A rigorous first principles Boltzmann-Peierls equation (BPE) for phonon transport approach is employed to examine the lattice thermal conductivity, ${\kappa }_L$, of strained and unstrained graphene. First principles calculations show that the out-of-plane, flexural acoustic phonons provide the dominant contribution to ${\kappa }_L$ of graphene for all strains, temperatures, and system sizes considered, supporting a previous prediction that used an optimized Tersoff empirical interatomic potential. For the range of finite system sizes considered, we show that the ${\kappa }_L$ of graphene is relatively insensitive to strain. This provides validation for use of the BPE approach to calculate ${\kappa }_L$ for unstrained graphene, which has recently been called into question. The temperature and system size dependence of the calculated ${\kappa }_L$ of graphene is in good agreement with experimental data. The enhancement of ${\kappa }_L$ with isotopic purification is found to be relatively small due to strong anharmonic phonon-phonon scattering. This work provides insight into the nature of phonon thermal transport in graphene, and it demonstrates the power of first principles thermal transport techniques.

The crystal structure of FePt nanoparticles of mean size of 6 nm produced by gas-phase condensation is characterized using a combination of high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field (HAADF) imaging in scanning transmission electron microscopy (STEM). These FePt nanoparticles are found to be chemically ordered, decahedral shaped, and Pt enriched at the surfaces. The experimentally determined crystallographic lattice constants and distribution of Fe and Pt atoms are compared with first-principles calculations of ordered decahedral FePt nanoparticles to confirm the discovery of a unique decahedral structure with Fe/Pt ordering and Pt surface segregation.

The formation of metalorganic hybrid interfaces is determined by the fine balance between molecule-substrate and molecule-molecule interactions at the interface. Here, we report on a systematic investigation of interfaces between a metal surface and organic monolayer films that consist of two different molecular species, one donor and one acceptor of electronic charge. Our x-ray standing wave data show that in heteromolecular structures, the molecules tend to align themselves to an adsorption height between those observed in the respective homomolecular structures. We attribute this alignment effect to a substrate-mediated charge transfer between the molecules, which causes a mutual enhancement of their respective donor and acceptor characters. We argue that this effect is of general validity for $\pi$-conjugated molecules adsorbing on noble metal surfaces.

We systematically investigate temperature- and spectrally dependent optical reflectivity dynamics in $A$As${}_2$Fe${}_2$, ($A=\text{Ba}$, Sr, and Eu), iron-based superconductors parent spin-density wave (SDW) compounds. Two different relaxation processes are identified. The behavior of the slower process, which is strongly sensitive to the magnetostructural transition, is analyzed in the framework of the relaxation-bottleneck model involving magnons. The results are compared to recent time-resolved angular photoemission results (TR-ARPES) and possible alternative assignment of the slower relaxation to the magnetostructural order parameter relaxation is discussed.

The results of an experimental study of the energy spectrum of a valence band in a HgTe quantum well of width $d<6.3$ nm with normal spectrum in the presence of a strong spin-orbit splitting are reported. The analysis of the temperature, magnetic field, and gate voltage dependences of the Shubnikov–de Haas oscillations allows us to restore the energy spectrum of the two valence-band branches, which are split by the spin-orbit interaction. Comparison with a theoretical calculation shows that a six-band kP theory well describes all the experimental data in the vicinity of the top of the valence band.

Electric and thermoelectric properties of silicene nanoribbons doped with Al and P impurity atoms are investigated theoretically for both antiparallel and parallel orientations of the edge magnetic moments. In the former case, appropriately arranged impurities can lead to a net magnetic moment, and thus also to spin dependent transport and spin thermoelectric phenomena. In the latter case, in turn, spin thermoelectric effects also occur in the absence of impurities. To calculate spin density distribution and transport parameters we use ab initio numerical methods based on the density functional theory. The obtained results clearly show that the spin thermopower can be considerably enhanced by the impurities. This enhancement appears in certain regions of chemical potential.

