Two-dimensional hard core bosons suffer strong scattering in the high-temperature resistive state at half filling. The dynamical conductivity $\sigma \left(\omega \right)$ is calculated using nonperturbative tools such as continued fractions, series expansions, and exact diagonalization. We find a large temperature range with linearly increasing resistivity and broad dynamical conductivity, signaling a breakdown of Boltzmann-Drude quasiparticle transport theory. At zero temperature, a high-frequency peak in $\sigma \left(\omega \right)$ appears above a “Higgs mass” gap and corresponds to order-parameter magnitude fluctuations. We discuss the apparent similarity between conductivity of hard core bosons and phenomenological characteristics of cuprates, including the universal scaling of Homes et al. [Nature (London) 430, 539 (2004)].

Neutron-scattering and magnetization measurements have been performed on the stuffed pyrochlore system ${\text{Tb}}_{2+x}{\text{Ti}}_{2-2x}{\text{Nb}}_x{\text{O}}_7$. We find that despite the introduction of chemical disorder and increasingly antiferromagnetic interactions, a spin-glass transition does not occur for $T\ge 1.5\text{K}$ and cooperative paramagnetic behavior exists for all $x$. For $x=1$, ${\text{Tb}}_3{\text{NbO}}_7$, an antiferromagnetically ordered state coexisting with cooperative paramagnetic behavior is seen without applying any external fields or pressure, a situation advantageous for studying this cooperative behavior.

We consider a model for a spin field-effect molecular transistor, where a directed pure spin current is controlled by an external electric field. Inelastic scattering effects of such molecular device are discussed within a framework of full counting statistics for a multilevel molecular system. We propose that the heating of the molecular junction can be controlled by external electric and magnetic fields. Characteristic features of the model are demonstrated by numerical calculations.

Nuclear spin polarization is probed via hyperfine interaction with spin-split edge channels. By scanning the edge channels with a biased metal side gate, the local profile of nuclear spin polarization is imaged with a resolution of $\sim 14\text{nm}$, determined by the extension of the edge-state wave function. Distribution of the nuclear spins dynamically polarized by the edge channels is found to be localized in a region as narrow as 35 nm at the lowest current (0.1 nA) but broadens with increasing current. The decay process is studied in detail with the same technique and the temporal dynamics of the nuclear polarization are interpreted in terms of the interplay of diffusion, electron-nuclear spin interaction, and spin-lattice relaxation.

Enhanced sensitivity to lateral forces on the nominally flat and inert (0001) surface of graphite is demonstrated via room-temperature dynamic force microscopy using simultaneous excitation and FM detection of the lowest flexural and torsional cantilever resonance modes. The site-independent long-range tip-sample interaction causes no significant lateral force variations except near atomic steps but unprecedented sensitivity to short-range forces is achieved on flat terraces in the attractive range. Two-dimensional bimodal force vs distance maps confirm the stronger distance dependence of the torsional frequency shift compared to the flexural resonance shift. This agrees with model calculations based on theoretical expressions. The lateral force gradient is extracted from the measured torsional shift, and a lateral force variation in at most $\pm 20\text{pN}$ is obtained by integrating this gradient parallel to the surface. A further integration reveals a potential-energy variation in the attractive force range of only 3 meV.

