two-dimensional crystalline membranes, such as graphene and related compounds, in the low-temperature quantum regime $T\to 0$. A key role is played by the anharmonic coupling between in-plane and out-of-plane lattice modes that, in the quantum limit, has very different consequences from those in the classical regime. The role of retardation, namely of frequency dependence, in the effective anharmonic interactions turns out to be crucial in the quantum regime. We identify a crossover temperature, $T^{*}$, between the classical and quantum regimes, which is $\sim 70–90$ K for graphene. Below $T^{*}$, the heat capacity and thermal expansion coefficient decrease as power laws with decreasing temperature, tending to zero for $T\to 0$ as required by the third law of thermodynamics.

The very nature of the ground state of the pyrochlore compound ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ is much debated, because experimental results demonstrate evidence for either a disordered ground state or a long-range ordered ground state. Indeed, the delicate balance of exchange interactions and anisotropy is believed to lead to competing states, such as a quantum spin liquid state or a ferromagnetic state which may originate from an Anderson-Higgs transition. We present a detailed magnetization study demonstrating a first-order ferromagnetic transition at 245 and 150 mK in a powder and a single-crystal sample, respectively. Its first-order character is preserved up to applied fields of $\sim 200$ Oe. The transition stabilizes a ferromagnetic component and involves slow dynamics in the magnetization. Residual fluctuations are also evidenced, the presence of which might explain some of the discrepancies between previously published data for ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$.

The potential for tuning of magnetic properties and the exceptional uniformity are among the features that make amorphous magnetic materials attractive for technology. Here it is shown that the magnetization reversal in amorphous SmCo thin films takes place through the formation of giant magnetic domains, over a centimeter across. The domain structure is found to be dictated by the direction of the imprinted in-plane easy axis and the film boundaries. This is a consequence of the size of the anisotropy and the structural uniformity of the films, which also allows the movement of millimeter-long domain walls over distances of several millimeters. The results demonstrate the possibility of tailoring the magnetic domain structure in amorphous magnets over a wide range of length scales, up to centimeters. Moreover, they highlight an important consequence of the structural perfection of amorphous films.

It was recently observed that the two antiferromagnetically coupled sublattices of a rare earth-transition metal ferrimagnet can temporarily align ferromagnetically during femtosecond laser heating, but always with the transition metal aligning in the rare earth direction. This behavior has been attributed to the slower magnetization dynamics of the rare earth sublattice. The aim of this work was to assess how the difference in the speed of the transition metal and rare earth dynamics affects the formation of the transient ferromagneticlike state and consequently controls its formation. Our investigation was performed using extensive atomistic spin simulations and analytic micromagnetic theory of ferrimagnets, with analysis of a large area of parameter space such as initial temperature, Gd concentration, and laser fluence. Surprisingly, we found that at high temperatures, close to the Curie point, the rare earth dynamics become faster than those of the transition metal. Subsequently we show that the transient state can be formed with the opposite polarity, where the rare earth aligns in the transition metal direction. Our findings shed light on the complex behavior of this class of ferrimagnetic materials and highlight an important feature which must be considered, or even exploited, if these materials are to be used in ultrafast magnetic devices.

The effect of the long-range order (LRO) on the longitudinal $({\rho }_{xx},{\rho }_{zz})$ and anomalous Hall $({\rho }_{xy})$ resistivities as well as on the anisotropic magnetoresistance (AMR) in partially ordered $L{1}_0\mathrm{FePt}$ alloys is studied from first principles. The linear-response theory as formulated in the framework of the relativistic tight-binding linear muffin-tin orbital method which includes both the Fermi-surface and Fermi-sea terms is used. The effect of disorder is treated by means of the coherent-potential approximation. The main result is a weak dependence of the anomalous Hall conductivity ${\sigma }_{xy}$ on the LRO, which is, however, compatible with resistivities ${\rho }_{xx}$ and ${\rho }_{xy}$, which both depend strongly on the disorder present in the system. The resistivity and the AMR are predicted to increase with increasing degree of the LRO. We also investigate the effect of spin fluctuations on studied quantities using a simple model of the spin disorder. We have found good agreement between the theory and the recent experiment.

We study a one-dimensional helical system with random Rashba spin-orbit coupling. Using renormalization group methods, we derive a consistent set of flow equations governing the important control parameters of the backscattering process. Thereby, we prove the existence of disorder-induced two-particle backscattering that can even be nonlocal in space. This analysis allows us to derive the scaling form of the conductance at low temperatures. We find that two-particle backscattering due to random spin-orbit coupling differs from the one off a single Rashba impurity by both the scaling of the conductance with the temperature and the relevance of the backscattering operators.

