Pentacene adsorbed on the $\mathrm{Cu}\left(110\right)-\mathrm{c}(6\times 2)\mathrm{O}$ surface has been investigated using scanning tunneling microscopy and spectroscopy and density functional theory calculations. The molecules lie almost flat on the surface and are anchored via the end carbon ring to a Cu atom of the topmost surface layer, leading to a break of the mirror symmetry of pentacene parallel to the short molecular axis. As a result the molecular electronic structure of the frontier orbitals exhibits a local and gentle modification around the anchoring site but is essentially unaffected in the remaining part of the molecule. The enhancement of the conductance peak related to the second highest occupied molecular orbital (HOMO-2) at the anchoring site, however, is suggestive of a resonance with a surface state or with the Cu $d$-band.

The orbital angular momentum and the polarization of light are physical quantities widely investigated for classical and quantum information processing. In this work we propose to take advantage of strong light-matter coupling, circular-symmetric confinement, and transverse-electric transverse-magnetic splitting to exploit states where these two degrees of freedom are combined. To this end we develop a model based on a spin-orbit Poincaré hypersphere. Then we consider the example of semiconductor polariton systems and demonstrate full ultrafast Stark control of spin-orbit states. Moreover, by controlling states on three different spin-orbit spheres and switching from one sphere to another we demonstrate the control of different logic bits within one single physical system.

In this paper the phase diagram of Dirac semimetals is studied within a lattice Monte Carlo simulation. In particular, we concentrate on the dynamical chiral symmetry breaking which results in a semimetal-insulator transition. Using numerical simulation, we determine the values of the critical coupling constant of the semimetal-insulator transition for different values of the anisotropy of the Fermi velocity. This measurement allows us to draw a tentative phase diagram for Dirac semimetals. It turns out that within the Dirac model with Coulomb interaction both ${\mathrm{Na}}_3\mathrm{Bi}$ and ${\mathrm{Cd}}_3{\mathrm{As}}_2$, known experimentally to be Dirac semimetals, would lie deep in the insulating region of the phase diagram. This result probably shows a decisive role of screening of the interelectron interaction in real materials, similar to the situation in graphene.

A tendency to form benzenelike molecular orbitals has recently been shown to be a common feature of the $4d$ and $5d$ transition metal oxides with a honeycomb lattice. This tendency competes with other interactions such as the spin-orbit coupling and Hubbard correlations and can be partially or completely suppressed. In the calculations, ${\mathrm{SrRu}}_2{\mathrm{O}}_6$ presents the cleanest case of well-formed molecular orbitals so far; however, direct experimental evidence for or against this proposition has been missing. In this paper, we show that combined photoemission and optical studies can be used to identify molecular orbitals in ${\mathrm{SrRu}}_2{\mathrm{O}}_6$. Symmetry-driven election selection rules suppress optical transitions between certain molecular orbitals, while photoemission and inverse photoemission measurements are insensitive to them. Comparing the photoemission and optical conductivity spectra, one should be able to observe clear signatures of molecular orbitals.

Recent work has shown that electron spins in germanium (Ge) nanoscale transistors can be electrically tuned and have encouraging coherence times. Based on a complete and validated theory of Ge-donor electron states, we propose that Ge spin qubits could have significant advantages over silicon (Si) in the implementation of a donor-based quantum processor architecture. Our work shows that the intrinsic features of the Ge band structure allow for a speedup of selective (local) one-qubit gates of up to two orders of magnitude as compared to Si. Further, we find that fast, robust two-qubit gates in Ge pose less stringent fabrication constraints than in Si devices: Ge donors spaced three times farther apart than in Si show comparable exchange couplings, allowing more space for readout and control gates. In addition, for realistic position uncertainty in donor placement, Ge:P spin couplings have a $33\%$ chance of being within an order of magnitude of the largest coupling, compared with only $10\%$ in Si:P. It is therefore possible that a Ge-based platform would enable fast, parallel, and robust architectures for quantum computing.

The structural morphology of complex dielectric media determines their functionalities by driving the statistical properties of the electromagnetic fields. Our controlled experiments and full electromagnetic calculations that go beyond common dipolar approximations demonstrate that the specific characteristics of disorder lead to non-Rayleigh statistics of detected intensity, which can be directly accessed in the near field of random media and can be unambiguously related to the short-range correlations of disorder.