We study the low-energy dynamics of $S=1/2$ antiferromagnetic Heisenberg clusters constructed by diluting a square lattice at vacancy concentration $p$ at and below the percolation threshold $p^{\ast }\approx 0.407$. The finite-size scaling behavior of the average excitation gap, $⟨\Delta ⟩\sim L^{-z}$, where $L$ is the cluster length, is obtained using quantum Monte Carlo results for an upper bound ${\Delta }^{\ast }$ to $\Delta$, derived from sum rules. At the percolation threshold, we obtain a dynamic exponent $z=3.6\pm 0.1\approx 2D_f$ for clusters with singlet $\left(S=0\right)$ ground state. Here $D_f=91/48$ is the fractal dimensionality of the percolating cluster. We argue that this large dynamic exponent—roughly twice that expected for quantum-rotor excitations—is a consequence of weakly interacting localized effective magnetic moments, which form due to local sublattice imbalance. This picture is supported by an extremal-value analysis of local spectral gaps, which delivers an exponent relation (between $z$ and two exponents characterizing the local-gap distribution) reproduced by our simulation data. However, the average $⟨{\Delta }^{\ast }⟩$ over all clusters, which have mostly ground-state spin $S>0$, scales with a smaller exponent than for the $S=0$ clusters alone; $z\approx 1.5D_f$. Lanczos exact diagonalization for small clusters show that typically, $S\to S-1$ in the lowest-energy excitations while the dominant spectral weight originates from $S\to S+1$ excitations. Thus, the scaling of $⟨{\Delta }^{\ast }⟩$ for clusters with ground state $S>0$ does not reflect the lowest-energy excitations but the higher $S\to S+1$ excitations. This result can be understood within a valence bond picture. To further explore the scenario of localized moments, we introduce a classical dimer-monomer aggregation model to study the distribution of nearest-neighbor sites forming dimers (which are the objects used in mapping to the quantum-rotor model) and unpaired spins (monomers). The monomers are localized and, thus, effective magnetic moments should form in the spin system. We also study the lowest triplet excitation of $S=0$ clusters using quantum Monte Carlo calculations in the valence bond basis. The triplet is concentrated at some of the classical monomer regions, confirming the mechanism of moment formation. The number of spins (and moment regions) affected by the excitation scales as a nontrivial power of the cluster size. For a dimer-diluted bilayer Heisenberg model with weak interlayer coupling (where the system remains Néel ordered), there is no sublattice imbalance. In this case we find $z\approx D_f$, consistent with quantum-rotor excitations. For a single layer at $p<p^{\ast }$ we find $z\approx 2=D$, which indicates that the weakly interacting localized moment mechanism is valid only exactly at the percolation point. There is a crossover behavior close to the percolation point.

In the cuprate superconductors, Nernst and torque magnetization experiments have provided evidence that the disappearance of the Meissner effect at $T_c$ is caused by the loss of long-range phase coherence, rather than the vanishing of the pair condensate. Here we report a series of torque magnetization measurements on single crystals of ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ (LSCO), ${\text{Bi}}_2{\text{Sr}}_{2-y}{\text{La}}_y{\text{CuO}}_6$ (Bi 2201), ${\text{Bi}}_2{\text{Sr}}_2{\text{CaCu}}_2{\text{O}}_{8+\delta }$, and optimal ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_7$. Some of the measurements were taken to fields as high as 45 T. Focusing on the magnetization above $T_c$, we show that the diamagnetic term $M_d$ appears at an onset temperature $T_{onset}^M$ high above $T_c$. We construct the phase diagram of both LSCO and Bi 2201 and show that $T_{onset}^M$ agrees with the onset temperature of the vortex Nernst signal $T_{onset}^{\nu }$. Our results provide thermodynamic evidence against a recent proposal that the high-temperature Nernst signal in LSCO arises from a quasiparticle contribution in a charge-ordered state.

Inelastic neutron scattering is used to probe the collective spin excitations of the near optimally doped superconductor $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ $\left(x=0.065\right)$. Previous measurements on the antiferromagnetically ordered parents of this material show a strongly anisotropic spin-wave velocity. Here we measure the magnetic excitations up to 80 meV and show that a similar anisotropy persists for superconducting compositions. The dispersive mode measured here connects directly with the spin resonance previously observed in this compound. When placed on an absolute scale, our measurements show that the local- or wave-vector-integrated susceptibility is larger in magnitude than that of the ordered parents over the energy range probed.

The microwave surface impedance of the proposed negative-$U$ induced superconductor Tl(1.4%)PbTe has been measured for a number of single crystals at several frequencies from 4.5 to 15.2 GHz in both the zero-field and mixed superconducting states. For crystals with the best-quality surfaces, $R_s$ and $X_s$ at low temperatures showed no significant power-law $T$ dependence. From the low-temperature values of $X_s$, we extracted $\lambda \left(0\right)=1.9\pm 0.2\mu \text{m}$, in good agreement with rigid-band model predictions. A coherence peak was observed in ${\sigma }_1$ below $T_c$ consistent with BCS predictions. However, the observed temperature dependence of the superfluid fraction ${\left[\lambda \left(0\right)/\lambda \left(T\right)\right]}^2$ fitted BCS predictions best if a somewhat larger value of $\lambda \left(0\right)=2.7\mu \text{m}$ was assumed, which may be associated with remanent surface defects in even our best sample. In addition to investigating the field dependence of the microwave properties in the mixed state, a $B_{c3}$ transition region was observed. The absence of a significant power-law temperature dependence at low temperatures in our best sample and the observation of a $B_{c3}$ transition are strongly suggestive of conventional $s$-wave superconductivity.

