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Anisotropic hopping in a toy Hofstadter model was recently invoked to explain a rich and surprising Landau spectrum measured in twisted bilayer graphene away from the magic angle. Suspecting that such anisotropy could arise from unintended uniaxial strain, we extend the Bistritzer-MacDonald model to include uniaxial heterostrain. We find that such strain strongly influences band structure, shifting the three otherwise-degenerate van Hove points to different energies. Coupled to a Boltzmann magnetotransport calculation, this reproduces previously-unexplained non-saturating $B^2$ magnetoresistance over broad ranges of density near filling $\nu=\pm 2$, and predicts subtler features that had not been noticed in the experimental data. In contrast to these distinctive signatures in longitudinal resistivity, the Hall coefficient is barely influenced by strain, to the extent that it still shows a single sign change on each side of the charge neutrality point -- surprisingly, this sign change no longer occurs at a van Hove point. The theory also predicts a marked rotation of the electrical transport principal axes as a function of filling even for fixed strain and for rigid bands. More careful examination of interaction-induced nematic order versus strain effects in twisted bilayer graphene could thus be in order., Comment: 11 pages main text (4 figures) + 8 pages supplementary material (11 figures)

Ideally, quantum anomalous Hall systems should display zero longitudinal resistance. Yet in experimental quantum anomalous Hall systems elevated temperature can make the longitudinal resistance finite, indicating dissipative flow of electrons. Here, we show that the measured potentials at multiple locations within a device at elevated temperature are well-described by solution of Laplace's equation, assuming spatially-uniform conductivity, suggesting non-equilibrium current flows through the two-dimensional bulk. Extrapolation suggests that at even lower temperatures current may still flow primarily through the bulk rather than, as had been assumed, through edge modes. An argument for bulk current flow previously applied to quantum Hall systems supports this picture., Comment: 6 pages, 4 figures, plus supplemental materials

A lateral Josephson junction in which the surface of a 3D topological insulator serves as the weak link should support topologically protected excitations related to Majorana fermions. The resulting $4\pi$-periodic current-phase relationship could be detected under high-frequency excitation by the suppression of odd Shapiro steps. Here, we demonstrate such devices through the self-formation of a Pd-Te superconducting layer from a telluride topological insulator, and observe suppressed first and third Shapiro steps. Other devices, including those where the Pd-Te layer is bolstered by an additional Al layer, show no suppression of Shapiro steps, a difference supported by simulations. Though we rule out the known trivial causes of suppressed Shapiro steps in our devices, we nevertheless argue that corroborating measurements and disorder-aware theoretical descriptions of these systems are needed before confidently claiming the observation of Majorana states., Comment: 10 pages, 5 figures, plus Supplemental Material

The ferromagnetic phase of Co$_3$Sn$_2$S$_2$ is widely considered to be a topological Weyl semimetal, with evidence for momentum-space monopoles of Berry curvature from transport and spectroscopic probes. As the bandstructure is highly sensitive to the magnetic order, attention has focused on anomalies in magnetization, susceptibility and transport measurements that are seen well below the Curie temperature, leading to speculation that a "hidden" phase coexists with ferromagnetism. Here we report spatially-resolved measurements by Kerr effect microscopy that identify this phase. We find that the anomalies coincide with a deep minimum in domain wall (DW) mobility, indicating a crossover between two regimes of DW propagation. We demonstrate that this crossover is a manifestation of a 2D phase transition that occurs within the DW, in which the magnetization texture changes from continuous rotation to unidirectional variation. We propose that the existence of this 2D transition deep within the ferromagnetic state of the bulk is a consequence of a giant quality factor for magnetocrystalline anisotropy unique to this compound. This work broadens the horizon of the conventional binary classification of DWs into Bloch and N\'eel walls, and suggests new strategies for manipulation of domain walls and their role in electron and spin transport., Comment: 17+6 pages, revised version, to appear in Nature Communications

