Raman spectroscopy has been widely used to measure thermal conductivity ($\kappa$) of two-dimensional (2D) materials such as graphene. This method is based on a well-accepted assumption that different phonon polarizations are in near thermal equilibrium. However, in this paper, we show that, in laser-irradiated single-layer graphene, different phonon polarizations are in strong nonequilibrium, using predictive simulations based on first principles density functional perturbation theory and a multitemperature model. We first calculate the electron cooling rate due to phonon scattering as a function of the electron and phonon temperatures, and the results clearly illustrate that optical phonons dominate the hot electron relaxation process. We then use these results in conjunction with the phonon scattering rates computed using perturbation theory to develop a multitemperature model and resolve the spatial temperature distributions of the energy carriers in graphene under steady-state laser irradiation. Our results show that electrons, optical phonons, and acoustic phonons are in strong nonequilibrium, with the flexural acoustic (ZA) phonons showing the largest nonequilibrium to other phonon modes, mainly due to their weak coupling to other carriers in suspended graphene. Since ZA phonons are the main heat carriers in graphene, we estimate that neglecting this nonequilibrium leads to underestimation of thermal conductivity in experiments at room temperature by a factor of 1.35 to 2.6, depending on experimental conditions and assumptions used. Underestimation is also expected in Raman measurements of other 2D materials when the optical-acoustic phonon coupling is weak.

Mixed magnetism (the coexistence of strong and weak magnetism in one material) is regarded as the origin of the giant magnetocaloric effect (GMCE). A good example is ${(\mathrm{Mn},\mathrm{Fe})}_2(\mathrm{Si},\mathrm{P})$, which is established as one of the best magnetocaloric materials available. Tuning the material properties are essential for optimizing its performance, and a straightforward way to do that is by doping. In this article, an ab initio electronic structure method was used to calculate the structure and magnetic properties of $3d$-transition-metal-doped ${(\mathrm{Mn},\mathrm{Fe})}_2(\mathrm{Si},\mathrm{P})$ materials for magnetocaloric applications (transition metals are Cr, Co, Mn, Ni, Cu). For a steady performance, the material should be mechanically stable. A detailed analysis of the elastic constants shows that the mechanical stability of the ${(\mathrm{Mn},\mathrm{Fe})}_2(\mathrm{Si},\mathrm{P})$ system increases significantly by doping with boron without affecting the magnetic properties. Insights of the influence of doping enable future studies to understand and predict better magnetocaloric materials.

We demonstrate how magnetic droplet soliton pairs, nucleated by two separated nanocontact (NC) spin torque oscillators, can merge into a single droplet soliton. A detailed description of the magnetization dynamics of this merger process is obtained by micromagnetic simulations: A droplet pair with a steady-state in-phase spin precession is generated through the spin-transfer torque effect underneath two separate NCs, followed by a gradual expansion of the droplets’ volume and the out-phase of magnetization on the inner side of the two droplets, resulting in the droplets merging into a larger droplet. This merger occurs only when the NC separation is smaller than a critical value. A transient breathing mode is observed before the merged droplet stabilizes into a steady precession state. The precession frequency of the merged droplet is lower than that of the droplet pair, consistent with its larger size. Merged droplets can again break up into droplet pairs at high enough magnetic field with a strong hysteretic response.

On the basis of the Usadel equation we study a multiterminal Josephson junction. This junction is composed by “magnetic” superconductors ${\mathrm{S}}_{\text{m}}$, which have singlet pairing and are separated from the normal n wire by spin filters so that the Josephson coupling is caused only by fully polarized triplet components. We show that there is no interaction between triplet Cooper pairs with antiparallel total spin orientations. The presence of an additional singlet superconductor S attached to the n wire leads to a finite Josephson current $I_{\text{Q}}$ with an unusual current-phase relation. The density of states in the n wire for different orientations of spins of Cooper pairs is calculated. We derive a general formula for the current $I_{\text{Q}}$ in a multiterminal Josephson contact and apply this formula for analysis of two four-terminal Josephson junctions of different structures. It is shown in particular that both the “nematic” and the “magnetic” cases can be realized in these junctions. In a two-terminal structure with parallel filter orientations and in a three-terminal structure with antiparallel filter orientations of the “magnetic” superconductors with attached additional singlet superconductor, we find a nonmonotonic temperature dependence of the critical current. Also, in these structures, the critical current shows a Riedel peak like dependence on the exchange field in the “magnetic” superconductors. Although there is no current through the S/n interface due to orthogonality of the singlet and triplet components, the phase of the order parameter in the superconuctor S is shown to affect the Josephson current in a multiterminal structure.

