We investigated the voltages obtained in a thin Pt strip on a permalloy film which was subject to in-plane temperature gradients and magnetic fields. The voltages detected by thin W tips or bond wires showed a purely symmetric effect with respect to the external magnetic field which can be fully explained by the planar Nernst effect. To verify the influence of the contacts, measurements in vacuum and atmosphere were compared and gave similar results. We explain that a slightly in-plane tilted temperature gradient only shifts the field direction dependence but does not cancel out the observed effects. Additionally, the anomalous Nernst effect could be induced by using thick Au tips which generated a heat current perpendicular to the sample plane. The effect can be manipulated by varying the temperature of the Au tips. These measurements are discussed concerning their relevance in transverse spin Seebeck effect measurements.

Chiral magnets, such as MnSi, display a rich finite temperature phase diagram in an applied magnetic field. The most unusual of the phases encountered is the so-called A Phase characterized by a triangular lattice of skyrmion tubes. Its existence cannot be captured within a mean-field treatment of a Landau-Ginzburg functional, but thermal fluctuations to Gaussian order are required to stabilize it. In this paper, we go beyond Gaussian order in a fully nonperturbative study of a three-dimensional lattice spin model using classical Monte Carlo simulations. We demonstrate that the A Phase is, indeed, stabilized by thermal fluctuations, and furthermore, we reproduce the full phase diagram found in experiments. The thermodynamic signatures of the helimagnetic transition upon cooling from the paramagnet are qualitatively consistent with experimental findings and lend further support to the Brazovskiĭ scenario, which describes a fluctuation-driven first-order transition due to the abundance of soft modes.

In nanoscale junctions the interaction between charge carriers and the local vibrations results in renormalization, damping, and heating of the vibrational modes. Here we formulate a nonequilibrium Green's function based theory to describe such effects. Studying a generic junction model with an off-resonant electronic level, we find a strong bias dependence of the frequency renormalization and vibrational damping accompanied by pronounced nonlinear vibrational heating in junctions with intermediate values of the coupling to the leads. Combining our theory with ab initio calculations, we furthermore show that the bias dependence of the Raman shifts and linewidths observed experimentally in an oligo(3)-phenylenevinylene (OPV3) junction [Ward et al., Nat. Nanotechnol. 6, 33 (2011)] may be explained by a combination of dynamic carrier screening and molecular charging.

We present a detailed theoretical analysis of biexciton state generation in InAs-GaAs quantum dots by strong, chirped laser pulses. Specifically, we derive an accurate analytical expression, which not only provides a clear physical picture of the process, but also allows identifications of laser parameter regimes where efficient biexciton generation should be possible, even at temperatures up to 80 K. The results are confirmed by numerical simulations, in very good agreement with the model proposed. A clear choice of parameters is proposed, which might pave the way towards the optimal design of high-fidelity sources of entangled photon pairs based on individual quantum dots.

The hexaferrites are known to exhibit a wide range of magnetic structures, some of which are connected to important technological applications and display magnetoelectric properties. We present data on the low magnetic field structures stabilized in a Y-type hexaferrite as observed by resonant soft x-ray diffraction. The helical spin block arrangement that is present in zero applied magnetic field becomes fanlike as a field is applied in plane. The propagation vectors associated with each fan structure are studied as a function of magnetic field, and a new magnetic phase is reported. Mean field calculations indicate this phase should stabilize close to the boundary of the previously reported phases.

Through relativistic photoemission calculations for the Au(111) surface state at the Fermi level, we study the photon energy dependence of circular dichroism. The dichromatic signal ($D_S$) pattern changes 23 times with photon energies between 7 and 100 eV, and we have found 13 different patterns in the $k_{\parallel }$ map at the Fermi level for the $D_S$ from the Au(111) surface state with normal incidence light. We show that the photon energy dependence of $D_S$ is very complex even in the simplest case. The sign change in the circular dichroism as a function of photon energy is related to the relative phases of the complex expansion coefficients of different outgoing partial waves in a time-reversed low-energy electron diffraction state. With off-normal incidence, the $z$ component of the incoming photon field is dominant, and the fine structure seen in the $D_S$ in the normal incidence case is lost very rapidly, moving from a normal to an off-normal incidence. We also report that the Rashba split surface state of Au(111) has a significant component of $d$-type orbital due to relativistic effects and the computational setup used.