The flat bands resulting from moir\'e superlattices in magic-angle twisted bilayer graphene (MATBG) and ABC-trilayer graphene aligned with hexagonal boron nitride (ABC-TLG/hBN) have been shown to give rise to fascinating correlated electron phenomena such as correlated insulators and superconductivity. More recently, orbital magnetism associated with correlated Chern insulators was found in this class of layered structures centered at integer multiples of n0, the density corresponding to one electron per moir\'e superlattice unit cell. Here we report the experimental observation of ferromagnetism at fractional filling of a flat Chern band in an ABC-TLG/hBN moir\'esuperlattice. The ferromagnetic state exhibits prominent ferromagnetic hysteresis behavior with large anomalous Hall resistivity in a broad region of densities, centered in the valence miniband at n = -2.3 n0. This ferromagnetism depends very sensitively on the control parameters in the moir\'e system: not only the magnitude of the anomalous Hall signal, but also the sign of the hysteretic ferromagnetic response can be modulated by tuning the carrier density and displacement field. Our discovery of electrically tunable ferromagnetism in a moir\'e Chern band at non-integer filling highlights the opportunities for exploring new correlated ferromagnetic states in moir\'e heterostructures., Comment: 12 pages, 4 figures

We present low-temperature transport measurements of a gate-tunable thin film topological insulator system that features high mobility and low carrier density. Upon gate tuning to a regime around the charge neutrality point, we infer an absence of strong localization even at conductivities well below $e^2/h$, where two dimensional electron systems should conventionally scale to an insulating state. Oddly, in this regime the localization coherence peak lacks conventional temperature broadening, though its tails do change dramatically with temperature. Using a model with electron-impurity scattering, we extract values for the disorder potential and the hybridization of the top and bottom surface states., Comment: 6 pages, 4 figures, with 11 pages of supplementary information

When two sheets of graphene are stacked at a small twist angle, the resulting flat superlattice minibands are expected to strongly enhance electron-electron interactions. Here we present evidence that near three-quarters ($3/4$) filling of the conduction miniband these enhanced interactions drive the twisted bilayer graphene into a ferromagnetic state. We observe emergent ferromagnetic hysteresis, with a giant anomalous Hall (AH) effect as large as $10.4\ \mathrm{k\Omega}$ and signs of chiral edge states in a narrow density range around an apparent insulating state at $3/4$. Surprisingly, the magnetization of the sample can be reversed by applying a small DC current. Although the AH resistance is not quantized and dissipation is significant, we suggest that the system is an incipient Chern insulator., Comment: 18 pages, 4 figures

We study the non-equilibrium dynamics of correlations in quantum lattice models in the presence of long-range interactions decaying asymptotically as a power law. For exponents larger than the lattice dimensionality, a Lieb-Robinson-type bound effectively restricts the spreading of correlations to a causal region, but allows supersonic propagation. We show that this decay is not only sufficient but also necessary. Using tools of quantum metrology, for any exponents smaller than the lattice dimension, we construct Hamiltonians giving rise to quantum channels with capacities not restricted to a causal region. An analytical analysis of long-range Ising models illustrates the disappearance of the causal region and the creation of correlations becoming distance-independent. Numerical results obtained using matrix product state methods for the XXZ spin chain reveal the presence of a sound cone for large exponents, and supersonic propagation for small ones. In all models we analyzed the fast spreading of correlations follows a power law, but not the exponential increase of the long-range Lieb-Robinson bound., Comment: 7 pages, 6 figures; Fixed another typo in equations (3) and (4) and added a footnote

The integer and fractional quantum Hall states are known to break down at high dc bias, exhibiting deviation from the ideal incompressible behavior. We measure breakdown of the \nu = 2, 3, 4, 5 integer and the \nu = 4/3 and 5/3 fractional states in a quantum point contact (QPC) of lithographic width ~600 nm. Dependence of the critical current on magnetic field, QPC gate voltage, and QPC width are presented. Of particular interest, the critical current of the 4/3 and 5/3 fractional states shows the opposite dependence on QPC width compared to the integer states. This previously unobserved result is not explained by current theories of breakdown.

