Giant, positive, and near-temperature-independent linear magnetoresistance (LMR), as large as 340%, was observed in graphene foam with a three-dimensional flexible network. Careful analysis of the magnetoresistance revealed that Shubnikov–de Haas (SdH) oscillations occurred at low temperatures and decayed with increasing temperature. The average classical mobility ranged from 300 (2 K) to 150 (300 K) $\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, which is much smaller than that required by the observed SdH oscillations. To understand the mechanism behind the observation, we performed the same measurements on the microsized graphene sheets that constitute the graphene foam. Much more pronounced SdH oscillations superimposed on the LMR background were observed in these microscaled samples, which correspond to a quantum mobility as high as $26,500\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Moreover, the spatial mobility fluctuated significantly from $64,200\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ to $1370\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, accompanied by a variation of magnetoresistance from near 20,000% to less than 20%. The presence of SdH oscillations actually excludes the possibility that the observed LMR originated from the extreme quantum limit, because this would demand all electrons to be in the first Landau level. Instead, we ascribe the large LMR to the second case of the classical Parish and Littlewood model, in which spatial mobility fluctuation dominates electrical transport. This is an experimental confirmation of the Parish and Littlewood model by measuring the local mobility randomly (by measuring the microsized graphene sheets) and finding the spatial mobility fluctuation.

By considering a nanoscale Aharonov-Bohm (AB) interferometer consisting of a laterally coupled double dot coupled to the source and drain electrodes, we investigate the AB phase dependence of the bonding and antibonding states and the transport currents via the bonding and antibonding state channels. The relations of the AB phase dependence between the quantum states and the associated transport current components are analyzed, which provides useful information for the reconstruction of quantum states through the measurement of the transport current in such systems. We also obtain the validity of the experimental analysis [given in T. Hatano et al., Phys. Rev. Lett. 106, 076801 (2011)] that bonding state currents in different energy configurations are almost the same. With the coherent properties in the quantum dot states as well as in the transport currents, we also provide a way to manipulate the bonding and antibonding states through the AB magnetic flux.

We examine the Casimir free energy and pressure of magnetic metal films, which are free standing in vacuum, sandwiched between two dielectric plates and deposited on either nonmagnetic or magnetic metallic plates. All calculations are performed using both the Drude and plasma model approaches to the Lifshitz theory. According to our results, the Casimir free energies and pressures calculated using both theoretical approaches are significantly different in the magnitude and sign even for thin films of several tens of nanometers thickness. Thus, for the Ni film of 47 nm thickness deposited on a Fe plate the obtained magnitudes of the Casimir free energy differ by the factor of 5866. We show that the Casimir free energy and pressure of a magnetic film calculated using the plasma model approach do not possess the classical limit but exponentially fast drop to zero with increasing film thickness. If the Drude model approach is used, the classical limit is reached for magnetic films of about 150 nm thickness, but the Casimir free energy remains nonzero in the limit of ideal metal, contrary to expectations. For the plasma model approach the Casimir free energy of a film vanishes in this case. Numerical computations are performed for the magnetic films made of Ni, nonmagnetic plates made of Cu and Al, and magnetic plates made of Fe using the tabulated optical data for the complex indexes of refraction of all metals. The obtained results can be used for a discrimination between the plasma and Drude model approaches in the Casimir physics and in the investigation of stability of thin films.

When a strongly correlated quantum dot is tunnel coupled to a superconductor, it leads to the formation of Shiba bound states inside the superconducting gap. They have been measured experimentally in a superconductor-quantum dot-normal lead setup. Side coupling the quantum dot to a topological superconducting wire that supports Majorana bound states at its ends, drastically affects the structure of the Shiba states and induces supplementary in-gap states. The anomalous coupling between the Majorana bound states and the quantum dot gives rise to a characteristic imbalance in the spin-resolved spectral functions for the dot operators. These are clear fingerprints for the existence of Majorana fermions and they can be detected experimentally in transport measurements. In terms of methods employed, we have used analytical approaches combined with the numerical renormalization group approach.