Zak phases in two types of acoustic metamaterials with negative density or negative modulus are studied. Different from phononic crystals based on Bragg scattering, extraordinary performance in acoustic metamaterials originates from local resonance. We find that the Zak phases in metamaterials are determined by the resonances of scatterers, side pipes, or membranes. Additionally, it is observed that the topological transition points, symmetrical properties of edge states, input impedances, and reflection phases in forbidden bands are all related to the resonances of the scatterers, and the relationship can be used to create surface states at the interface of two types of acoustic metamaterials.

We present macroscopic and neutron diffraction data on multiferroic lightly Co doped ${\mathrm{Ni}}_3{\mathrm{V}}_2{\mathrm{O}}_8$. The magnetic $H-T$ phase diagrams have been derived from magnetization and electric polarization measurements with field directions parallel to the principal crystallographic axes. While the phase diagram for $H\parallel b$ is very similar to that of the parent compound ${\mathrm{Ni}}_3{\mathrm{V}}_2{\mathrm{O}}_8$ for the commonly involved phases, the zero-field phases in ${\left({\mathrm{Ni}}_{0.9}{\mathrm{Co}}_{0.1}\right)}_3{\mathrm{V}}_2{\mathrm{O}}_8$ show a stronger instability for applied magnetic fields along the $a$ or $c$ axis. Neutron single-crystal diffraction revealed the magnetic structure of the field-induced phase for $H\parallel c$ with a collinear spin alignment along the $a$ and $b$ axes for the two magnetically inequivalent sites. A pronounced irreversibility has been observed for the transition between the zero-field spin cycloid and the field-induced phase, which is manifested in a propagation vector change from $\mathbf{q}=\left(\mathrm{0.322\; 0\; 0}\right)$ to $\mathbf{q}=\left(\mathrm{0.306\; 0\; 0}\right)$, with slight modifications of the magnetic structure after reentering the zero-field phase. The reentrant phase is characterized by a significantly larger $b$ component of the cross-tie site spin, therefore showing remanent features of the high-field phase. For $H\parallel a$ the magnetization data reveal anomalies, one of which was proved to reflect a field-induced transition from the cycloidally to the sinusoidally modulated magnetic structure.

We use a strong-disorder renormalization group (SDRG) method and ground-state quantum Monte Carlo (QMC) simulations to study $S=1/2$ spin chains with random couplings, calculating disorder-averaged spin and dimer correlations. The QMC simulations demonstrate logarithmic corrections to the power-law decaying correlations obtained with the SDRG scheme. The same asymptotic forms apply both for systems with standard Heisenberg exchange and for certain multispin couplings leading to spontaneous dimerization in the clean system. We show that the logarithmic corrections arise in the valence-bond (singlet pair) basis from a contribution that cannot be generated by the SDRG scheme. In the model with multispin couplings, where the clean system dimerizes spontaneously, random singlets form between spinons localized at domain walls in the presence of disorder. This amorphous valence-bond solid is asymptotically a random-singlet state and only differs from the random-exchange Heisenberg chain in its short-distance properties.

We present a systematic study of the phase diagram of ${\mathrm{LiHo}}_x{\mathrm{Y}}_{1-x}{\mathrm{F}}_4$ ($0.25\le x\le 1$) Ising ferromagnets obtained from neutron scattering measurements and mean-field calculations. We show that while the thermal phase transition decreases linearly with dilution, as predicted by mean-field theory, the critical transverse field at the quantum critical point is suppressed much faster. This behavior is related to competition between off-diagonal dipolar coupling and quantum fluctuations that are tuned by doping and applied field, respectively. In this paper, we quantify the deviation of the experimental results from mean-field predictions, with the aim that this analysis can be used in future theoretical efforts towards a quantitative description.

We demonstrate theoretically that atomistic spin dynamics simulations of topological magnon insulators (TMIs) provide access to the magnon-mediated transport of both spin and heat. The TMIs, modeled by kagome ferromagnets with Dzyaloshinskii-Moriya interaction, exhibit nonzero transverse-current correlation functions from which conductivities are derived for the entire family of magnon Hall effects. Both longitudinal and transverse conductivities are studied in dependence on temperature and on an external magnetic field. A comparison between theoretical and experimental results for Cu(1,3-benzenedicarboxylate), a recently discovered TMI, is drawn.