Some models of the 5/2 fractional quantum Hall state predict that the quasi-particles, which carry the charge, have non-Abelian statistics: exchange of two quasi-particles changes the wave function more dramatically than just the usual change of phase factor. Such non-Abelian statistics would make the system less sensitive to decoherence, making it a candidate for implementation of topological quantum computation. We measure quasi-particle tunneling as a function of temperature and DC bias between counter-propagating edge states. Fits to theory give e*, the quasi-particle effective charge, close to the expected value of e/4 and g, the strength of the interaction between quasi-particles, close to 3/8. Fits corresponding to the various proposed wave functions, along with qualitative features of the data, strongly favor the Abelian 331 state.

We present a contactless method for measuring charge in a thin film of amorphous germanium (a-Ge) with a nanoscale silicon MOSFET charge sensor. This method enables the measurement of conductance of the a-Ge film even in the presence of blocking contacts. At high bias voltage, the resistance of the contacts becomes negligible and a direct measurement of current gives a conductance that agrees with that from the measurement of charge. This charge-sensing technique is used to measure the temperature- and field-dependence of the conductance, and they both agree with a model of Mott variable-range hopping. From the model, we obtain a density of states at the Fermi energy of 1.6 x 10^18 eV^-1 cm^-3 and a localization length of 1.06 nm. This technique enables the measurement of conductance as low as 10^-19 S., Comment: 8 pages, 4 figures

We measure charge transport in hydrogenated amorphous silicon (a-Si:H) using a nanometer scale silicon MOSFET as a charge sensor. This charge detection technique makes possible the measurement of extremely large resistances. At high temperatures, where the a-Si:H resistance is not too large, the charge detection measurement agrees with a direct measurement of current. The device geometry allows us to probe both the field effect and dispersive transport in the a-Si:H using charge sensing and to extract the density of states near the Fermi energy., Comment: 4 pages, 4 figures

The fidelity metric has recently been proposed as a useful and elegant approach to identify and characterize both quantum and classical phase transitions. We study this metric on the manifold of thermal states for the Lipkin-Meshkov-Glick (LMG) model. For the isotropic LMG model, we find that the metric reduces to a Fisher-Rao metric, reflecting an underlying classical probability distribution. Furthermore, this metric can be expressed in terms of derivatives of the free energy, indicating a relation to Ruppeiner geometry. This allows us to obtain exact expressions for the (suitably rescaled) metric in the thermodynamic limit. The phase transition of the isotropic LMG model is signalled by a degeneracy of this (improper) metric in the paramagnetic phase. Due to the integrability of the isotropic LMG model, ground state level crossings occur, leading to an ill-defined fidelity metric at zero temperature., Comment: 18 pages, 3 figures

Theory predicts that quasiparticle tunneling between the counter-propagating edges in a fractional quantum Hall state can be used to measure the effective quasiparticle charge e* and dimensionless interaction parameter g, and thereby characterize the many-body wavefunction describing the state. We report measurements of quasiparticle tunneling in a high mobility GaAs two dimensional electron system in the fractional quantum Hall state at nu=5/2 using a gate-defined constriction to bring the edges close together. We find the dc-bias peaks in the tunneling conductance at different temperatures collapse onto a single curve when scaled, in agreement with weak tunneling theory. Various models for the \nu=5/2 state predict different values for g. Among these models, the non-abelian states with e*=1/4 and g=1/2 are most consistent with the data., Comment: supporting material: http://marcuslab.harvard.edu/Radu_SOMs.pdf

Using real-time charge sensing and gate pulsing techniques we measure the ratio of the rates for tunneling into the excited and ground spin states of a single-electron AlGaAs/GaAs quantum dot in a parallel magnetic field. We find that the ratio decreases with increasing magnetic field until tunneling into the excited spin state is completely suppressed. However, we find that by adjusting the voltages on the surface gates to change the orbital configuration of the dot we can restore tunneling into the excited spin state and that the ratio reaches a maximum when the dot is symmetric., Comment: 4 pages, 3 figures