A fluctuational electrodynamics-based formalism for calculating near-field radiative heat transfer between objects of arbitrary size and shape and an infinite surface is presented. The surface interactions are treated analytically via Sommerfeld's theory of electric dipole radiation above an infinite plane. The volume integral equation for the electric field is discretized using the thermal discrete dipole approximation (T-DDA). The framework is verified against exact results in the sphere-surface configuration and is applied to analyze near-field radiative heat transfer between a complex-shaped probe and an infinite plane, both made of silica. It is found that, when the probe tip size is approximately equal to or smaller than the gap $d$ separating the probe and the surface, coupled localized surface phonon (LSPh)-surface phonon-polariton (SPhP) mediated heat transfer occurs. In this regime, the net spectral heat rate exhibits four resonant modes due to LSPhs along the minor axis of the probe, while the net total heat rate in the near field follows a $d^{-0.3}$ power law. Conversely, when the probe tip size is much larger than the separation gap $d$, heat transfer is mediated by SPhPs, resulting in two resonant modes in the net spectral heat rate, corresponding to those of a single emitting silica surface, while the net total heat rate approaches a $d^{-2}$ power law. It is also demonstrated that a complex-shaped probe can be approximated by a prolate spheroidal electric dipole when the thermal wavelength is larger than the major axis of the spheroidal dipole and when the separation gap $d$ is much larger than the radius of curvature of the dipole tip facing the surface.

We compare extended x-ray absorption fine structure (EXAFS) data at the Zn K edge for a low concentration of Zn (0.7 mol%) in a stoichiometric crystal with that for higher Zn concentrations (nominally 5 and 9 mol%) in congruent ${\mathrm{LiNbO}}_3$ (LNO). Note that stoichiometric and congruent LNO have significantly different optical properties. We find no significant difference in the local structure about Zn out to 4 Å for the two types of crystals and different dopant levels. Although some earlier theoretical models suggest a self-compensation model with 75% of Zn on a Li site and 25% Zn on Nb, we find no clear evidence for a significant fraction of Zn on the Nb site, and estimate at most 2%–3% of Zn might be ${\mathrm{Zn}}_{Nb}$.

We develop a general model to investigate the dynamics of charge carriers in vacancy endowed honeycomb two-dimensional nanolattices. As a fundamental application, results concerning the influence of vacancies placed on different sites of semiconducting armchair graphene nanoribbons (AGNR) over the transport of polarons are presented. It is observed that the positioning of vacancies plays a major role over the scattering of the charge carriers, in the sense that their overall mobility is determined by where the defect is allocated. By considering different structural configurations of the system, the arising polaron can either move freely or be reflected. Therefore, our work provides a phenomenological understanding of the underlying mechanism responsible for the change of conductivity experienced by systems in which structural defects are present, a fact that has been reported for different nanostructures of the same symmetry. Because vacancies are one of the most common kinds of defects and are, in practice, unavoidable, the kind of description proposed in the present paper is crucial to correctly address transport and electronic properties in more realistic electronic devices based on two-dimensional nanolattices.

An acoustic metasurface carpet cloak based on membrane-capped cavities is proposed and investigated numerically. This design has been chosen for allowing ultrathin geometries, although adapted to airborne sound frequencies in the range of 1 kHz ($\lambda \approx 30$ cm), surpassing the designs reported in the literature in terms of thinness. A formulation of generalized Snell's laws is first proposed, mapping the directions of the incident and reflected waves to the metasurface phase function. This relation is then applied to achieve a prescribed wavefront reflection direction, for a given incident direction, by controlling the acoustic impedance grading along the metasurface. The carpet cloak performance of the proposed acoustic metasurface is then assessed on a triangular bump obstacle, generally considered as a baseline configuration in the literature.

