The magnetic ordering structure of ${\mathrm{GdPO}}_4$ is determined at $T=60$ mK by the diffraction of hot neutrons with wavelength $\lambda =0.4696$ Å. It corresponds to a noncollinear antiferromagnetic arrangement of the Gd moments with propagation vector $\mathbf{k}=(1/2,0,1/2)$. This arrangement is found to minimize the dipole-dipole interaction and the crystal-field anisotropy energy, the magnetic superexchange being much smaller. The intensity of the magnetic reflections decreases with increasing temperature and vanishes at $T\approx 0.8$ K, in agreement with the magnetic ordering temperature $T_N=0.77$ K, as reported in previous works based on heat capacity and magnetic susceptibility measurements. The magnetocaloric parameters have been determined from heat capacity data at constant applied fields up to 7 T, as well as from isothermal magnetization data. The magnetocaloric effect, for a field change $\Delta B=0-7\mathrm{T}$, reaches $-\Delta S_T=375.8\mathrm{mJ}/{\mathrm{cm}}^3{\mathrm{K}}^{-1}$ at $T=2.1$ K, largely exceeding the maximum values reported to date for Gd-based magnetic refrigerants.

The challenging problem of skew scattering for Hall effects in dilute ferromagnetic alloys, with intertwined effects of spin-orbit coupling, magnetism, and impurity scattering, is studied here from first principles. Our main aim is to identify chemical trends and work out simple rules for large skew scattering in terms of the impurity and host states at the Fermi surface, with particular emphasis on the interplay of the spin and anomalous Hall effects in one and the same system. The predicted trends are benchmarked by referring to three different ab initio methods based on different approximations with respect to the electronic structure and transport properties.

We show, in several important and general cases, that a low variational energy density of a trial state is possible even when the trial state represents a different phase from the ground state. Specifically, we ask whether the ground-state energy density of a Hamiltonian whose ground state is in phase A can be approximated to arbitrary accuracy by a wave function, which represents a different phase B. We show this is indeed the case when A has discrete symmetry breaking order in one dimension or topological order in two dimensions, while B is disordered. We argue that, if reasonable conditions of physicality are imposed upon the trial wave function, then this is not possible when A has discrete symmetry breaking in dimensions greater than one and B is symmetric. Some other situations are also discussed.

The proximity effect describes the extension of superconductivity into a metal, which is in contact with a superconductor. Scanning tunneling spectroscopy is used to probe this effect in a two-atomic-layer Pb/Ag nonsuperconducting film, surrounding a superconducting Pb nanoisland, with sub-nm spatial resolution. Here we show a nontrivial dependence of the length scales of the superconducting proximity effect on magnetic field. Surprisingly, we find that the magnetic field does not affect the induced superconductivity up to 0.3 T, but a breakdown of the proximity effect is induced for fields around 0.6 T, with a $1/H^p$ dependence with $p\cong 7$. The unexpected robustness of the induced superconductivity in the presence of magnetic fields is ascribed to the high electronic diffusivity in the metallic wetting layer.

We report ballistic transport commensurability minima in the magnetoresistance of $\nu =3/2$ composite fermions (CFs). The CFs are formed in high-quality two-dimensional electron systems confined to wide GaAs quantum wells and subjected to an in-plane, unidirectional periodic potential modulation. We observe a slight asymmetry of the CF commensurability positions with respect to $\nu =3/2$, which we explain quantitatively by comparing three CF density models and concluding that the $\nu =3/2$ CFs are likely formed by the minority carriers in the upper energy spin state of the lowest Landau level. Our data also allow us to probe the shape and size of the CF Fermi contour. At a fixed electron density of $\simeq 1.8\times {10}^{11}$ ${\mathrm{cm}}^{-2}$, as the quantum well width increases from 30 to 60 nm, the CFs show increasing spin polarization. We attribute this to the enhancement of the Zeeman energy relative to the Coulomb energy in wider wells where the latter is softened because of the larger electron layer thickness. The application of an additional parallel magnetic field ($B_{\left|\right|}$) leads to a significant distortion of the CF Fermi contour as $B_{\left|\right|}$ couples to the CFs' out-of-plane orbital motion. The distortion is much more severe compared to the $\nu =1/2$ CF case at comparable $B_{\left|\right|}$. Moreover, the applied $B_{\left|\right|}$ further spin-polarizes the $\nu =3/2$ CFs as deduced from the positions of the commensurability minima.