A distinguishing feature of fractional quantum Hall (FQH) states is a singular behavior of equilibrium densities at boundaries. In contrast to states at integer filling fraction, such quantum liquids posses an additional dipole moment localized near edges. It enters observable quantities such as universal dispersion of edge states and Lorentz shear stress. For a Laughlin state, this behavior is seen as a peak, or overshoot, in the single-particle density near the edge, reflecting a general tendency of electrons in FQH states to cluster near edges. We compute the singular edge behavior of the one-particle density by a perturbative expansion carried out around a completely filled Landau level. This correction is shown to fully capture the dipole moment and the major features of the overshoot observed numerically. Furthermore, it exhibits the Stokes phenomenon with the Stokes line at the boundary of the droplet, decaying like a Gaussian inside and outside the liquid with different decay lengths. In the limit of vanishing magnetic length, the shape of the overshoot is a singular double layer with a capacity that is a universal function of the filling fraction. Finally, we derive the edge dipole moment of Pfaffian FQH states. The result suggests an explicit connection between the magnitude of the dipole moment and the bulk odd viscosity.

We study resonant inelastic x-ray scattering (RIXS) peaks corresponding to low-energy particle-hole excited states of metallic FeTe and semimetallic ${\mathrm{TiSe}}_2$ for photon incident energy tuned near the $L_3$ absorption edge of Fe and Ti, respectively. We show that the cross-section amplitudes are well described within a renormalization-group theory where the effect of the core electrons is captured by effective dielectric functions expressed in terms of the the atomic scattering parameters $f_1$ of Fe and Ti. This method can be used to extract the dynamical structure factor from experimental RIXS spectra in metallic systems.

The ground state of frustrated (antiferromagnetic) triangular molecular magnets is characterized by two total-spin $S=1/2$ doublets with opposite chirality. According to a group theory analysis [M. Trif et al., Phys. Rev. Lett. 101, 217201 (2008)], an external electric field can efficiently couple these two chiral spin states, even when the spin-orbit interaction (SOI) is absent. The strength of this coupling, $d$, is determined by an off-diagonal matrix element of the dipole operator, which can be calculated by ab initio methods [M. F. Islam et al., Phys. Rev. B 82, 155446 (2010)]. In this work, we propose that Coulomb-blockade transport experiments in the cotunneling regime can provide a direct way to determine the spin-electric coupling strength. Indeed, an electric field generates a $d$-dependent splitting of the ground-state manifold, which can be detected in the inelastic cotunneling conductance. Our theoretical analysis is supported by master-equation calculations of quantum transport in the cotunneling regime. We employ a Hubbard-model approach to elucidate the relationship between the Hubbard parameters $t$ and $U$, and the spin-electric coupling constant $d$. This allows us to predict the regime in which the coupling constant $d$ can be extracted from experiment.

In the current work we present the complete results for the measurement of normal Casimir force between a shallow and smooth sinusoidally corrugated gold coated sphere and a plate at various angles between the corrugations using an atomic force microscope. All measured data were compared with the theoretical approach using the proximity force approximation and theory based on derivative expansion. In both cases real material properties of the surfaces and nonzero temperature were taken into account. Special attention is paid to the description of electrostatic interactions between corrugated surfaces at different angles between corrugations and samples preparation and characterization. The measured forces are found to be in good agreement with the theory including correlation effects of geometry and material properties and deviate significantly from the predictions of the proximity force approximation approach. This provides the quantitative confirmation for the observation of diffraction-type effects that are disregarded within the PFA approach. The obtained results open new opportunities for control of the Casimir effect in micromechanical systems.

For carbon nanotubes irradiated with 140 keV He ions, the intensity ratio of the $D$ to $G$ Raman modes initially increases and then decreases with increasing ion fluence. A model is proposed to describe the competing effects between the reduction in crystal size, which increases the $D$ band intensity with increasing fluence as a long-range order effect, and the reduction in the number of sixfold rings available for $D$ mode vibration, which decreases the $D$ band intensity with increasing fluence as a short-range order effect. The latter effect dominates when the length of the short-range order becomes comparable to the unit-cell dimensions as a result of ion bombardment. This suggests that the appearance of the maximum $D$ to $G$ intensity ratio coincides with the onset of structural amorphization. The model is supported experimentally by examination of irradiated nanotubes with transmission electron microscopy, and theoretically by Raman calculation based on density functional perturbation theory. A universal formula is proposed to describe $D/G$ behaviors under arbitrary ion irradiation.

Conventional and spin-related thermoelectric effects in transport through a magnetic tunnel junction with a large-spin impurity, such as a magnetic molecule or atom, embedded into the corresponding barrier are studied theoretically in the linear-response regime. The impurity is described by the giant spin Hamiltonian, with both uniaxial and transverse magnetic anisotropy taken into account. Owing to the presence of the transverse component of magnetic anisotropy, the spin of a tunneling electron can be reversed during scattering on the impurity, even in the low-temperature regime. This reversal appears due to the exchange interaction of tunneling electrons with the magnetic impurity. We calculate Seebeck and spin Seebeck coefficients, and analyze their dependence on various parameters of the spin impurity and tunnel junction. In addition, conventional and spin figures of merit as well as the electronic contribution to heat conductance are considered. We also show that pure spin current can be driven by a spin bias applied to the junction with spin impurity, even if no electron transfer between the electrodes can take place. The underlying mechanism employs single-electrode tunneling processes (electrode-spin exchange interaction) and the impurity as an intermediate reservoir of angular momentum.