Angle-resolved upper critical field $H_{c2}$ provides an efficient tool to probe the gap symmetry of unconventional superconductors. We revisit the behavior of in-plane $H_{c2}$ in $d$-wave superconductors by considering both the orbital effect and Pauli paramagnetic effect. After carrying out systematic analysis, we show that the maxima of $H_{c2}$ could be along either nodal or antinodal directions of a $d$-wave superconducting gap, depending on the specific values of a number of tuning parameters. This behavior is in contrast to the common belief that the maxima of in-plane $H_{c2}$ are along the direction where the superconducting gap takes its maximal value. Therefore, identifying the precise $d$-wave gap symmetry through fitting experiments results of angle-resolved $H_{c2}$ with model calculations at a fixed temperature, as widely used in previous studies, is difficult and practically unreliable. However, our extensive analysis of angle-resolved $H_{c2}$ show that there is a critical temperature $T^{*}$: in-plane $H_{c2}$ exhibits its maxima along nodal directions at $T<T^{*}$ and along antinodal directions at $T^{*}<T<T_c$. The concrete value of $T^{*}$ may change as other parameters vary, but the existence of $\pi /4$ shift of $H_{c2}$ at $T^{*}$ appears to be a general feature. Thus a better method to identify the precise $d$-wave gap symmetry is to measure $H_{c2}$ at a number of different temperatures, and examine whether there is a $\pi /4$ shift in its angular dependence at certain $T^{*}$. We further show that Landau level mixing does not change this general feature. However, in the presence of Fulde-Ferrell-Larkin-Ovchinnikov state, the angular dependence of $H_{c2}$ becomes quite complicated, which makes it more difficult to determine the gap symmetry by measuring $H_{c2}$. Our results indicate that some previous studies on the gap symmetry of ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ are unreliable and need to be reexamined, and also provide a candidate solution to an experimental discrepancy in the angle-resolved $H_{c2}$ in ${\mathrm{CeCoIn}}_5$.

We formulate and apply a low-energy transport theory for hybrid quantum devices containing junctions of topological superconductor (TS) wires and conventional normal (N) or superconducting (S) leads. We model TS wires as spinless $p$-wave superconductors and derive their boundary Keldysh Green's function, capturing both the Majorana end state and continuum quasiparticle excitations in a unified manner. We also specify this Green's function for a finite-length TS wire. Junctions connecting different parts of the device are described by the standard tunneling Hamiltonian. Using this Hamiltonian approach, one also has the option to include many-body interactions in a systematic manner. For N-TS junctions, we provide the current-voltage $\left(I\text{−}V\right)$ characteristics at arbitrary junction transparency and give exact results for the shot-noise power and the excess current. For TS-TS junctions, analytical results for the thermal noise spectrum and for the $I\text{−}V$ curve in the high-transparency low-bias regime are presented. For S-TS junctions, we compute the entire $I\text{−}V$ curve and clarify the conditions for having a finite Josephson current.

We report a first-principles study of the superconducting critical temperature and other properties of Fe-based superconductors taking into account, on equal footing, phonon, charge, and spin-fluctuation mediated Cooper pairing. We show that in FeSe this leads to a modulated $\mathrm{s}\pm$ gap symmetry and that the antiferromagnetic paramagnons are the leading mechanism for superconductivity in FeSe, overcoming the strong repulsive effect of both phonons and charge pairing.

Coherent spin-wave generation by focused ultrashort laser pulse irradiation was investigated for a permalloy thin film at micrometer scale using an all-optical space- and time-resolved magneto-optical Kerr effect microscope. The spin-wave packet propagating perpendicular to the magnetization direction was clearly observed; however, that propagating parallel to the magnetization direction was not observed. The propagation length, group velocity, center frequency, and packet width of the observed spin-wave packet were evaluated and quantitatively explained in terms of the propagation of a magnetostatic spin wave driven by the ultrafast change of an out-of-plane demagnetization field induced by the focused-pulse laser.