We have determined the number of transport channels and the value of the transmission coefficients of a single molecular junction by measuring the multiple Andreev reflections (MARs) with a scanning tunneling microscope (STM). By precisely positioning a Nb STM tip to a single ${\mathrm{C}}_{60}$ molecule on Pb(111), a single molecular junction was fabricated in which the ${\mathrm{C}}_{60}$ molecule connects with the two superconducting electrodes. From the subharmonic gap structures arising from MARs in the current-voltage characteristics together with the tunneling spectrum of the ${\mathrm{C}}_{60}$ molecule, we found that unoccupied molecular orbitals of ${\mathrm{C}}_{60}$ extending to the Fermi level provide three electronic transport channels in the molecular junction. We also found that the transmission coefficients depend on the contact geometry of the molecule. These results demonstrate that the combination of the STM imaging with the MARs measurement provides an effective path for investigating the electronic transport properties through a single molecule sandwiched by two superconducting electrodes.

We show that the chiral kagome ice manifold exhibits an anomalous integer quantum Hall effect (IQHE) when coupled to itinerant electrons. Although electron-mediated interactions select a magnetically ordered ground state, the full ice manifold can coexist with the IQHE over a range of finite temperatures. The degenerate ice states provide a natural realization of power-law correlated flux disorder, for which the spectral gap of the system remains robust. The quantized (up to exponentially small finite-temperature corrections) Hall conductance persists over a wide range of electron densities due to the disorder-induced localization of electronic states.

We compute the nonzero temperature conductivity of conserved flavor currents in conformal field theories (CFTs) in 2+1 space-time dimensions. At frequencies much greater than the temperature, $\hslash \omega \gg k_{B}T$, the $\omega$ dependence can be computed from the operator product expansion (OPE) between the currents and operators, which acquire a nonzero expectation value at $T>0$. Such results are found to be in excellent agreement with quantum Monte Carlo studies of the O(2) Wilson-Fisher CFT. Results for the conductivity and other observables are also obtained in vector $1/N$ expansions. We match these large $\omega$ results to the corresponding correlators of holographic representations of the CFT: the holographic approach then allows us to extrapolate to small $\hslash \omega /\left(k_{B}T\right)$. Other holographic studies implicitly only used the OPE between the currents and the energy-momentum tensor, and this yields the correct leading large $\omega$ behavior for a large class of CFTs. However, for the Wilson-Fisher CFT, a relevant “thermal” operator must also be considered, and then consistency with the Monte Carlo results is obtained without a previously needed ad hoc rescaling of the $T$ value. We also establish sum rules obeyed by the conductivity of a wide class of CFTs.

The spin-Seebeck effect (SSE) in platinum (Pt) and tantalum (Ta) on yttrium iron garnet has been investigated by both externally heating the sample (using an on-chip Pt heater on top of the device) and by current-induced heating. For SSE measurements, external heating is the most common method to obtain clear signals. Here we show that also by current-induced heating it is possible to directly observe the SSE, separate from the also present spin-Hall magnetoresistance (SMR) signal, by using a lock-in detection technique. Using this measurement technique, the presence of additional second-order signals at low applied magnetic fields and high heating currents is revealed. These signals are caused by current-induced magnetic fields (Oersted fields) generated by the used ac current, resulting in dynamic SMR signals.

We discuss control of the quantum-transport properties of a mesoscopic device by connecting it in a coherent feedback loop with a quantum-mechanical controller. We work in a scattering approach and derive results for the combined scattering matrix of the device-controller system and determine the conditions under which the controller can exert ideal control on the output characteristics. As a concrete example we consider the use of feedback to optimize the conductance of a chaotic quantum dot and investigate the effects of controller dimension and decoherence. In both respects we find that the performance of the feedback geometry is well in excess of that offered by a simple series configuration.