We report electrical transport measurements of arrays of PbSe nanocrystals forming the channels of field effect transistors. We measure the current in these devices as a function of source-drain voltage, gate voltage and temperature. Annealing is necessary to observe measurable current after which a simple model of hopping between intrinsic localized states describes the transport properties of the nanocrystal solid. We find that the majority carriers are holes, which are thermally released from acceptor states. At low source-drain voltages, the activation energy for the conductivity is given by the energy required to generate holes plus the activation over barriers resulting from site disorder. At high source-drain voltages the activation energy is given by the former only. The thermal activation energy of the zero-bias conductance indicates that the Fermi energy is close to the highest-occupied valence level, the 1Sh state, and this is confirmed by field-effect measurements, which give a density of states of approximately eight per nanocrystal as expected from the degeneracy of the 1Sh state., Comment: 8 pages,10 figures

We demonstrate electrical control of the spin relaxation time T_1 between Zeeman split spin states of a single electron in a lateral quantum dot. We find that relaxation is mediated by the spin-orbit interaction, and by manipulating the orbital states of the dot using gate voltages we vary the relaxation rate W= (T_1)^-1 by over an order of magnitude. The dependence of W on orbital confinement agrees with theoretical predictions and from these data we extract the spin-orbit length. We also measure the dependence of W on magnetic field and demonstrate that spin-orbit mediated coupling to phonons is the dominant relaxation mechanism down to 1T, where T_1 exceeds 1s., Comment: 4 pages, 3 figures

Recent theories suggest that the excitations of certain quantum Hall states may have exotic braiding statistics which could be used to build topological quantum gates. This has prompted an experimental push to study such states using confined geometries where the statistics can be tested. We study the transport properties of quantum point contacts (QPCs) fabricated on a GaAs/AlGaAs two dimensional electron gas that exhibits well-developed fractional quantum Hall effect, including at bulk filling fraction 5/2. We find that a plateau at effective QPC filling factor 5/2 is identifiable in point contacts with lithographic widths of 1.2 microns and 0.8 microns, but not 0.5 microns. We study the temperature and dc-current-bias dependence of the 5/2 plateau in the QPC, as well as neighboring fractional and integer plateaus in the QPC while keeping the bulk at filling factor 3. Transport near QPC filling factor 5/2 is consistent with a picture of chiral Luttinger liquid edge-states with inter-edge tunneling, suggesting that an incompressible state at 5/2 forms in this confined geometry.

We present measurements of the rates for an electron to tunnel on and off a quantum dot, obtained using a quantum point contact charge sensor. The tunnel rates show exponential dependence on drain-source bias and plunger gate voltages. The tunneling process is shown to be elastic, and a model describing tunneling in terms of the dot energy relative to the height of the tunnel barrier quantitatively describes the measurements., Comment: 4 pages, 4 figures

We measure the relaxation rate $W \equiv T_{1}^{-1}$ of a single electron spin in a quantum dot at magnetic fields from 7 T down to 1.75 T, much lower than previously measured. At 1.75 T we find that $T_{1}$ is 170 ms. We find good agreement between our measurements and theoretical predictions of the relaxation rate caused by the spin-orbit interaction, demonstrating that spin-orbit coupling can account for spin relaxation in quantum dots., Comment: 4 pages, 3 figures

An artificial atom with four electrons is driven through a singlet-triplet transition by varying the confining potential. In the triplet, a Kondo peak with a narrow dip at drain-source voltage V_ds=0 is observed. The low energy scale V_ds* characterizing the dip is consistent with predictions for the two-stage Kondo effect. The phenomenon is studied as a function of temperature T and magnetic field B, parallel to the two-dimensional electron gas. The low energy scales T* and B* are extracted from the behavior of the zero-bias conductance and are compared to the low energy scale V_ds* obtained from the differential conductance. Good agreement is found between kT* and |g|muB*, but eV_ds* is larger, perhaps because of nonequilibrium effects., Comment: 7 pages, 7 figures. Added labels on Fig. 3f and one reference

