arXiv subjects

Per Hedegard

Publications and source records attributed to Per Hedegard.

14 recordsLinked to original sources

arXiv2009

Electrical manipulation of spin states in a single electrostatically gated transition-metal complex

We demonstrate an electrically controlled high-spin (S=5/2) to low-spin (S=1/2) transition in a three-terminal device incorporating a single Mn2+ ion coordinated by two terpyridine ligands. By adjusting the gate-voltage we reduce the terpyridine moiety and thereby strengthen the ligand-field on the Mn-atom. Adding a single electron thus stabilizes the low-spin configuration and the corresponding sequential tunnelling current is suppressed by spin-blockade. From low-temperature inelastic cotunneling spectroscopy, we infer the magnetic excitation spectrum of the molecule and uncover also a strongly gate-dependent singlet-triplet splitting on the low-spin side. The measured bias-spectroscopy is shown to be consistent with an exact diagonalization of the Mn-complex, and an interpretation of the data is given in terms of a simplified effective model.

cond-mat.mes-hall↗

arXiv2009

Magnetic field-induced soft mode in spin-gapped high-Tc superconductors

We present an explanation of the dynamical in-gap spin mode in LSCO induced by an applied magnetic field H as recently observed by J. Chang et al. Our model consists of a phenomenological spin-only Hamiltonian, and the softening of the spin mode is caused by vortex pinning of dynamical stripe fluctuations which we model by a local ordering of the exchange interactions. The spin gap vanishes experimentally around H=7T which in our scenario corresponds to the field required for overlapping vortex regions.

cond-mat.supr-con↗

arXiv2006

Recursion method for the quasiparticle structure of a single vortex with induced magnetic order

We use a real-space recursion method to calculate the local density of states (LDOS) within a model that contains both d-wave superconducting and antiferromagnetic order. We focus on the LDOS in the superconducting phase near single vortices with either normal or antiferromagnetic cores. Furthermore, we study the low-energy quasiparticle structure when magnetic vortices operate as pinning centers for surrounding unidirectional spin density waves (stripes). We calculate the Fourier transformed LDOS and show how the energy dependence of relevant Fourier components can be used to determine the nature of the magnetic field-induced order, and predict field-induced LDOS features that can be tested by future scanning tunneling microscopy (STM) experiments.

cond-mat.supr-con↗

arXiv2005

Charge transport through image charged stabilized states in a single molecule single electron transistor device

The present paper gives an elaborate theoretical description of a new molecular charge transport mechanism applying to a single molecule trapped between two macroscopic electrodes in a solid state device. It is shown by a Hubbard type model of the electronic and electrostatic interactions, that the close proximity of metal electrodes may allow electrons to tunnel from the electrode directly into a very localized image charge stabilized states on the molecule. Due to this mechanism, an exceptionally large number of redox states may be visited within an energy scale which would normally not allow the molecular HOMO-LUMO gap to be transversed. With a reasonable set of parameters, a good fit to recent experimental values may be obtained. The theoretical model is furthermore used to search for the physical boundaries of this effect, and it is found that a rather narrow geometrical space is available for the new mechanism to be effective: In the specific case of oligophenylenevinylene molecules recently explored in such devices several atoms in the terminal benzene rings need to be at van der Waal's distance to the electrode in order for the mechanism to be effective. The model predicts, that chemisorption of the terminal benzene rings too gold electrodes will impede the image charge effect very significantly because the molecule is pushed away from the electrode by the covalent thiol-gold bond.

cond-mat.mes-hall↗

arXiv2004

Spin dynamics in the stripe phase of the cuprates

Within a model that supports stripe spin and charge order coexisting with a d$_{x^2-y^2}$-wave superconducting phase, we study the self-consistently obtained electronic structure and the associated transverse dynamical spin susceptibility. In the coexisting phase of superconducting and static stripe order, the resulting particle-hole continuum can strongly damp parts of the low-energy spin wave branches. This provides insight into recent inelastic neutron scattering data revealing the dispersion of the low-energy collective magnetic modes of lanthanum based cuprate superconductors.

cond-mat.supr-con↗

arXiv2003

Quantum interference between multiple impurities in anisotropic superconductors

We study the quantum interference between impurities in d-wave superconductors within a potential scattering formalism that easily applies to multiple impurities. The evolution of the low-energy local density of states for both magnetic and nonmagnetic short-ranged scatterers are studied as a function of the spatial configuration of the impurities. Further we discuss the influence of subdominant bulk superconducting order parameters on the interference pattern from multiple impurities.

cond-mat.supr-con↗

arXiv2002

Checkerboard local density of states in striped domains pinned by vortices

Within a Green's function formalism we calculate the electronic structure around static extended magnetic and non-magnetic perturbations in a d-wave superconductor. In partucular, we discuss recent elastic neutron scattering and scanning tunneling experiments on High-T_c cuprates exposed to an applied magnetic field. A physical picture consisting of antiferromagnetic vortex cores operating as pinning centers for surrounding stripes is qualitatively consistent with the neutron data provided the stripes have the usual antiphase modulation. The low energy electronic structure in such a region reveals a checkerboard interference pattern consistent with recent scanning tunneling experiments.