The superconducting phase of hydrogen sulfide at $T_c=200$ K observed by Drozdov and collaborators at pressures around 200 GPa is simple bcc $Im\overline{3}m {\mathrm{H}}_3\mathrm{S}$ from a combination of theoretical and experimental confirmation. The various “extremes” that are involved—high pressure implying extreme reduction of volume, extremely high H phonon energy scale around 1400 K, extremely high temperature for a superconductor—necessitates a close look at new issues raised by these characteristics in relation to high $T_c$ itself. First principles methods are applied to analyze the ${\mathrm{H}}_3\mathrm{S}$ electronic structure, beginning with the effect of sulfur and then focusing on the origin and implications of the two van Hove singularities (vHs) providing an impressive peak in the density of states near the Fermi energy. Implications arising from strong coupling Migdal-Eliashberg theory are studied. It becomes evident that electron spectral density smearing due to virtual phonon emission and absorption must be accounted for in a correct understanding of this unusual material and to obtain accurate theoretical predictions. Means for increasing $T_c$ in ${\mathrm{H}}_3\mathrm{S}$-like materials are noted.

Trions and biexcitons in anisotropic two-dimensional materials are investigated within an effective mass theory. Explicit results are obtained for phosphorene and arsenene, materials that share features such as a direct quasiparticle gap and anisotropic conduction and valence bands. Trions are predicted to have remarkably high binding energies and an elongated electron-hole structure with a preference for alignment along the armchair direction, where the effective masses are lower. We find that biexciton binding energies are also notably large, especially for monolayer phosphorene, where they are found to be twice as large as those for typical monolayer transition metal dichalcogenides.

A three-terminal Josephson junction biased at opposite voltages can sustain a phase-sensitive dc current carrying three-body static phase coherence, known as the “quartet current.” We calculate the zero-frequency current noise cross correlations and answer the question of whether this current is noisy (like a normal current in response to a voltage drop) or noiseless (like an equilibrium supercurrent in response to a phase drop). A quantum dot with a level at energy ${\epsilon }_0$ is connected to three superconductors $S_a,S_b$, and $S_c$ with gap $\mathrm{\Delta }$, biased at $V_a=V,V_b=-V$, and $V_c=0$, and with intermediate contact transparencies. At zero temperature, nonlocal quartets (in the sense of four-fermion correlations) are noiseless at subgap voltage in the nonresonant dot regime ${\epsilon }_0/\mathrm{\Delta }\gg 1$, which is demonstrated with a semianalytical perturbative expansion of the cross correlations. Noise reveals the absence of granularity of the superflow splitting from $S_c$ towards $(S_a,S_b)$ in the nonresonant dot regime, in spite of finite voltage. In the resonant dot regime ${\epsilon }_0/\mathrm{\Delta }\lesssim 1$, cross correlations measured in the $(V_a,V_b)$ plane should reveal an “anomaly” in the vicinity of the quartet line $V_a+V_b=0$, related to an additional contribution to the noise, manifesting the phase sensitivity of cross correlations under the appearance of a three-body phase variable. Phase-dependent effective Fano factors $F_{\phi }$ are introduced, defined as the ratio between the amplitudes of phase modulations of the noise and the currents. At low bias, the Fano factors $F_{\phi }$ are of order unity in the resonant dot regime ${\epsilon }_0/\mathrm{\Delta }\lesssim 1$, and they are vanishingly small in the nonresonant dot regime ${\epsilon }_0/\mathrm{\Delta }\gg 1$.