Since any realistic electronic device has some degree of disorder, predicting disorder effects in quantum transport is a critical problem. Here, we report the theory of nonequilibrium coherent potential approximation (NECPA) for analyzing disorder effects in nonequilibrium quantum transport of nanoelectronic devices. The NECPA is formulated by contour-ordered nonequilibrium Green's function where the disorder average is carried out within the coherent potential approximation on the complex-time contour. We have derived a set of rules that supplement the celebrated Langreth theorem and, as a whole, the generalized Langreth rules allow us to derive NECPA equations for real-time Green's functions. The solution of NECPA equations provides the disorder-averaged nonequilibrium density matrix as well as other relevant quantities for quantum transport calculations. We establish the excellent accuracy of NECPA by comparing its results to brute force numerical calculations of disordered tight-binding models. Moreover, the connection of NECPA equations which are derived on the complex-time contour to the nonequilibrium vertex correction theory which is derived on the real-time axis is made. As an application, we demonstrate that NECPA can be combined with density functional theory to enable analysis of nanoelectronic device physics from atomistic first principles.

We present a detailed experimental study on Mo${}_{1-x}$Si${}_x$ thin films, an archetypal alloy system combining metallic and semiconductor materials. The correlations between structure and elastic response are comprehensively investigated. We focus on assessing trends for understanding the evolution of elastic properties upon Si alloying in relation to the structural state (crystalline vs amorphous), bonding character (metallic vs covalent), and local atomic environment. By combining picosecond ultrasonics and Brillouin light scattering techniques, a complete set of effective elastic constants and mechanical moduli ($B$, $G$, $E$) is provided in the whole compositional range, covering bcc solid solutions ($x$ < 0.20) and the amorphous phase (0.20 < $x$ < 1.0). A softening of the shear and Young moduli and a concomitant decrease of the Debye temperature is revealed for crystalline alloys, with a significant drop being observed at $x$ ∼ 0.2 corresponding to the limit of crystal lattice stability. Amorphous alloys exhibit a more complex elastic response, related to variations in coordination number, atomic volume, and bonding state, depending on Si content. Finally, distinct evolutions of the G/B ratio as a function of Cauchy pressure are reported for crystalline and amorphous alloys, enabling us to identify signatures of ductility vs brittleness in the features of the local atomic environment. This work paves the way to design materials with improved mechanical properties by appropriate chemical substitution or impurity incorporation during thin-film growth.

Recently, in certain flat band lattice systems at commensurate fillings, fractional quantum Hall states have been found, which have anyonic excitations. We study such systems away from commensuration, i.e., the ground state of an anyon gas in such a system. The presence of the underlying lattice allows access to an entirely new regime where the anyon kinetic energy can be larger than their interaction energy. Within the flux-attachment approach, using a mean field and then adding fluctuations, we find several possible superfluid states. Two have intrinsic topological order: fractionalized quasiparticles with a fusion structure of ${\left({\mathbb{Z}}_2\right)}^4$ and ${\left({\mathbb{Z}}_8\right)}^2$, respectively, and a third has no fractionalized excitations similar to a BCS-type state. This represents a mechanism for superconductivity driven purely by strong repulsion and complex hopping of electrons.