We introduce an efficient method to calculate the ground state of one-dimensional lattice models with periodic boundary conditions. The method works in the representation of matrix product states (MPS), related to the density matrix renormalization group (DMRG) method. It improves on a previous approach by Verstraete et al. We introduce a factorization procedure for long products of MPS matrices, which reduces the computational effort from $m^5$ to $m^3$, where $m$ is the matrix dimension, and $m\simeq 100–1000$ in typical cases. We test the method on the $S=\frac{1}{2}$ and $S=1$ Heisenberg chains. It is also applicable to nontranslationally invariant cases. The method makes ground-state calculations with periodic boundary conditions about as efficient as traditional DMRG calculations for systems with open boundaries.

We have investigated the effect of scattering on intrinsic anomalous Hall effect (AHE) for typical itinerant ferromagnets of Fe, Co, Ni, and their impurity-doped specimens in terms of the Hall Lorenz ratio $L_{xy}={\kappa }_{xy}/{\sigma }_{xy}T$, the ratio of thermal $\left({\kappa }_{xy}\right)$ to electrical $\left({\sigma }_{xy}\right)$ Hall conductivities divided by temperature $\left(T\right)$. We show that Lorenz ratio for the intrinsic anomalous Hall current (AHC) $\left(L_{xy}{}^{\text{A}}\right)$ is almost constant and coincides with $L_0=2.44\times {10}^{-8}\text{W}\Omega /{\text{K}}^2$ (Wiedemann-Franz law) in the clean region (e.g., ${\rho }_{xx}=1–3\times {10}^{-6}\Omega \text{cm}$), indicating the scattering-free nature of intrinsic AHE. On the other hand, the Lorenz ratio for the AHC begins to decrease from $L_{xy}=L_0$ with increasing ${\rho }_{xx}$ above $3–6\times {10}^{-6}\Omega \text{cm}$, which indicates the crossover from the scattering-free to scattering-dependent nature of AHE. The scattering rate $\left(\hslash /\tau \right)$ corresponding to the crossover resistivity is comparable with the gap magnitude formed at the band anticrossing point caused by the spin-orbit interaction.

We present time-dependent density-matrix renormalization-group (DMRG) results for strongly interacting one-dimensional fermionic systems at finite temperature. When interactions are strong the characteristic spin energy can be greatly suppressed relative to the characteristic charge energy, allowing for the possibility of spin-incoherent Luttinger liquid physics when the temperature is high compared to the spin energy, but small compared to the charge energy. Using DMRG we compute the spectral properties of the $t\text{−}J$ model at arbitrary temperatures with respect to both spin and charge energies. We study the full crossover from the Luttinger liquid regime to the spin-incoherent regime, focusing on small $J/t$, where the signatures of spin-incoherent behavior are more manifest. Our method allows us to access the analytically intractable regime where temperature is on the order of the spin energy, $T\sim J$. Our results should be helpful in the interpretation of experiments that may be in the crossover regime, $T\sim J$, and apply to one-dimensional cold atomic gases where finite-temperature effects are appreciable. The technique may also be used to guide the development of analytical approximations for the crossover regime.

In this work we demonstrate control of the spin-torque resonance frequency of a domain wall (DW) in a notched wire through the lithographic engineering of the shape and profile of the notch. By modeling the magnetization dynamics of a current-driven domain wall in a nanometric constrained geometry, an almost harmonic pinning potential has been designed, and we experimentally demonstrate the operation of domain wall resonators with well-defined, selectable resonance frequencies, suitable for application in zero magnetic field. These results show that the DW may be treated to a good approximation as a quasiparticle acted on by a restoring force that may be derived from the notch shape in a physically transparent manner.

We calculate ground-state total energies and single-particle excitation energies of seven pi-conjugated molecules described with the semiempirical Pariser-Parr-Pople model using self-consistent many-body perturbation theory at the GW level and exact diagonalization. For the total energies GW captures around 65% of the ground-state correlation energy. The lowest lying excitations are overscreened by GW leading to an underestimation of electron affinities and ionization potentials by 0.15 eV on average corresponding to $\sim 3$%. One-shot ${\text{G}}_0{\text{W}}_0$ calculations starting from Hartree-Fock reduce the screening and improve the low-lying excitation energies. The effect of the GW self-energy on the molecular excitation energies is shown to be similar to the inclusion of final-state relaxations in Hartree-Fock theory. We discuss the breakdown of the GW approximation in systems with short-range interactions (Hubbard models) where correlation effects dominate over screening/relaxation effects. Finally we illustrate the important role of the derivative discontinuity of the true exchange-correlation functional by computing the exact Kohn-Sham levels of benzene.