The topological protected electronic states on the surface of a topological insulator can progressively change their Fermi cross section from circular to a snowflake shape as the chemical potential is increased above the Dirac point because of an hexagonal warping term in the Hamiltonian. Another effect of warping is to change the spin texture which exists when a finite gap is included by magnetic doping, although the in-plane spin component remains locked perpendicular to momentum. It also changes the orbital magnetic moment, the matrix element for optical absorption, and the circular dichroism. We find that the Fermi surface average of $z$ component of spin is closely related to the value of the Berry phase. This holds even when the Hamiltonian includes a subdominant nonrelativistic quadratic in momentum term (which provides particle-hole asymmetry) in addition to the dominant relativistic Dirac term. There is also a qualitative correlation between $\langle S_z\rangle$ and the dichroism. For the case when the chemical potential falls inside the gap between valence and conduction band, the Hall conductivity remains quantized and unaffected in value by the hexagonal warping term.

We analyze theoretically charge transport in Coulomb coupled graphene waveguides (GWGs). The GWGs are defined using antidot lattices, and the lateral geometry bypasses many technological challenges of earlier designs. The drag resistivity ${\rho }_D$, which is a measure of the many-particle interactions between the GWGs, is computed for a range of temperatures and waveguide separations. It is demonstrated that for $T>0.1T_F$ the drag is significantly enhanced due to plasmons, and that in the low-temperature regime a complicated behavior may occur. In the weak coupling regime the dependence of drag on the interwaveguide separation $d$ follows ${\rho }_D\sim d^{-n}$, where $n\simeq 6$.

We report on the magnetic and structural characterization of high lattice-mismatched [Dy${}_{2nm}$/Sc${}_{t_{\text{Sc}}}$] superlattices, with variable Sc thickness $t_{\text{Sc}}$= 2–6 nm. We find that the characteristic in-plane effective hexagonal magnetic anisotropy K${}_6^{6,ef}$ reverses sign and undergoes a dramatic reduction, attaining values of $\approx$13–24 kJm${}^{-3}$, when compared to K${}_6^6=-0.76$ MJm${}^{-3}$ in bulk Dy. As a result, the basal plane magnetic anisotropy is dominated by a uniaxial magnetic anisotropy (UMA) unfound in bulk Dy, which amounts to $\approx$175–142 kJm${}^{-3}$. We attribute the large downsizing in K${}_6^{6,ef}$ to the compression epitaxial strain, which generates a competing sixfold magnetoelastic (MEL) contribution to the magnetocrystalline (strain-free) magnetic anisotropy. Our study proves that the in-plane UMA is caused by the coupling between a giant symmetry-breaking MEL constant M${}_{\gamma ,2}^2\approx 1$ GPa and a morphic orthorhombiclike strain ${\varepsilon }_{\gamma ,1}\approx {10}^{-4}$, whose origin resides on the arising of an in-plane anisotropic strain relaxation process of the pseudoepitaxial registry between the nonmagnetic bottom layers in the superstructure. This investigation shows a broader perspective on the crucial role played by epitaxial strains at engineering the magnetic anisotropy in multilayers.

Using a generalized Jordan-Wigner transformation combined with the defining representation of the SO($N$) spin, we map the SO($N$) bilinear-biquadratic (BLBQ) spin chain into the $N$-color bosonic particle model. We find that, when the Jordan-Wigner transformation disentangles the symmetry-protected topological entanglement, this bosonic model becomes negative-sign-free in the context of quantum Monte Carlo simulation. For the SO(3) case, moreover, the Kennedy-Tasaki transformation for the $S=1$ BLBQ chain, which is also a topological disentangler, derives the same bosonic model through the dimer-R bases. We present the temperature dependence of the energy, entropy, and string order parameter for the SO($N=3,4,5$) BLBQ chains by a world-line Monte Carlo simulation for the $N$-color bosonic particle model.

We report the effect of applied pressures on magnetic and superconducting order in single crystals of the aliovalent La-doped iron pnictide material ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. Using electrical transport, elastic neutron scattering, and resonant tunnel diode oscillator measurements on samples under both quasihydrostatic and hydrostatic pressure conditions, we report a series of phase diagrams spanning the range of substitution concentrations for both antiferromagnetic and superconducting ground states that include pressure-tuning through the antiferromagnetic (AFM) superconducting critical point. Our results indicate that the observed superconducting phase with a maximum transition temperature of $T_c=47$ K is intrinsic to these materials, appearing only upon suppression of magnetic order by pressure-tuning through the AFM critical point. Thus, the superconducting phase appears to exist exclusively in juxtaposition to the antiferromagnetic phase in a manner similar to the oxygen- and fluorine-based iron-pnictide superconductors with the highest transition temperatures reported to date. Unlike the lower-$T_c$ systems, in which superconductivity and magnetism usually coexist, the tendency for the highest-$T_c$ systems to show noncoexistence provides an important insight into the distinct transition temperature limits in different members of the iron-based superconductor family.