A Kondo peak in the differential conductance of a single-electron transistor is measured as a function of both magnetic field and the Kondo temperature. We observe that the Kondo splitting decreases logarithmically with Kondo temperature and that there exists a critical magnetic field Bc below which the Kondo peak does not split, in qualitative agreement with theory. However, we find that the magnitude of the prefactor of the logarithm is larger than predicted and is independent of B, in contradiction with theory. Our measurements also suggest that the value of Bc is smaller than predicted., Comment: 6 pages, 5 figures. Replaced with version submitted to PRB that includes corrected theoretical predictions, additonal analysis, and other suggested modifications

We report the observation of a magnetic-field-induced transition between magnetically disordered and ordered phases in slightly under-doped La(2-x)SrxCuO4 with x=0.144. Static incommensurate spin-density-wave order is induced above a critical field of about 3 T, as measured by elastic neutron scattering. Our results allow us to constrain the location of a quantum critical point on the phase diagram., Comment: 10 pages, 2 figures; discussion on the location of a quantum critical point is revised

We measure the spin splitting in a magnetic field $B$ of localized states in single-electron transistors using a new method, inelastic spin-flip cotunneling. Because it involves only internal excitations, this technique gives the most precise value of the Zeeman energy $\Delta = \ZeemanE$. In the same devices we also measure the splitting with $B$ of the Kondo peak in differential conductance. The Kondo splitting appears only above a threshold field as predicted by theory. However, the magnitude of the Kondo splitting at high fields exceeds $2 \ZeemanE$ in disagreement with theory., Comment: Replaced with version published in Phys. Rev. Lett

We have measured the specific heat and magnetization {\it versus} temperature in a single crystal sample of superconducting La$_{2}$CuO$_{4.11}$ and in a sample of the same material after removing the excess oxygen, in magnetic fields up to 15 T. Using the deoxygenated sample to subtract the phonon contribution, we find a broad peak in the specific heat, centered at 50 K. This excess specific heat is attributed to fluctuations of the Cu spins possibly enhanced by an interplay with the charge degrees of freedom, and appears to be independent of magnetic field, up to 15 T. Near the superconducting transition $T_{c}$($H$=0)= 43 K, we find a sharp feature that is strongly suppressed when the magnetic field is applied parallel to the crystallographic c-axis. A model for 3D vortex fluctuations is used to scale magnetization measured at several magnetic fields. When the magnetic field is applied perpendicular to the c-axis, the only observed effect is a slight shift in the superconducting transition temperature., Comment: 8 pages, 8 figures

We report neutron scattering measurements of the structure and magnetism of stage-4 La_2CuO_{4+y} with T_c ~42 K. Our diffraction results on a single crystal sample demonstrate that the excess oxygen dopants form a three-dimensional ordered superlattice within the interstitial regions of the crystal. The oxygen superlattice becomes disordered above T ~ 330 K, and a fast rate of cooling can freeze-in the disordered-oxygen state. Hence, by controlling the cooling rate, the degree of dopant disorder in our La_2CuO_{4+y} crystal can be varied. We find that a higher degree of quenched disorder reduces T_c by ~ 5 K relative to the ordered-oxygen state. At the same time, the quenched disorder enhances the spin density wave order in a manner analogous to the effects of an applied magnetic field., Comment: 4 figures included in text; submitted to PRB

A novel model of transport is proposed to explain power law current transients and memory phenomena observed in partially ordered arrays of semiconducting nanocrystals. The model describes electron transport by a stationary Levy process of transmission events and thereby requires no time dependence of system properties. The waiting time distribution with a characteristic long tail gives rise to a nonstationary response in the presence of a voltage pulse. We report on noise measurements that agree well with the predicted non-Poissonian fluctuations in current, and discuss possible mechanisms leading to this behavior., Comment: 7 pages, 2 figures