Thin epitaxial films of Tb metal were grown on a clean W(110) substrate in ultrahigh vacuum and studied in situ by low-energy electron microscopy. Annealed films present magnetic contrast in spin-polarized low-energy electron microscopy. The energy dependence of the electron reflectivity was determined and a maximum value of its spin asymmetry of about 1% was measured. The magnetization direction of the Tb films is in-plane. Upon raising the temperature, no change in the domain distribution is observed, while the asymmetry in the electron reflectivity decreases when approaching the critical temperature, following a power law $\sim {(1-T/T_C)}^{\beta }$ with a critical exponent $\beta$ of 0.39.

Electric and magnetic properties of multiferroic ${\text{GdMn}}_2{\text{O}}_5$ in external magnetic fields were investigated to map out the magnetoelectric phases in this material. Due to strong magnetoelectric coupling, the dielectric permittivity is highly sensitive to phase boundaries in ${\text{GdMn}}_2{\text{O}}_5$, which allowed us to construct the field-temperature phase diagrams. Several phase transitions are observed which are strongly field dependent with respect to field orientation and strength. The phase diagram for a magnetic field along the crystallographic $a$ axis corresponds well to a polarization step, as induced by ${90}^{\circ }$ rotation of Gd magnetic moments. Our results support the model of two ferroelectric sublattices, Mn-Mn and Gd-Mn, with strong $R$-Mn ($4f-3d$) interaction for the polarization in $R{\text{Mn}}_2{\text{O}}_5$.

Pairs fluctuation supercurrents and inverse lifetimes in the pseudogap regime are reported. These were measured on epitaxial c-axis junctions of the cuprates, with a ${\text{PrBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }$ barrier sandwiched in between two ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }$ or doped ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_y$ electrodes, with or without magnetic fields parallel to the a-b planes. All junctions had a $T_c\left(\text{high}\right)\approx 85–90$ K and a $T_c\left(\text{low}\right)\approx 50–55$ K electrodes, allowing us to study pairs fluctuation supercurrents and inverse lifetimes in between these two temperatures. In junctions with a pseudogap electrode under zero field, an excess current due to pair fluctuations was observed which persisted at temperatures above $T_c\left(\text{low}\right)$, in the pseudogap regime, and up to about $T_c\left(\text{high}\right)$. No such excess current was observed in junctions without an electrode with a pseudogap. The measured conductance spectra at temperatures above $T_c\left(\text{low}\right)$ were fitted using a modified fluctuations model by Scalapino [Phys. Rev. Lett. 24, 1052 (1970)] of a junction with a serial resistance. We found that in the pseudogap regime, the conductance vs voltage consists of a narrow peak sitting on top of a very broad peak. This yielded two distinct pairs fluctuation lifetimes in the pseudogap electrode which differ by an order of magnitude up to about $T_c\left(\text{high}\right)$. Under in-plane fields, these two lifetime values remain separated in two distinct groups, which varied with increasing field moderately. We also found that detection of Amperian pairing [Phys. Rev. X 4, 031017 (2014)] in our cuprate junctions is not feasible, due to Josephson vortices penetration into the superconducting electrodes which drove the necessary field above the depairing field.

Strontium titanate is a low-temperature, non–Bardeen-Cooper-Schrieffer superconductor that superconducts to carrier concentrations lower than in any other system and exhibits avoided ferroelectricity at low temperatures. Neither the mechanism of superconductivity in strontium titanate nor the importance of the structure and dielectric properties for the superconductivity are well understood. We studied the effects of twin structure on superconductivity in a 5.5-nm-thick layer of niobium-doped ${\mathrm{SrTiO}}_3$ embedded in undoped ${\mathrm{SrTiO}}_3$. We used a scanning superconducting quantum interference device susceptometer to image the local diamagnetic response of the sample as a function of temperature. We observed regions that exhibited a superconducting transition temperature $T_c\gtrsim 10$% higher than the temperature at which the sample was fully superconducting. The pattern of these regions varied spatially in a manner characteristic of structural twin domains. Some regions are too wide to originate on twin boundaries; therefore, we propose that the orientation of the tetragonal unit cell with respect to the doped plane affects $T_c$. Our results suggest that the anisotropic dielectric properties of ${\mathrm{SrTiO}}_3$ are important for its superconductivity and need to be considered in any theory of the mechanism of the superconductivity.