We study the underlying chemical, electronic, and magnetic properties of a number of magnetite-based thin films. The main focus is placed onto ${\mathrm{Fe}}_3{\mathrm{O}}_4$(001)/NiO bilayers grown on MgO(001) and Nb-${\mathrm{SrTiO}}_3$(001) substrates. We compare the results with those obtained on pure ${\mathrm{Fe}}_3{\mathrm{O}}_4$(001) thin films. It is found that the magnetite layers are oxidized and ${\mathrm{Fe}}^{3+}$ dominates at the surfaces due to maghemite ($\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$) formation, which decreases with increasing magnetite layer thickness. For layer thicknesses of around 20 nm and above, the cationic distribution is close to that of stoichiometric ${\mathrm{Fe}}_3{\mathrm{O}}_4$. At the interface between NiO and ${\mathrm{Fe}}_3{\mathrm{O}}_4$ we find the Ni to be in a divalent valence state, with unambiguous spectral features in the Ni $2p$ core level x-ray photoelectron spectra typical for NiO. The formation of a significant ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ interlayer can be excluded by means of x-ray magnetic circular dichroism. Magneto-optical Kerr effect measurements reveal significant higher coercive fields compared to magnetite thin films grown on MgO(001), and an altered in-plane easy axis pointing in the $\langle 100\rangle$ direction. We discuss the spin magnetic moments of the magnetite layers and find that a thickness of 20 nm or above leads to spin magnetic moments close to that of bulk magnetite.

We use finite size scaling to study Ising spin glasses in two spatial dimensions. The issue of universality is addressed by comparing discrete and continuous probability distributions for the quenched random couplings. The sophisticated temperature dependency of the scaling fields is identified as the major obstacle that has impeded a complete analysis. Once temperature is relinquished in favor of the correlation length as the basic variable, we obtain a reliable estimation of the anomalous dimension and of the thermal critical exponent. Universality among binary and Gaussian couplings is confirmed to a high numerical accuracy.

We explore the emergence of chiral magnetism in one-dimensional monatomic Mn, Fe, and Co chains deposited at the Pt(664) step edge carrying out an ab initio study based on density functional theory (DFT). The results are analyzed employing several models: (i) a micromagnetic model, which takes into account the Dzyaloshinskii-Moriya interaction (DMI) besides the spin stiffness and the magnetic anisotropy energy, and (ii) the Fert-Levy model of the DMI for diluted magnetic impurities in metals. Due to the step-edge geometry, the direction of the Dzyaloshinskii vector ($\mathrm{D}$ vector) is not predetermined by symmetry and points in an off-symmetry direction. For the Mn chain we predict a long-period cycloidal spin-spiral ground state of unique rotational sense on top of an otherwise atomic-scale antiferromagnetic phase. The spins rotate in a plane that is tilted relative to the Pt surface by ${62}^{\circ }$ towards the upper step of the surface. The Fe and Co chains show a ferromagnetic ground state since the DMI is too weak to overcome their respective magnetic anisotropy barriers. An analysis of domain walls within the latter two systems reveals a preference for a Bloch wall for the Fe chain and a Néel wall of unique rotational sense for the Co chain in a plane tilted by ${29}^{\circ }$ towards the lower step. Although the atomic structure is the same for all three systems, not only the size but also the direction of their effective $\mathrm{D}$ vectors differ from system to system. The latter is in contradiction to the Fert-Levy model. Due to the considered step-edge structure, this work provides also insight into the effect of roughness on DMI at surfaces and interfaces of magnets. Beyond the discussion of the monatomic chains we provide general expressions relating ab initio results to realistic model parameters that occur in a spin-lattice or in a micromagnetic model. We prove that a planar homogeneous spiral of classical spins with a given wave vector rotating in a plane whose normal is parallel to the $\mathrm{D}$ vector is an exact stationary state solution of a spin-lattice model for a periodic solid that includes Heisenberg exchange and DMI. In the vicinity of a collinear magnetic state, assuming that the DMI is much smaller than the exchange interaction, the curvature and slope of the stationary energy curve of the spiral as a function of the wave vector provide directly the values of the spin stiffness and the spiralization required in micromagnetic models. The validity of the Fert-Levy model for the evaluation of micromagnetic DMI parameters and for the analysis of ab initio calculations is explored for chains. The results suggest that some care has to be taken when applying the model to infinite periodic one-dimensional systems.