A novel method to image charge is used to measure the diffusion coefficient of electrons in films of CdSe nanocrystals at room temperature. This method makes possible the study of charge transport in films exhibiting high resistances or very small diffusion coefficients., Comment: 4 pages, 4 jpg figures

We have measured the enhancement of the static incommensurate spin-density wave (SDW) order by an applied magnetic field in stage-4 and stage-6 samples of superconducting La2CuO(4+y). We show that the stage-6 La2CuO(4+y) (Tc=32 K) forms static long-range SDW order with the same wave-vector as that in the previously studied stage-4 material. We have measured the field dependence of the SDW magnetic Bragg peaks in both stage-4 and stage-6 materials at fields up to 14.5 T. A recent model of competing SDW order and superconductivity describes these data well., Comment: Published version. 6 pages. 1 figure added

We report sharp peaks in the differential conductance of a single-electron transistor (SET) at low temperature, for gate voltages at which charge fluctuations are suppressed. For odd numbers of electrons we observe the expected Kondo peak at zero bias. For even numbers of electrons we generally observe Kondo-like features corresponding to excited states. For the latter, the excitation energy often decreases with gate voltage until a new zero-bias Kondo peak results. We ascribe this behavior to a singlet-triplet transition in zero magnetic field driven by the change of shape of the potential that confines the electrons in the SET., Comment: 4 p., 4 fig., 5 new ref. Rewrote 1st paragr. on p. 4. Revised author list. More detailed fit results on page 3. A plotting error in the horizontal axis of Fig. 1b and 3 was corrected, and so were the numbers in the text read from those fig. Fig. 4 was modified with a better temperature calibration (changes are a few percent). The inset of this fig. was removed as it is unnecessary here. Added remarks in the conclusion. Typos are corrected

Transport properties of artificial solids composed of colloidal CdSe nanocrystals (NCs) are studied from 6 K to 250 K, before and after annealing. Annealing results in greatly enhanced dark and photocurrent in NC solids, while transmission electron microscopy (TEM) micrographs show that the inter-dot separation decreases. The increased current can be attributed to the enhancement of inter-dot tunneling caused by the decreased separation between NCs and by chemical changes in their organic cap. In addition, the absorption spectra of annealed solids are slightly red-shifted and broadened. These red-shifts may result from the change of the dielectric environment around the NCs. Our measurements also indicate that Coulomb interactions between charges on neighboring NCs play an important role in the tunneling current., Comment: 24 pages,4 figures, 1 table

This paper reports muon spin relaxation (MuSR) measurements of two single crystals of the title high-Tc cuprate systems where static incommensurate magnetism and superconductivity coexist. By zero-field MuSR measurements and subsequent analyses with simulations, we show that (1) the maximum ordered Cu moment size (0.36 Bohr magneton) and local spin structure are identical to those in prototypical stripe spin systems with the 1/8 hole concentration; (2) the static magnetism is confined to less than a half of the volume of the sample, and (3) regions with static magnetism form nano-scale islands with the size comparable to the in-plane superconducting coherence length. By transverse-field MuSR measurements, we show that Tc of these systems is related to the superfluid density, in the same way as observed in cuprate systems without static magnetism. We discuss a heuristic model involving percolation of these nanoscale islands with static magnetism as a possible picture to reconcile heterogeneity found by the present MuSR study and long-range spin correlations found by neutron scattering., Comment: 19 pages, 15 figures, submitted to Phys. Rev. B. E-mail: tomo@lorentz.phys.columbia.edu