Access to magnetic excitation spectra of single atoms deposited on surfaces is nowadays possible by means of low-temperature inelastic scanning tunneling spectroscopy. We present a first-principles method for the calculation of inelastic tunneling spectra utilizing the Korringa-Kohn-Rostoker Green function method combined with time-dependent density functional theory and many-body perturbation theory. The key quantity is the electron self-energy describing the coupling of the electrons to the spin excitation within the adsorbate. By investigating Cr, Mn, Fe, and Co adatoms on a Cu(111) substrate, we spin-characterize the spectra and demonstrate that their shapes are altered by the magnetization of the adatoms, of the tip and the orbital decay into vacuum. Our method also predicts spectral features more complex than the steps obtained by simpler models for the adsorbate (e.g., localized spin models).

Photoluminescence (PL) spectroscopy is a useful probe of excitonic interactions in optically excited nanostructures. Under intense optical excitation, the diffusion-annihilation of excitons in single-walled carbon nanotubes (SWCNTs) results in strong nonlinear PL. This behavior has been observed in a number of samples and has, until recently, been believed to be independent of excitation pathway. Contrary to this assumption, recent studies show that nonlinear PL in encapsulated SWCNTs, excited resonant to ${\mathrm{E}}_{22}$, is not dominated by diffusion-annihilation but instead by laser induced quenching sites. In this paper, we show that, unlike encapsulated SWCNTs, air-suspended SWCNT PL saturation is independent of excitation pathway, validating the use of a diffusion model for excitons generated via ${\mathrm{E}}_{22}$ excitation. In addition, we show that the diffusion of excitons in air-suspended SWCNTs is independent of atmospheric adsorbates, strengthening the assertion that in this system exciton diffusion is intrinsic and not disorder limited.

We calculate the energy bands for graphene monolayers when electrons move through a periodic electrostatic potential in the presence of a uniform perpendicular magnetic field. We clearly demonstrate the quantum fractal nature of the energy bands at reasonably low magnetic fields. We present results for the energy bands as functions of both wave number and magnetic flux through the unit cells of the resulting moiré superlattice. The effects due to pseudospin coupling and Landau orbit mixing by a strong scattering potential have been exhibited. At low magnetic fields when the Landau orbits are much larger than the period of the modulation, the Landau levels are only slightly broadened. This feature is also observed at extremely high magnetic fields. The density of states has been calculated and shows a remarkable self-similarity like the energy bands. We estimate that for modulation period of 10 nm the region where the Hofstadter butterfly is revealed is $B\le 2$ T.

We consider the Josephson current through the carbon nanotube quantum dot with twofold orbital degeneracy connected by two superconductor leads. We show that the removal of orbital and spin degeneracies due to strong spin-orbit coupling and external magnetic field has a significant influence on the subgap Andreev bound states and the $0\text{−}\pi$ transition of the Josephson current. The $0\text{−}\pi$ transition point is determined by the level crossing of the lowest branch of the bound states and the Fermi level, and is given by $[T_K-{\Delta }_{\text{SO}}/2-(\mu +1)B]/\Delta \sim 1$, with $T_K$ as the Kondo temperature, ${\Delta }_{\text{SO}}$ the spin-orbit coupling, $\Delta$ the superconducting gap, $\mu$ the ratio of orbital moment and Bohr magneton, and $B$ the Zeeman splitting loaded by the magnetic field. The interplay of the Kondo effect and superconductivity in such a quantum dot device provides a route to manipulate the $0\text{−}\pi$ transition of the Josephson current.

Topological protected surface states are one of the hallmarks of three-dimensional topological insulators. In this paper we theoretically analyze the gate-voltage effects on a quasi-3D layer of HgTe. We find that while the gapless surface states dominate the transport, as an external gate voltage is applied, the existence of bulk charge carriers is likely to occur. We also find that due to screening effects, physical properties that arise from the bottom surface are gate-voltage independent. Finally, we point out the experimental signatures that characterize these effects.

The photoemission spectra of four different metal quinoline molecules, namely, the prototypical tris(8-hydroxyquinolinato)-aluminum(III) $\left({\mathrm{Alq}}_3\right)$ complex and the related compounds ${\mathrm{Gaq}}_3,{\mathrm{Inq}}_3$, and tris-(9-hydroxyphenalen-1oate)-aluminum(III) [$\mathrm{Al}\left(\mathrm{OP}\right){}_3$] are compared to the electronic structure computed with different first-principles methods. In general, we found that, for ${\mathrm{Alq}}_3,{\mathrm{Gaq}}_3$, and ${\mathrm{Inq}}_3$, the molecular orbitals obtained with density functional theory and hybrid functionals represent a good approximation to the quasiparticle states. The same conclusion can be partially extended to the interesting case of $\mathrm{Al}\left(\mathrm{OP}\right){}_3$, although a direct comparison between theoretical and experimental results appears rather difficult for states, which are lower in energy than the first ten highest occupied molecular orbitals. Taking our results as a starting point we critically discuss the different available experimental data concerning the charge transport gap of ${\mathrm{Alq}}_3$.