We revisit the physics of a Kondo impurity coupled to a fermionic host with a diverging power-law density of states near the Fermi level, $\rho \left(\omega \right)\sim {\left|\omega \right|}^r$, with exponent $-1<r<0$. Using the analytical understanding of several fixed points, based partially on powerful mappings between models with bath exponents $r$ and $(-r)$, combined with accurate numerical renormalization group calculations, we determine thermodynamic quantities within the stable phases and also near the various quantum phase transitions. Antiferromagnetic Kondo coupling leads to strong screening with a negative zero-temperature impurity entropy, while ferromagnetic Kondo coupling can induce a stable fractional spin moment. We formulate the quantum field theories for all critical fixed points of the problem, which are fermionic in nature and allow for a perturbative renormalization-group treatment.

We use time-resolved two-color Kerr microscopy to study low-temperature electron spin diffusion in bulk GaAs with high spatial and temporal resolution. This method enables direct real-space imaging of the advancing spread of an optically excited electron spin packet by spin diffusion. We observe a high initial expansion rate of the spin packet, which is strongly reduced with increasing time. Comparison with continuous-wave Kerr spectroscopy demonstrates that this decrease is caused by the influence of the transient cooling of hot photocarriers on the electron spin diffusion coefficient.

The orbital-lattice coupled excitation dynamics in orbitally degenerate correlated systems is examined. We present a theoretical framework where both local vibronic excitations and superexchange-type intersite interactions are treated on an equal footing. We generalize the spin-wave approximation so as to take local vibronic states into account. The present method is valid from weak to strong Jahn-Teller coupling magnitudes. Two characteristic excitation modes coexist, a low-energy dispersive mode and a high-energy multipeak mode; these are identified as a collective vibronic mode and Flanck-Condon excitations in a single Jahn-Teller center modified by the intersite interactions, respectively. The present formalism covers vibronic dynamics in several orbital-lattice coupled systems.

Recent electron quantum optics experiments performed with on-demand single electron sources call for a mixed time/frequency approach to electronic quantum coherence. Here, we present a Wigner function representation of first-order electronic coherence and show that it provides a natural visualization of the excitations emitted by recently demonstrated single electron sources. It also gives a unified perspective on single particle and two particle interferometry experiments. In particular, we introduce a nonclassicality criterion for single electron coherence and discuss it in the context of Mach-Zehnder interferometry. Finally, the electronic Hanbury Brown and Twiss and the Hong-Ou-Mandel experiments are interpreted in terms of overlap of Wigner function, thus connecting them to signal processing.

Here we present a Brownian ratchet based on plasmonic interactions. By periodically turning on and off a laser beam that illuminates a periodic array of plasmonic nanostructures with broken spatial symmetry, the random thermal motion of a subwavelength dielectric bead is rectified into one direction. By means of the molecular dynamics technique we show a statistical directed drift in particle flow.

We measure the electrical resistance of $o$-TaS${}_3$ between 1 and 300 K under pressures up to 20 GPa. We observe a gradual decrease of the charge density wave transition temperature $T_{\mathrm{CDW}}$ with increasing pressure $P$ following a mean field quantum fluctuation power law $T_{\mathrm{CDW}}=215\mathrm{K}{\left[\left(P_C-P\right)/P_C\right]}^{\gamma }$ with a quantum critical point at a pressure $P_C=11.5\mathrm{GPa}$ and $\gamma \approx 0.5$. Around the quantum critical point (QCP) we observe a superconducting dome with a maximum superconducting transition temperature $T_c=3.1\mathrm{K}$. Such dome is similar to superconducting domes around other types of order suggesting that the QCP is directly responsible for the enhancement of superconductivity through a universal mechanism still not well understood.

We have studied the electrodynamic response of strongly disordered superconducting TiN films using microwave resonators, where the disordered superconductor is the resonating element in a high-quality superconducting environment of NbTiN. We describe the response assuming an effective pair-breaking mechanism modifying the density of states and compare this to local tunneling spectra obtained using scanning tunneling spectroscopy. For the least disordered film ($k_{\mathrm{F}}l=8.7$, $R_{\mathrm{s}}=13\Omega$), we find good agreement, whereas for the most disordered film ($k_{\mathrm{F}}l=0.82$, $R_{\mathrm{s}}=4.3\mathrm{k}\Omega$), there is a strong discrepancy, which signals the breakdown of a model based on uniform properties.