We investigate the capability of density functional theory (DFT) to appropriately describe the spin susceptibility, ${\chi }_s$, and the intervalley electron-phonon coupling in ${\mathrm{Li}}_x\mathrm{ZrNCl}$. At low doping, ${\mathrm{Li}}_x\mathrm{ZrNCl}$ behaves as a two-dimensional two-valley electron gas, with parabolic bands. In such a system, ${\chi }_s$ increases with decreasing doping because of the electron-electron interaction. We show that DFT with local functionals (LDA/GGA) is not capable of reproducing this behavior. The use of exact exchange in Hartree-Fock (HF) or in DFT hybrid functionals enhances ${\chi }_s$. HF, B3LYP, and PBE0 approaches overestimate ${\chi }_s$, whereas the range-separated HSE06 functional leads to results similar to those obtained in the random phase approximation (RPA) applied to a two-valley two-spin electron gas. Within HF, ${\mathrm{Li}}_x\mathrm{ZrNCl}$ is even unstable towards a ferromagnetic state for $x<0.16$. The intervalley phonons induce an imbalance in the valley occupation that can be viewed as the effect of a pseudomagnetic field. Thus, similarly to what happens for ${\chi }_s$, the electron-phonon coupling of intervalley phonons is enhanced by the electron-electron interaction. Only hybrid DFT functionals capture such an enhancement and the HSE06 functional reproduces the RPA results presented in M. Calandra et al. [Phys. Rev. Lett. 114, 077001 (2015)]. These results imply that the description of the susceptibility and electron-phonon coupling with a range-separated hybrid functional would be important also in other two-dimensional weakly doped semiconductors, such as transition-metal dichalcogenides and graphene.

A beam of linearly polarized light transmitted through magnetically biased graphene can have its axis of polarization rotated by several degrees after passing the graphene sheet. This large Faraday effect is due to the action of the magnetic field on graphene's charge carriers. As deformations of the graphene membrane result in pseudomagnetic fields acting on the charge carriers, the effect of random mesoscopic corrugations (ripples) can be described as the exposure of graphene to a random pseudomagnetic field. We aim to clarify the interplay of these typically sample inherent fields with the external magnetic bias field and the resulting effect on the Faraday rotation. In principle, random gauge disorder can be identified from a combination of Faraday angle and optical spectroscopy measurements.

The continuous quantum phase transition between noninteracting, time-reversal symmetric topological and trivial insulators in three dimensions is described by the massless Dirac fermion. We address the stability of this quantum critical point against short range electronic interactions by using renormalization group analysis and mean field theory. For sufficiently weak interactions, we show that the nature of the direct transition remains unchanged. Beyond a critical strength of interactions we find that either (i) there is a direct first order transition between two time reversal symmetric insulators or (ii) the direct transition is eliminated by an intervening time reversal and inversion odd “axionic” insulator. We also demonstrate the existence of an interaction driven first order quantum phase transition between topological and trivial gapped states in lower dimensions.

The topological pumping proposed in 1980s and recently realized by cold atom experiments is revisited from the view point of the bulk-edge correspondence. For a system with boundaries, a different form of the pumped charge is derived by the Berry connection in the temporal gauge that corresponds to the shift of the center of mass (c.m.). Even with boundaries, the pumped charge is carried by the bulk and its quantization is guaranteed by the discontinuities of the c.m. associated with the edge states. This is a modified Laughlin argument based on the local $U\left(1\right)$ invariance, although the physics behind it is quite different.

Efficient conversion of photons into electrical current in two-dimensional semiconductors requires, as a first step, the dissociation of the strongly bound excitons into free electrons and holes. Here we calculate the dissociation rates and energy shift of excitons in monolayer ${\mathrm{MoS}}_2$ as a function of an applied in-plane electric field. The dissociation rates are obtained as the inverse lifetime of the resonant states of a two-dimensional hydrogenic Hamiltonian which describes the exciton within the Mott-Wannier model. The resonances are computed using complex scaling, and the effective masses and screened electron-hole interaction defining the hydrogenic Hamiltonian are computed from first principles. For field strengths above 0.1 V/nm the dissociation lifetime is shorter than 1 ps, which is below the lifetime associated with competing decay mechanisms. Interestingly, encapsulation of the ${\mathrm{MoS}}_2$ layer in just two layers of hexagonal boron nitride ($h\mathrm{BN}$), enhances the dissociation rate by around one order of magnitude due to the increased screening. This shows that dielectric engineering is an effective way to control exciton lifetimes in two-dimensional materials.