We investigate the proximity effect in diffusive superconducting hybrid structures with a spin-orbit (SO) coupling. Our study is focused on the singlet-triplet conversion and the generation of long-range superconducting correlations in ferromagnetic elements. We derive the quasiclassical equations for the Green's functions including the SO coupling terms in form of a background SU(2) field. With the help of these equations, we first present an interesting complete analogy between the spin diffusion process in normal metals and the generation of the triplet components of the condensate in a diffusive superconducting structure in the presence of SO coupling. From this analogy it turns out naturally that the SO coupling is an additional source of the long-range triplet component (LRTC) besides the magnetic inhomogeneities studied in the past. This analogy opens a range of possibilities for the generation and manipulation of the triplet condensate in hybrid structures. In particular we demonstrate that a normal metal with SO coupling can be used as source of LRTC if attached to a superconductor-ferromagnet bilayer. We also demonstrate an explicit connection between an inhomogeneous exchange field and SO coupling mechanisms for the generation of the LRTC and establish the conditions for the appearance of the LRTC in different geometries. Our work gives a global description of the singlet-triplet conversion in hybrid structures in terms of generic spin fields and our results are particularly important for the understanding of the physics underlying spintronic devices with superconductors.

Nonlinear magnetization dynamics in ferromagnetic nanoelements excited by a spin-polarized dc current is one of the most intensively studied phenomena in solid-state magnetism. Despite immense efforts, synchronization of oscillations induced in several such nanoelements [spin-torque-driven nano-oscillators (STNO)] still represents a major challenge from both the fundamental and technological points of view. In this paper we propose a system where synchronization of any number of STNOs, represented by magnetization vortices inside squared nanoelements, can be easily achieved. Using full-scale micromagnetic simulations we show that synchronization of these STNOs is extremely dynamically stable due to their very large coupling energy provided by the magnetodipolar interaction. Finally, we demonstrate that our concept allows robust synchronization of an arbitrary number of STNOs (arranged either as a one-dimensional chain or as a two-dimensional array), even when current supplying nanocontacts have a broad size distribution.

Powder neutron diffraction and magnetometry studies have been conducted to investigate the crystallographic and magnetic structure of ${\mathrm{Bi}}_{0.8}{\mathrm{La}}_{0.2}{\mathrm{Fe}}_{0.5}{\mathrm{Mn}}_{0.5}{\mathrm{O}}_3$. The compound stabilizes in the $Imma$ orthorhombic crystal symmetry in the measured temperature range of 5 to 380 K, with a transition to antiferromagnetic order at $T_{\mathrm{N}}\approx 240$ K. The spin cycloid present for BiFeO${}_3$ is found to be absent with 50% Mn${}^{3+}$ cation substitution, leading to $G$-type antiferromagnetic order with an enhanced out-of-plane canted ferromagnetic component, evident from measurable weak-ferromagnetic hysteresis. Structural modifications do not solely explain this behavior, indicating that modified electron exchange interactions must be taken into account. A classical spin simulation was developed to investigate the effect of random substitution in a disordered pseudocubic perovskite. The calculations took into account the nearest-neighbor, next-nearest-neighbor, and Dzyaloshinskii-Moriya interactions, along with the local spin anisotropy. Using this framework to extend the established Hamiltonian model for BiFeO${}_3$, we show that only certain types of perturbations at a magnetic defect and the surrounding molecular fields trigger a simultaneous collapse of cycloidal order and the emergence of the long-range weak-ferromagnetic component. By adopting values for the Mn molecular fields appropriate for $\mathit{RE}$MnO${}_3$ ($\mathit{RE}=$ rare earth), simulations of BiMn${}_{0.5}$Fe${}_{0.5}{\mathrm{O}}_3$ exhibit the key magnetic properties of our experimental observations.