cond-mat.supr-con↗

arXiv2002

Electronic checkerboard pattern in striped racetrack domains: a consistent picture of recent neutron and STM experiments

We discuss recent elastic neutron scattering and scanning tunneling experiments on high-T_c cuprates exposed to an applied magnetic field. In particular we show that a physical picture consisting of antiferromagnetic vortex cores operating as pinning centers for surrounding stripes is qualitatively consistent with the neutron data provided the stripes have the usual antiphase modulation. Further, we calculate the electronic structure in such a region using a T-matrix method, and find a checkerboard interference pattern consistent with recent scanning tunneling experiments.

cond-mat.supr-con↗

arXiv2001

Magnetic Vortices in High Temperature Superconductors

It is suggested that modes, observed in recent neutron scattering experiments by Lake {\it et al.}, on La$_{2-x}$Sr$_x$CuO$_4$ in strong magnetic fields ($\approx$ 7 T), are due to the existence of antiferromagnetic moments associated with the cores of vortices generated by the field. These moments form one-dimensional chains along the $c$-axis (the vortex axis), which at finite temperatures are disordered. At temperatures higher than 10 K the correlation length gets shorter than the lattice parameter, resulting in no scattering from coherent spin-waves above that temperature. The bandwidth of the spin-waves is estimated to be $\approx$ 4 meV in accordance with the observations.

cond-mat.supr-con↗

arXiv2000

The magnetic neutron scattering resonance of high-T_c superconductors in external magnetic fields: an SO(5) study

The magnetic resonance at 41 meV observed in neutron scattering studies of Y Ba_2 Cu_3 O_7 holds a key position in the understanding of high-T_c superconductivity. Within the SO(5) model for superconductivity and antiferromagnetism, we have calculated the effect of an applied magnetic field on the neutron scattering cross-section of the magnetic resonance. In the presence of vortices, the neutron scattering cross-section shows clear signatures of not only the fluctuations in the superconducting order parameter ψ, but also the modulation of the phase of ψdue to vortices. In reciprocal space we find that i) the scattering amplitude is zero at (pi/a,pi/a), ii) the resonance peak is split into a ring with radius pi/d centered at (pi/a,pi/a), d being the vortex lattice constant, and consequently, iii) the splitting pi/d scales with the magnetic field as sqrt{B}.

cond-mat.supr-con↗

arXiv1995

Quasiparticle structure and coherent propagation in the $t-J_{z}-J_{\perp}$ model

Numerical studies, from variational calculation to exact diagonalization, all indicate that the quasiparticle generated by introducing one hole into a two-dimensional quantum antiferromagnet has the same nature as a string state in the $t-J_{z}$ model. Based on this observation, we attempt to visualize the quasiparticle formation and subsequent coherent propagation at low energy by studying the generalized $t-J_{z}-J_{\perp}$ model in which we first diagonalize the $t-J_{z}$ model and then perform a {\em degenerate} perturbation in $J_{\perp}$. We construct the quasiparticle state and derive an effective Hamiltonian describing the coherent propagation of the quasiparticle and its interaction with the spin wave excitations in the presence of the Néel order. We expect that qualitative properties of the quasiparticle remain intact when analytically continuing $J_{\perp}$ from the anisotropic $J_{\perp} < J_{z}$ to the isotropic $J_{\perp} = J_{z}$ limit, despite the fact that the spin wave excitations change from gapful to gapless. Extrapolating to $J_{\perp}=J_{z}$, our quasiparticle dispersion and spectral weight compare well with the exact numerical results for small clusters.

cond-mat↗

arXiv1995

Composite quasiparticle formation and the low-energy effective Hamiltonians of the one- and two-dimensional Hubbard Model

We investigate the effect of hole doping on the strong-coupling Hubbard model at half-filling in spatial dimensions $D\ge 1$. We start with an antiferromagnetic mean-field description of the insulating state, and show that doping creates solitons in the antiferromagnetic background. In one dimension, the soliton is topological, spinless, and decoupled from the background antiferromagnetic fluctuations at low energies. In two dimensions and above, the soliton is non-topological, has spin quantum number 1/2, and is strongly coupled to the antiferromagnetic fluctuations. We derive the effective action governing the quasiparticle motion, study the properties of a single carrier, and comment on a possible description at finite concentration.

cond-mat↗

arXiv1994

Solution of the Boltzmann equation in a random magnetic field

A general framework for solving the Boltzmann equation for a 2-dimensional electron gas (2DEG) in random magnetic fields is presented, when the random fields are included in the driving force. The formalism is applied to some recent experiments, and a possible extension to composite fermions at $ν=1/2$ is discussed.

cond-mat↗

arXiv1994

Magnetoresistance of a 2-dimensional electron gas in a random magnetic field

We report magnetoresistance measurements on a two-dimensional electron gas (2DEG) made from a high mobility GaAs/AlGaAs heterostructure, where the externally applied magnetic field was expelled from regions of the semiconductor by means of superconducting lead grains randomly distributed on the surface of the sample. A theoretical explanation in excellent agreement with the experiment is given within the framework of the semiclassical Boltzmann equation.

cond-mat↗