Current-induced domain wall motion in the presence of the Dzyaloshinskii-Moriya interaction (DMI) is experimentally and theoretically investigated in heavy-metal/ferromagnet bilayers. The angular dependence of the current-induced torque and the magnetization structure of Dzyaloshinskii domain walls are described and quantified simultaneously in the presence of in-plane fields. We show that the DMI strength depends strongly on the heavy metal, varying by a factor of 20 between Ta and Pa, and that strong DMI leads to wall distortions not seen in conventional materials. These findings provide essential insights for understanding and exploiting chiral magnetism for emerging spintronics applications.

Magnetostructural phase transitions in ${\mathrm{Ba}}_{1-x}{A}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ (A = K, Na) materials are discussed for both magnetically and orbitally driven mechanisms, using a symmetry analysis formulated within the Landau theory of phase transitions. Both mechanisms predict identical orthorhombic space-group symmetries for the nematic and magnetic phases observed over much of the phase diagram, but they predict different tetragonal space-group symmetries for the newly discovered reentrant tetragonal phase in ${\mathrm{Ba}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ $(x\sim 0.24–0.28)$. In a magnetic scenario, magnetic order with moments along the $c$ axis, as found experimentally, does not allow any type of orbital order, but in an orbital scenario, we have determined two possible orbital patterns, specified by $P4/mnc{1}^{\prime }$ and $I{4221}^{\prime }$ space groups, which do not require atomic displacements relative to the parent $I4/mmm{1}^{\prime }$ symmetry and, in consequence, are indistinguishable in conventional diffraction experiments. We demonstrate that the three possible space groups are, however, distinct in resonant x-ray Bragg diffraction patterns created by Templeton & Templeton scattering. This provides an experimental method of distinguishing between magnetic and orbital models.

We present an experimental realization of a Coulomb blockade refrigerator (CBR) based on a single-electron transistor (SET). In the present structure, the SET island is interrupted by a superconducting inclusion to permit charge transport while preventing heat flow. At certain values of the bias and gate voltages, the current through the SET cools one of the junctions. The measurements follow the theoretical model down to $\sim 80$ mK, which was the base temperature of the current measurements. The observed cooling increases rapidly with decreasing temperature, in agreement with the theory, reaching about a 15 mK drop at the base temperature. The CBR appears as a promising electronic cooler at temperatures well below 100 mK.

The ultrastrong light-matter interaction regime was investigated in metallic and superconducting complementary split-ring resonators coupled to the cyclotron transition of two-dimensional electron gases. The subwavelength light confinement and the large optical dipole moment of the cyclotron transition yield record high normalized coupling rates of up to $\frac{{\Omega }_R}{{\omega }_c}=0.87$. We observed a blue-shift of both polaritons due to the diamagnetic term of the interaction Hamiltonian.

We have performed electrical transport experiments on individual carbon nanotubes (CNTs) in situ in a transmission electron microscope using the liquid-metal contact method (LMC method), which consists of immersing a CNT placed on the apex of a metallic tip into a drop of liquid mercury (Hg). In the literature, this method has been mostly employed without visualization (ex situ) to show the ballistic- and quantum-conductance properties of different kinds of CNTs. We show that on the one hand the in situ LMC method is well suited to create low-resistance contacts with the CNTs but on the other hand the ballistic and quantum conductance measured by the ex situ LMC method is likely to give false positives for three reasons: (a) the CNTs are likely to be removed from the tip surface through contact with the Hg, (b) occurring Hg-tip surface nanocontacts are likely to be mistaken for quantum-conductor CNTs, and (c) occurring Hg nanomenisci are likely to be mistaken for ballistic-conductor CNTs. These findings have strong consequences for the interpretation of previously reported works.

The $s_z=0$ spin configuration of two electrons confined at a double quantum dot (DQD) encodes the singlet-triplet qubit (STQ). We introduce the inverted STQ (ISTQ) that emerges from the setup of two quantum dots (QDs) differing significantly in size and out-of-plane magnetic fields. The strongly confined QD has a two-electron singlet ground state, but the weakly confined QD has a two-electron triplet ground state in the $s_z=0$ subspace. Spin-orbit interactions act nontrivially on the $s_z=0$ subspace and provide universal control of the ISTQ together with electrostatic manipulations of the charge configuration. GaAs and InAs DQDs can be operated as ISTQs under realistic noise conditions.