Pressure effects on the superconductors ${\mathrm{V}}_3$Pn${\mathrm{N}}_x$ (Pn = P and As) with the two-dimensional anti-post-perovskite structure were investigated. It was found that with increasing pressure, the superconducting transition temperature ($T_c$) decreases with a negative differential pressure ($P$) coefficient $d{T}_c$/dP = −0.19 and −0.013 K/GPa for Pn = P and As, respectively. This is in marked contrast to the positive differential pressure coefficients reported for other vanadium-based superconductors with a three-dimensional crystal structure. The observed change in $T_c$ under pressure is revealed to be closely correlated with the evolution of the normal-state resistivity. The results support the phonon-mediated superconductivity in ${\mathrm{V}}_3$Pn${\mathrm{N}}_x$.

We present a detailed investigation of the electronic phase diagram of effectively charge compensated ${\text{Ba}}_{1-x}$${\text{K}}_x$(${\text{Fe}}_{1-y}$${\text{Co}}_y$)${}_2$${\text{As}}_2$ with $x/2\approx y$. Our experimental study by means of x-ray diffraction, Mössbauer spectroscopy, muon spin relaxation and ac-susceptibility measurements on polycrystalline samples is complemented by density functional electronic structure calculations. For low substitution levels of $x/2\approx y\le 0.13$, the system displays an orthorhombically distorted and antiferromagnetically ordered ground state. The low-temperature structural and magnetic order parameters are successively reduced with increasing substitution level. We observe a linear relationship between the structural and the magnetic order parameter as a function of temperature and substitution level for $x/2\approx y\le 0.13$. At intermediate substitution levels in the range between 0.13 and 0.19, we find superconductivity with a maximum $T_c$ of 15 K coexisting with static magnetic order on a microscopic length scale. For higher substitution levels $x/2\approx y\ge 0.25$, a tetragonal nonmagnetic ground state is observed. Our DFT calculations yield a significant reduction of the Fe $3d$ density of states at the Fermi energy and a strong suppression of the ordered magnetic moment in excellent agreement with experimental results. The appearance of superconductivity within the antiferromagnetic state can by explained by the introduction of disorder due to nonmagnetic impurities to a system with a constant charge carrier density.

We consider two-dimensional metals near a Pomeranchuk instability which breaks ${90}^{\circ }$ lattice rotation symmetry. Such metals realize strongly coupled non-Fermi liquids with critical fluctuations of an Ising-nematic order. At low temperatures, impurity scattering provides the dominant source of momentum relaxation and, hence, a nonzero electrical resistivity. We use the memory matrix method to compute the resistivity of this non-Fermi liquid to second order in the impurity potential, without assuming the existence of quasiparticles. Impurity scattering in the $d$-wave channel acts as a random “field” on the Ising-nematic order. We find contributions to the resistivity with a nearly linear temperature dependence, along with more singular terms; the most singular is the random-field contribution which diverges in the limit of zero temperature.

We show that negative-index metamaterials can generate chiral electromagnetic fields, which can be used to enhance molecular chiroptical signals by inducing strong local magnetic fields. Compared to circularly polarized light (CPL), the enhanced chiral field generated by a double-fishnet negative-index metamaterial shows a 3.5-fold enhancement of the volume-averaged optical chirality and possesses the same handedness as that of incident CPL, thereby forming a globally enhanced chiral field. By analyzing near-field configurations, the mechanism of the optical chirality enhancement by double-fishnet negative-index metamaterial is elucidated. We thus anticipate that these metamaterials have potential applications in chiroptical spectroscopy.

We verify that the exchange bias effect unexpectedly emerges in a single ${\mathrm{LaMnO}}_{3-\delta }$ film, one of the most studied correlated oxides. We combine x-ray absorption spectroscopy results, which serves as a fingerprint of the electronic structure, with microstructure characterizations and magnetization data to explore the origin of the exchange bias behavior. Taken together, these measurements provide compelling evidence that the formation of a Mn${}^{2+}$ component associated with the double exchange between Mn${}^{2+}$-O-Mn${}^{3+}$ produces robust ferromagnetism in the upper part of the film, which is exchanged coupled with the antiferromagnetic bottom part that is dominated by Mn${}^{3+}$. Thus, the Mn ions are found to be distributed unevenly in the depth profile, with vertical electronic phase separation, in contrast to the often accepted view of the lateral phase separation in manganites. The observation of exchange bias in a chemically single film paves the way for interface engineering induced by vertical electronic phase separation towards technological applications.