We report a detailed study, using neutron scattering, transport and magnetization measurements, of the interplay between superconducting (SC) and spin density wave (SDW) order in La2CuO(4+y). Both kinds of order set in below the same critical temperature. However, the SDW order grows with applied magnetic field, whereas SC order is suppressed. Most importantly, the field dependence of the SDW Bragg peak intensity has a cusp at zero field, as predicted by a recent theory of competing SDW and SC order. This leads us to conclude that there is a repulsive coupling between the two order parameters. The question of whether the two kinds of order coexist or microscopically phase separate is discussed., Comment: Version accepted for publication in Phys. Rev. B. Improved discussion in connection with the muSR results

We report a neutron scattering study of the long-wavelength dynamic spin correlations in the model two-dimensional $S=1/2$ square lattice Heisenberg antiferromagnets Sr$_2$CuO$_2$Cl$_2$ and Sr$_2$Cu$_3$O$_4$Cl$_2$. The characteristic energy scale, $\omega_0 (T/J)$, is determined by measuring the quasielastic peak width in the paramagnetic phase over a wide range of temperature ($0.2 \alt T/J \alt 0.7$). The obtained values for $\omega_0 (T/J)$ agree {\it quantitatively} between the two compounds and also with values deduced from quantum Monte Carlo simulations. The combined data show scaling behavior, $\omega \sim \xi^{-z}$, over the entire temperature range with $z=1.0(1)$, in agreement with dynamic scaling theory., Comment: 4 pages, 3 figures, to be published in Phys. Rev. Lett

A recent paper by A. R. Lima, P. M. C. de Oliveira, and T. J. P. Penna [cond-mat/0002176; J. Stat. Phys. 99:691 (2000)], seems to contain at least two mistakes which deserve comment, one concerning the numerical data, the other being of a conceptual kind., Comment: submitted to J. Stat. Phys

We report a neutron scattering study on the tetragonal compound Sr_2Cu_3O_4Cl_2, which has two-dimensional (2D) interpenetrating Cu_I and Cu_{II} subsystems, each forming a S=1/2 square lattice quantum Heisenberg antiferromagnet (SLQHA). The mean-field ground state is degenerate, since the inter-subsystem interactions are geometrically frustrated. Magnetic neutron scattering experiments show that quantum fluctuations lift the degeneracy and cause a 2D Ising ordering of the Cu_{II} subsystem. Due to quantum fluctuations a dramatic increase of the Cu_I out-of-plane spin-wave gap is also observed. The temperature dependence and the dispersion of the spin-wave energy are quantitatively explained by spin-wave calculations which include quantum fluctuations explicitly. The values for the nearest-neighbor superexchange interactions between the Cu_I and Cu_{II} ions and between the Cu_{II} ions are determined experimentally to be J_{I-II} = -10(2)meV and J_{II}= 10.5(5)meV, respectively. Due to its small exchange interaction, J_{II}, the 2D dispersion of the Cu_{II} SLQHA can be measured over the whole Brillouin zone with thermal neutrons, and a novel dispersion at the zone boundary, predicted by theory, is confirmed. The instantaneous magnetic correlation length of the Cu_{II} SLQHA is obtained up to a very high temperature, T/J_{II}\approx 0.75. This result is compared with several theoretical predictions as well as recent experiments on the S=1/2 SLQHA., Comment: Figures and equations are rearranged

We report single crystal growth and magnetic susceptibility and neutron diffraction studies of the S=1/2 quasi-1D antiferromagnet CaCu2O3. The structure of this material is similar to that of the prototype two-leg spin-ladder compound SrCu2O3. However, the Cu-O-Cu bond angle in the ladder rungs in CaCu2O3 is equal to 123 deg, and therefore the magnetic interaction along the rungs is expected to be much weaker in this material. At high temperatures, the magnetic susceptibility of CaCu2O3 can be decomposed into a contribution from 1D antiferromagnetic chains of finite-size chain segments together with a weak Curie contribution. The intrachain magnetic exchange constant, determined from the magnetic susceptibility measurements, is 2000 K. CaCu2O3 undergoes a Neel transition at T_N=25 K with ordering wavevector of (0.429(5), 0.5, 0.5). The magnetic structure is incommensurate in the direction of the frustrated interchain interaction. Weak commensurate (0.5, 0.5, 0.5) magnetic peaks are also observed below T_N. Application of a magnetic field induces a metamagnetic transition at which the incommensurability of the magnetic structure is substantially reduced. The material possesses only short-range magnetic order above the transition field., Comment: 12 pages, 10 embedded figures