We calculate single-particle excitation energies for a series of 34 molecules using fully self-consistent GW, one-shot ${\text{G}}_0{\text{W}}_0$, Hartree-Fock (HF), and hybrid density-functional theory (DFT). All calculations are performed within the projector-augmented wave method using a basis set of Wannier functions augmented by numerical atomic orbitals. The GW self-energy is calculated on the real frequency axis including its full frequency dependence and off-diagonal matrix elements. The mean absolute error of the ionization potential (IP) with respect to experiment is found to be 4.4, 2.6, 0.8, 0.4, and 0.5 eV for DFT-PBE, DFT-PBE0, HF, ${\text{G}}_0{\text{W}}_0\left[\text{HF}\right]$, and self-consistent GW, respectively. This shows that although electronic screening is weak in molecular systems, its inclusion at the GW level reduces the error in the IP by up to 50% relative to unscreened HF. In general GW overscreens the HF energies leading to underestimation of the IPs. The best IPs are obtained from one-shot ${\text{G}}_0{\text{W}}_0$ calculations based on HF since this reduces the overscreening. Finally, we find that the inclusion of core-valence exchange is important and can affect the excitation energies by as much as 1 eV.

Vacancies are the dominant point defects in perovskite oxides, however, detecting and identifying the nature of vacancy defects in thin films remains challenging. This can be achieved using electron-beam methods but concentrations of several percent are required. Here we use a high-flux positron beam, providing high statistics positron lifetime measurements, to identify vacancies in laser ablated ${\text{SrTiO}}_3$ on ${\text{SrTiO}}_3$. The method is capable of subparts per million sensitivity and when combined with density-functional theory provides local structure information. The positron lifetime spectrum depth profile detects the presence of large vacancy clusters in a surface layer, a uniform distribution of Sr vacancies through the bulk of the film and resolves the interface with the substrate.

We report a complete study of spin-wave transduction combining microwave measurements performed over a frequency range of [1–15] GHz on Permalloy strips ($\text{thickness}=10–20\text{nm}$, $\text{width}=2–8\mu \text{m}$) and an accurate modeling of the experiment. This technique has been used recently to assess the spin polarization of the electrical current by measuring a current-induced spin-wave Doppler shift. We present here an electromagnetism calculation based on the magnetostatic wave theory combined with the Gilbert form of the damping that can be used directly as an optimization tool for future spin-wave experiments.

We have measured spin-transfer-induced dynamics in magnetic nanocontact devices having a perpendicularly magnetized Co/Ni free layer and an in-plane magnetized CoFe fixed layer. The frequencies and powers of the excitations agree well with the predictions of the single-domain model and indicate that the excited dynamics correspond to precessional orbits with angles ranging from zero to $90°$ as the applied current is increased at a fixed field. From measurements of the onset current as a function of applied field strength we estimate the magnitude of the spin torque asymmetry parameter $\Lambda \approx 1.5$. By combining these with spin torque ferromagnetic resonance measurements, we also estimate the spin-wave radiation loss in these devices.

We report a detailed study of the effects that crystalline disorder has on the magnetic relaxation and quantum tunneling of ${\text{Mn}}_{12}$ benzoate clusters. Thanks to the absence of interstitial molecules in the crystal structure of this molecular compound, we have been able to isolate the influence of long-range crystalline disorder. For this, we compare results obtained under two extreme situations: a crystalline sample and a nearly amorphous material. The results show that crystalline disorder affects little the anisotropy, magnetic relaxation, and quantum tunneling of these materials. It follows that disorder is not a necessary ingredient for the existence of magnetic quantum tunneling. The results unveil, however, a subtle influence of crystallinity via the modification of the symmetry of dipole-dipole interactions. The faster tunneling rates measured for the amorphous material are accounted for by a narrower distribution of dipolar bias in this material, as compared with the crystalline sample.

We report a combined study of the Nernst coefficient $\left(\nu \right)$, Hall effect, and thermoelectric power of ${\text{CaFe}}_2{\text{As}}_2$ and ${\text{CaFe}}_{1.92}{\text{Co}}_{0.08}{\text{As}}_2$ single crystals. The absolute value of $\nu$ in both samples is enhanced, probably due to ambipolar flow of electronlike and holelike quasiparticles. The onset of spin-density-wave (SDW) order in ${\text{CaFe}}_2{\text{As}}_2$ causes further rapid rise of the Nernst coefficient. On the contrary, in the ${\text{CaFe}}_{1.92}{\text{Co}}_{0.08}{\text{As}}_2$ crystal we do not see any feature of $\nu \left(T\right)$, which could be clearly associated with SDW fluctuations. In this Co-doped sample there is also no noticeable increase in of $\nu$ in the vicinity of superconducting transition despite the expectation of such due to vortex movement.