In magnetoplasmonics, it is possible to tailor the magneto-optical properties of nanostructures by exciting surface plasmon polaritons (SPPs). Thus far, magnetoplasmonic effects have been considered to be static. Here, we describe ultrafast manifestations of magnetoplasmonics by observing the nontrivial evolution of the transverse magneto-optic Kerr effect within 45-fs pulses reflected from an iron-based magnetoplasmonic crystal. The effect occurs for resonant SPP excitations, displays opposite time derivative signs for different slopes of the resonance, and is explained with the magnetization-dependent dispersion relation of SPPs.

Structural and physical properties determined by measurements on large single crystals of the anisotropic ferromagnet MnBi are reported. The findings support the importance of magnetoelastic effects in this material. X-ray diffraction reveals a structural phase transition at the spin reorientation temperature $T_{SR}=90$ K. The distortion is driven by magnetoelastic coupling, and upon cooling transforms the structure from hexagonal to orthorhombic. Heat capacity measurements show a thermal anomaly at the crystallographic transition, which is suppressed rapidly by applied magnetic fields. Effects on the transport and anisotropic magnetic properties of the single crystals are also presented. Increasing anisotropy of the atomic displacement parameters for Bi with increasing temperature above $T_{SR}$ is revealed by neutron diffraction measurements. It is likely that this is directly related to the anisotropic thermal expansion in MnBi, which plays a key role in the spin reorientation and magnetocrystalline anisotropy. The identification of the true ground-state crystal structure reported here may be important for future experimental and theoretical studies of this permanent magnet material, which have to date been performed and interpreted using only the high-temperature structure.

The destruction and formation of equilibrium multidomain patterns in permalloy $\left({\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}\right)$ microsquares has been captured using pump-probe x-ray magnetic circular dichroism (XMCD) spectromicroscopy at a new full-field magnetic transmission soft x-ray microscopy endstation with subnanosecond time resolution. The movie sequences show the dynamic magnetization response to intense Oersted field pulses of approximately 200-ps root mean square (rms) duration and the magnetization reorganization to the ground-state domain configuration. The measurements display how a vortex flux-closure magnetization distribution emerges out of a nonequilibrium uniform single-domain state. During the destruction of the initial vortex pattern, we have traced the motion of the central vortex core that is ejected out of the microsquare at high velocities exceeding 1 km/s. A reproducible recovery into a defined final vortex state with stable chirality and polarity could be achieved. Using an additional external bias field, the transient reversal of the square magnetization direction could be monitored and consistently reproduced by micromagnetic simulations.

The hydrogen molecule ${\mathrm{H}}_2$ (or “hidden” hydrogen) in ZnO is studied by Raman scattering spectroscopy. It is shown that ${\mathrm{H}}_2$ is practically a free rotator stable up to $700{}^{\circ }\mathrm{C}$, which is formed by two mobile interstitial hydrogen atoms. The concentration profile of the hydrogen molecule anticorrelates with hydrogen substituting for oxygen (${\mathrm{H}}_{\mathrm{O}}$) created at the sample surface during the high temperature hydrogenation. The ${\mathrm{H}}_2$ dissociation at elevated temperatures in the bulk regenerates interstitial (${\mathrm{H}}_{\mathrm{BC}}$) hydrogen, whereas the formation of ${\mathrm{H}}_{\mathrm{O}}$ and hydrogen out-diffusion are dominant decay channels of the molecule near the surface. The latter mechanisms are responsible for the lower stability of ${\mathrm{H}}_2$ in the subsurface region. An ortho-para conversion between the nuclear spin states 1 and 0 of the molecule at 79 K was found to occur within 7.5 h, whereas the back conversion at room temperature occurs faster than 0.5 h. A shift in frequency of the ${\mathrm{H}}_2$ local vibrational mode with annealing time and temperature is associated with the thermal anneal of lattice imperfections. The coupling of transitions between rotational states of the molecule and lattice phonons influences the ro-vibrational properties. Interstitial lattice sites and/or larger vacancy clusters are preferred trapping centers for ${\mathrm{H}}_2$ in ZnO.