Using an advanced technique combining pulsed laser deposition growth of $\mathrm{LaAl}{\mathrm{O}}_3$, $\mathrm{LaTi}{\mathrm{O}}_3$, and $\mathrm{SrTi}{\mathrm{O}}_3$ we effectively constructed half-integer unit cell number $\mathrm{LaAl}{\mathrm{O}}_3\text{−}\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures where all interfaces are of LaO-$\mathrm{Ti}{\mathrm{O}}_2$ type, and where a two-dimensional electron gas (2DEG) forms a symmetric $n$-type bilayer. Using ultrahigh-vacuum scanning tunneling microscopy we investigated the properties of the surface 2DEG in these heterostructures. Our results indicate that the surface 2DEG is strongly, within one unit cell, confined at the interface. Tunneling spectroscopy of the surface 2DEG reveals thickness-dependent band-gap changes attributed to the quantum size effect.

The modification of Ba in BaCl compounds from alkaline-metal to transition- and half-metal behavior is explored. High-pressure structural changes in BaCl are predicted using an ab initio structure search method. Dynamically stable bcc and $R-3m$ forms of BaCl are predicted at 15 and 10 GPa, respectively. The BaCl forms are more stable than elemental Ba plus $\mathrm{BaC}{\mathrm{l}}_2$ above ∼10 GPa. Ba in stable BaCl adopts transition-metal properties via an $s-d$ transition. At ambient pressure the fcc structure is ferromagnetic, and the bcc structure is half metallic and ferromagnetic. The transition-metal electronic structure found is sufficient to support superconductivity, with $T_c$ as high as 3.4 K near ambient pressure.

We explore the stability of far-from-equilibrium metastable states of a three-dimensional Coulomb glass at zero temperature by studying charge avalanches triggered by a slowly varying external electric field. Surprisingly, we identify a sharply defined dynamical (“depinning”) phase transition from stationary to nonstationary charge displacement at a critical value of the external electric field. Using particle-conserving dynamics, scale-free system-spanning avalanches are observed only at the critical field. We show that the qualitative features of this depinning transition are completely different for an equivalent short-range model, highlighting the key importance of long-range interactions for nonequilibrium dynamics of Coulomb glasses.

We report a study of the electrical transport properties of single crystals of $\mathrm{P}{\mathrm{r}}_4\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2\mathrm{T}{\mathrm{e}}_{1-x}{\mathrm{O}}_4$, a recently discovered iron-based superconductor. Resistivity, Hall effect, and magnetoresistance are measured in a broad temperature range revealing the role of electrons as dominant charge carriers. The significant temperature dependence of the Hall coefficient and the violation of Kohler's law indicate multiband effects in this compound. The upper critical field and the magnetic anisotropy are investigated in fields up to 16 T, applied parallel and perpendicular to the crystallographic $c$ axis. Hydrostatic pressure up to 2 GPa linearly increases the critical temperature and the resistivity residual ratio. A simple two-band model is used to describe the transport and magnetic properties of $\mathrm{P}{\mathrm{r}}_4\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2\mathrm{T}{\mathrm{e}}_{1-x}{\mathrm{O}}_4$. The model can successfully explain the strongly temperature-dependent negative Hall coefficient and the high magnetic anisotropy, assuming that the mobility of electrons is higher than that of holes.

We present a systematic investigation of the electrical, structural, and antiferromagnetic properties for the series of $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x-y}{\mathrm{Co}}_x{\mathrm{Rh}}_y\right)}_2{\mathrm{As}}_2$ compounds with fixed $x\approx 0.027$ and $0\le y\le 0.035$. We compare our results for the Co-Rh doped $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x-y}{\mathrm{Co}}_x{\mathrm{Rh}}_y\right)}_2{\mathrm{As}}_2$ compounds with the Co doped $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ compounds. We demonstrate that the electrical, structural, antiferromagnetic, and superconducting properties of the Co-Rh doped compounds are similar to the properties of the Co doped compounds. We find that the overall behaviors of $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x-y}{\mathrm{Co}}_x{\mathrm{Rh}}_y\right)}_2{\mathrm{As}}_2$ and $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ compounds are very similar when the total number of extra electrons per Fe/$\text{TM}$ $(\text{TM}=\text{transition}$ metal) site is considered, which is consistent with the rigid band model. Despite the similarity, we find that the details of the transitions, for example, the temperature difference between the structural and antiferromagnetic transition temperatures and the incommensurability of the antiferromangetic peaks, are different between $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x-y}{\mathrm{Co}}_x{\mathrm{Rh}}_y\right)}_2{\mathrm{As}}_2$ and $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ compounds.