To elucidate the pseudogap phase diagram including the overdoped state of high transition temperature (high-$T_c$) cuprates, we must understand the origin of the positive out-of-plane magnetoconductivity observed in these compounds. For this purpose, the out-of-plane resistivity ${\rho }_c(T,H)$ of an overdoped ${\mathrm{Bi}}_{1.6}{\mathrm{Pb}}_{0.4}{\mathrm{Sr}}_2{\mathrm{CaCu}}_{1.96}{\mathrm{Fe}}_{0.04}{\mathrm{O}}_{8+\delta }$ (Bi-2212) single crystal is measured under pulsed magnetic fields up to 60 T. We show that the superconducting density-of-states depletion effect, in addition to the pseudogap effect, clearly appears below the superconducting fluctuation regime, and the contribution becomes dominant in the superconducting state.

Several experimental and theoretical arguments have been made in favor of a $d$-wave symmetry for the superconducting state in some Fe-based materials. It is a common belief that a $d$-wave gap in the Fe-based superconductors must have nodes on the Fermi surfaces centered at the $\mathrm{\Gamma }$ point of the Brillouin zone. Here we show that, while this is the case for a single Fermi surface made out of a single orbital, the situation is more complex if there is an even number of Fermi surfaces made out of different orbitals. In particular, we show that for the two $\mathrm{\Gamma }$-centered hole Fermi surfaces made out of $d_{xz}$ and $d_{yz}$ orbitals, the nodal points still exist near $T_c$ along the symmetry-imposed directions, but are are displaced to momenta between the two Fermi surfaces. If the two hole pockets are close enough, pairs of nodal points can merge and annihilate at some $T<T_c$, making the $d$-wave state completely nodeless. These results imply that photoemission evidence for a nodeless gap on the $d_{xz}/d_{yz}$ Fermi surfaces of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ does not rule out $d$-wave gap symmetry in this material, while a nodeless gap observed on the $d_{xy}$ pocket in ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ is truly inconsistent with the $d$-wave gap symmetry.

The interfacial Dzyaloshinskii-Moriya interaction (DMI) in ultrathin magnetic thin film heterostructures provides a new approach for controlling spin textures on mesoscopic length scales. Here we investigate the dependence of the interfacial DMI constant $D$ on a Pt wedge insertion layer in Ta/CoFeB/Pt(wedge)/MgO thin films by observing the asymmetric spin-wave dispersion using Brillouin light scattering. Continuous tuning of $D$ by more than a factor of 3 is realized by inserting less than one monolayer of Pt. The observations provide new insights for designing magnetic thin film heterostructures with tailored $D$ for controlling skyrmions and magnetic domain-wall chirality and dynamics.

We demonstrate the control of Gilbert damping in 65-nm-thick yttrium iron garnet (YIG) films using a spin-polarized current generated by a direct current through a nanocontact, spin filtered by a thin Co layer. The magnetodynamics of both the YIG and the Co layers can be excited by a pulse-modulated microwave current injected through the nanocontact and the response detected as a lock-in amplified voltage over the device. The spectra show three clear peaks, two associated with the ferromagnetic resonance (FMR) in each layer, and an additional Co mode with a higher wave vector proportional to the inverse of the nanocontact diameter. By varying the sign and magnitude of the direct nanocontact current, we can either increase or decrease the linewidth of the YIG FMR peak consistent with additional positive or negative damping being exerted by the nonlocal spin current injected into the YIG film. Our nanocontact approach thus offers an alternative route in the search for auto-oscillations in YIG films.

We present a comprehensive density-functional theory study of total energy and structural properties of As, Sb, and Bi in their A7 ground-state structure and in the bcc, fcc, and simple cubic (sc) modifications. We also investigate continuous structural transitions between these structures. The electronic structures and total energies are calculated both within the generalized gradient approximation (GGA) and local-density approximation (LDA) to the exchange-correlation energy as well as with and without inclusion of the spin-orbit coupling (SOC). The total energies of deformed structures are displayed in contour plots as functions of selected structural parameters and/or atomic volume; these plots are then used for understanding and interpreting structural parameters of As, Sb and Bi thin films on various substrates. Our calculated values of lattice parameters for (0001) thin films of Bi on Si(111) and Ge(111) substrates agree very well with available experimental data. In analogy with that, we suggest to investigate (0001) thin films of As on Ti(0001), Co(0001), Zn(0001) and Rh(111) substrates, of Sb on C(0001), Zn(0001), Al(111), Ag(111) and Au(111) substrates and of Bi on Co(0001), Al(111), Rh(111), Ba(111) and Pb(111) substrates. For these cases, we also predict the lattice parameters of the films. A large part of our results are theoretical predictions which may motivate experimentalists for a deeper study of these systems.