Femtosecond laser spectroscopy is an important tool for investigating fundamental processes of light-matter interaction. Here, we report on transient optical anisotropy in the multiferroic hexagonal manganite YMnO${}_3$ excited by linearly or circularly polarized laser pulses with $\sim$17 fs and $\sim$34 fs duration. Spectral dependencies of photoinduced optical rotation and ellipticity at different temperatures are analyzed by considering ultrafast population and relaxation processes near the interband transitions from the hybridized O($2p$)-Mn($3d$) to the Mn($3d$) states at photon energies around 1.6 eV. A Raman coherence time of $\sim$10 fs between the excited ${\Gamma }_5|x\rangle$ and ${\Gamma }_5|y\rangle$ states and a relaxation time of $\sim$500 fs between the ${\Gamma }_5|x,y\rangle$ and ${\Gamma }_1|g\rangle$ states for the electronic transition ${\Gamma }_1\to {\Gamma }_5$ are determined in that way. The ultrashort Raman coherence time is related to the strong electron Coulomb interaction for the ${\Gamma }_5|x,y\rangle$ states.

The low-temperature behavior of the magnetic insulator Cs${}_2$CuCl${}_4$ can be modeled by an anisotropic triangular lattice spin-$1/2$ Heisenberg antiferromagnet with two different exchange couplings $J$ and $J^{\prime }\approx J/3$. We show that in a wide range of magnetic fields the experimentally observed field dependence of the crossover temperature $T_c$ for spin-liquid behavior can be explained within a mean-field theory based on the representation of spin operators in terms of Majorana fermions. We also show that for small magnetic fields the specific heat and the spin susceptibility both exhibit a maximum as a function of temperature at $T_c=J/2$. In the spin-liquid regime, the Majorana fermions can only propagate along the direction of the strongest bond, in agreement with the dimensional reduction scenario advanced by Balents [Nature (London) 464, 199 (2010)].

In recent years, there has been an increased interest in octahedral rotations in perovskite materials, particularly on their response to strain in epitaxial thin films. The current theoretical framework assumes that rotations are affected primarily through the change in in-plane lattice parameters imposed by coherent heteroepitaxy on a substrate of different lattice constant. This model, which permits prediction of the thin-film rotational pattern using first-principles density functional theory, has not been tested quantitatively over a range of strain states. To assess the validity of this picture, coherent LaAlO${}_3$ thin films were grown on SrTiO${}_3$, NdGaO${}_3$, LaSrAlO${}_4$, NdAlO${}_3$, and YAlO${}_3$ substrates to achieve strain states ranging from $+$3.03$\%$ to $-$2.35$\%$. The out-of-plane and in-plane octahedral rotation angles were extracted from the intensity of superlattice reflections measured using synchrotron x-ray diffraction. Density functional calculations show that no measurable change in intrinsic defect concentration should occur throughout the range of accessible strain states. Thus, the measured rotation angles were compared with those calculated previously for defect-free films. [Hatt and Spaldin, Phys. Rev. B 82, 195402 (2010)]. Good agreement between theory and experiment was found, suggesting that the current framework correctly captures the appropriate physics in LaAlO${}_3$.

We demonstrate that the spin decoherence of nitrogen vacancy (NV) centers in diamond can be suppressed by a transverse magnetic field if the electron spin bath is the primary decoherence source. The NV spin coherence, created in “a decoherence-free subspace,” is protected by the transverse component of the zero-field splitting, increasing the spin-coherence time about twofold. The decoherence due to the electron spin bath is also suppressed at magnetic fields stronger than ∼25 G when applied parallel to the NV symmetry axis. Our method can be used to extend the spin-coherence time of similar spin systems for applications in quantum computing, field sensing, and other metrologies.