In this work, we will explore the properties of superconducting surfaces decorated by two-dimensional ferromagnetic adatom lattices. As discovered recently [Röntynen and Ojanen, Phys. Rev. Lett. 114, 236803 (2015)], in the presence of a Rashba spin-orbit coupling these systems may support topological superconductivity with complex phase diagrams and high Chern numbers. We show how the long-range hopping nature of the effective low-energy theory generically gives rise to a phase diagram covered by a Chern mosaic, a rich pattern of topological phases with large Chern numbers. We study different lattice geometries and the dependence of energy gaps and abundance of different phases as a function of system parameters. Our findings establish the studied system as one of the richest platforms for topological matter known to date.

Using ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ as motivation, we investigate finite-field properties of $XY$ pyrochlore antiferromagnets. In addition to a fluctuation-induced six-fold anisotropy present in zero field, an external magnetic field induces a combination of two-, three-, and six-fold clock terms as a function of its orientation, providing for rich and controllable magnetothermodynamics. For ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, we predict a phase transition in a weak magnetic field $\mathbf{H}\parallel \left[001\right]$. Re-entrant transitions are also found for $\mathbf{H}\parallel \left[111\right]$. We extend these results to the whole family the $XY$ pyrochlore antiferromagnets and show that presence and number of low-field transitions for different orientations can be used for locating a given material in the parameter space of anisotropic pyrochlores. Finite-temperature classical Monte Carlo simulations serve to confirm and illustrate these analytic predictions.

It was recently proposed that electron magnetic circular dichroism can be measured in scanning transmission electron microscopy with atomic resolution by tuning the phase distribution of an electron beam. Here, we describe the theoretical and practical aspects for the detection of out-of-plane and in-plane magnetization utilizing atomic size electron probes. We present the calculated optimized astigmatic probes and discuss how to achieve them experimentally.

We present a theoretical analysis of quasiparticle poisoning in Coulomb-blockaded Majorana fermion systems tunnel-coupled to normal-conducting leads. Taking into account finite-energy quasiparticles, we derive the effective low-energy theory and present a renormalization group analysis. We find qualitatively new effects when a quasiparticle state with very low energy is localized near a tunnel contact. For $M=2$ attached leads, such “dangerous” quasiparticle poisoning processes cause a spin $S=1/2$ single-channel Kondo effect, which can be detected through a characteristic zero-bias anomaly conductance peak in all Coulomb blockade valleys. For more than two attached leads, the topological Kondo effect of the unpoisoned system becomes unstable. A strong-coupling bosonization analysis indicates that at low energy the poisoned lead is effectively decoupled and hence, for $M>3$, the topological Kondo fixed point re-emerges, though now it involves only $M-1$ leads. As a consequence, for $M=3$, the low-energy fixed point becomes trivial corresponding to decoupled leads.

The low temperature properties of single level molecular quantum dots including both electron-electron and electron-vibration interactions, are theoretically investigated. The calculated differential conductance in the Kondo regime exhibits not only the zero bias anomaly but also side peaks located at bias voltages which coincide with multiples of the energy of vibronic mode $V\sim \hslash \mathrm{\Omega }/e$. We obtain that the evolution with temperature of the two main satellite conductance peaks follows the corresponding one of the Kondo peak when $\hslash \mathrm{\Omega }\gg k_B{T}_K, T_K$ being the Kondo temperature, in agreement with recent transport measurements in molecular junctions. However, we find that this is no longer valid when $\hslash \mathrm{\Omega }$ is of the order of a few times $k_B{T}_K$.