We report the temperature dependence of the zero-bias conductance of a single-electron transistor in the regime of weak coupling between the quantum dot and the leads. The Fano line shape, convoluted with thermal broadening, provides a good fit to the observed asymmetric Coulomb charging peaks. However, the width of the peaks increases more rapidly than expected from the thermal broadening of the Fermi distribution in a temperature range for which Fano interference is unaffected. The intrinsic width of the resonance extracted from the fits increases approximately quadratically with temperature. Above about 600 mK the asymmetry of the peaks decreases, suggesting that phase coherence necessary for Fano interference is reduced with increasing temperature., Comment: 6 pages, 4 figures. New references have been added to support the analysis

Typically, in order to obtain finite-size scaling laws for quantities in the microcanonical ensemble, an assumption is taken as a starting point. In this paper, consistency of such a Microcanonical Finite-Size Scaling Assumption with its commonly accepted canonical counterpart is shown, which puts Microcanonical Finite-Size Scaling on a firmer footing., Comment: 10 pages. Submitted to J. Stat. Phys

Using an electron spin resonance spectrometer covering a wide range of frequency and magnetic field, we have measured the low energy excitations of the S=1/2 tetragonal antiferromagnets, Sr_{2}CuO_{2}Cl_{2} and Sr_{2}Cu_{3}O_{4}Cl_{2}. Our observation of in-plane energy gaps of order 0.1 meV at zero external magnetic field are consistent with a spin wave calculation, which includes several kinds of quantum fluctuations that remove frustration. Results agree with other experiments and with exchange anisotropy parameters determined from a five band Hubbard model., Comment: 4 pages, 3 figures

Neutron scattering experiments reveal that a diagonal spin modulation, which is a one-dimensional modulation rotated away by 45$^\circ$ from that in the superconducting phase, occurs universally across the insulating spin-glass phase in La$_{2-x}$Sr$_x$CuO$_4$ (0.02$\le x\le$0.055). This establishes an intimate relation between the magnetism and the transport properties in the high-temperature copper oxide superconductors. Furthermore, it is found that the charge density per unit length estimated using a charge stripe model is almost constant throughout the phase diagram, even when the modulation rotates away by 45$^\circ$ at the superconducting boundary. However, at the lowest values for $x$ the density changes approaching 1 hole/Cu as in La$_{2-x}$Sr$_x$NiO$_4$. Magnetic excitation spectra suggest that magnetic correlations change from incommensurate to commensurate at $\omega\sim$7 meV and $T\sim$70 K, indicating a characteristic energy for the incommensurate structure of 6-7 meV., Comment: 7 pages, submitted to Phys. Rev. B

The energy spectrum of incommensurate spin fluctuations in superconducting Sr-doped La2CuO4 has been studied by inelastic neutron scattering experiments. An energy gap in the spin excitation spectrum is observed in the superconducting state of optimally doped (x=0.15) and slightly overdoped (x=0.18) samples. At temperatures well below Tc, the incommensurate peaks diminish rapidly in intensity with decreasing energy below ~8meV and merge into the background below ~3.5meV for x=0.15 and ~4.5meV for x=0.18. For both samples, the energy spectrum of the q-integrated spin susceptibility, x"(w), exhibits an enhancement around 7meV, which is caused by a broadening in the momentum width of the incommensurate peaks. The gap-like structure in the energy spectrum and the enhancement of x"(w) survive at Tc. On the other hand, for both underdoped (x=0.10) and heavily overdoped (x=0.25) samples there is neither a clear energy gap nor an enhancement of x"(w) below Tc., Comment: 15 pages, 8 figures, revtex