arXiv subjects

Richard A. Klemm

Publications and source records attributed to Richard A. Klemm.

At least 37 records · Page 2Linked to original sources

arXiv2013

Anomalous angular dependence of the upper critical induction of orthorhombic ferromagnetic superconductors with completely broken p-wave symmetry

We employ the Klemm-Clem transformations to map the equations of motion for the Green functions of a clean superconductor with a general ellipsoidal Fermi surface (FS) characterized by the effective masses $m_1, m_2$, and $m_3$ in the presence of an arbitrarily directed magnetic induction ${\bm B}=B(\sinθ\cosϕ,\sinθ\sinϕ,\cosθ)$ onto those of a spherical FS. We then obtain the transformed gap equation for a transformed pairing interaction $\tilde{V}(\hat{\tilde{\bm k}},\hat{\tilde{\bm k}}')$ appropriate for any orbital order parameter symmetry. We use these results to calculate the upper critical induction $B_{c2}(θ,ϕ)$ for an orthorhombic ferromagnetic superconductor with transition temperatures $T_{\rm Curie}>T_c$. We assume the FS is split by strong spin-orbit coupling, with a single parallel-spin ($\uparrow\uparrow$) pairing interaction of the \textit{p}-wave polar state form locked onto the $\hat{\bm e}_3$ crystal axis normal to the spontaneous magnetization ${\bm M}_0\perp\hat{\bm e}_3$ due to the ferromagnetism. The orbital harmonic oscillator eigenvalues are modified according to $B\rightarrow Bα$, where $α(θ,ϕ)=\sqrt{m_3/m}\sqrt{\cos^2θ+γ^{-2}(ϕ)\sin^2θ}$, $γ^2(ϕ)=m_3/(m_1\cos^2ϕ+m_2\sin^2ϕ)$ and $m=(m_1m_2m_3)^{1/3}$. At fixed $ϕ$, the order parameter anisotropy causes $B_{c2}$ to exhibit a novel $θ$-dependence, which for $γ^2(ϕ)>3$ becomes a double peak at $0^{\circ}<θ^{*}<90^{\circ}$ and at $180^{\circ}-θ^{*}$, providing a sensitive bulk test of the order parameter orbital symmetry in both phases of URhGe and in similar compounds still to be discovered.

cond-mat.supr-con↗

arXiv2012

Broadly Tunable Sub-terahertz Emission from Internal Branches of the Current-voltage Characteristics of Superconducting Bi2Sr2CaCu2O8+d Single Crystals

Continuous, coherent sub-terahertz radiation arises when a dc voltage is applied across a stack of the many intrinsic Josephson junctions in a Bi2Sr2CaCu2O8+d single crystal. The active junctions produce an equal number of I-V characteristic branches. Each branch radiates at a slightly tunable frequency obeying the ac Josephson relation. The overall output is broadly tunable and nearly independent of heating effects and internal cavity frequencies. Amplification by a surrounding external cavity to allow for the development of a useful high-power source is proposed.

cond-mat.supr-con↗

arXiv2011

Upper critical field of p-wave superconductors with orthorhombic symmetry

We extend the Scharnberg-klemm theory of $H_{c2}$ in p-wave superconductors with broken symmetry to cases of partially broken symmetry in an orthorhombic crystal, as is appropriate for the more exotic ferromagnetic superconductor UCoGe in strong applied magnetic fields. For some partially broken symmetry cases, $H_{c2}$ can mimic upward curvature in all three crystal axis directions, and reasonably good fits to some of the the low-field UCoGe data are obtained.

cond-mat.supr-con↗

arXiv2011

Upper critical field of p-wave ferromagnetic superconductors with orthorhombic symmetry

We extended the Scharnberg-Klemm theory of $H_{c2}(T)$ in p-wave superconductors with broken symmetry to cases of partially broken symmetry in an orthorhombic crystal, as is appropriate for the more exotic ferromagnetic superconductor UCoGe in strong magnetic fields. For some partially broken symmetry cases, $H_{c2}(T)$ can mimic upward curvature in all three crystal axis directions, and reasonably good fits to some of the UCoGe data are obtained.

cond-mat.supr-con↗

arXiv2010

Cavity mode waves during terahertz radiation from rectangular Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ mesas

We re-examined the angular dependence of the radiation from the intrinsic Josephson junctions in rectangular mesas of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, in order to determine if the cavity mode part of the radiation arises from waves across the width $w$ or along the length $\ell$ of the mesas, associated with ``hot spots'' [Wang {\it et al.}, Phys. Rev. Lett. {\bf 105}, 057002 (2010)]. We derived analytic forms for the angular dependence expected in both cases for a general cavity mode in which the width of the mesa corresponds to an integer multiple of one-half the wavelength of the radiation. Assuming the coherent radiation from the $ac$ Josephson current source and the cavity magnetic surface current density source combine incoherently, fits to the data of Kadowaki {\it et al.} [J. Phys. Soc. Jpn. {\bf 79}, 023703 (2010)] on a mesa with mean $\ell/w=5.17$ for both wave directions using two models for the incoherent combination were made, which correspond to standing and traveling waves, respectively. The results suggest that the combined output from the uniform $ac$ Josephson current source plus a cavity wave forming along the rectangle length is equally probable as that of the combined output from the uniform $ac$ Josephson current plus a cavity wave across the width. However, for mesas in which $n\ell/2w$ is integral, where $n$ is the index of the rectangular TM$^z_{n,0}$ mode, it is shown that standing cavity mode waves along the length of the mesa do not radiate in the $xz$ plane perpendicular to the length of the mesa, suggesting experiments on such mesas could help to resolve the question.

cond-mat.supr-con↗

arXiv2010

Geometrical Resonance Conditions for THz Radiation from the Intrinsic Josephson Junctions in Bi2Sr2CaCu2O8+d

Subterahertz radiation emitted from a variety of short rectangular-, square-, and disk-shaped mesas of intrinsic Josephson junctions fabricated from a Bi2Sr2CaCu2O8+d single crystal was studied from the observed I-V characteristics, far-infrared spectra, and spatial radiation patterns. In all cases, the radiation frequency satisfies the conditions both for the ac Josephson effect and for a mesa cavity resonance mode. The integer higher harmonics observed in all spectra imply that the ac Josephson effect plays the dominant role in the novel dual-source radiation mechanism.

cond-mat.supr-con↗

arXiv2009

Evidence for a Dual-Source Mechanism of THz Radiation from Rectangular Mesas of Single Crystalline \bm{$\mathrm{Bi_2Sr_2CaCu_2O_{8+δ}}$} Intrinsic Josephson Junctions

The THz radiation emitted from rectangular mesas of single-crystal $\mathrm{Bi_2Sr_2CaCu_2O_{8+δ}}$ was studied using angular distribution measurements and Fourier transform infrared spectroscopy. Unlike the recent theoretical predictions, the results provide strong evidence for a dual-source mechanism in which the uniform and non-uniform parts of the $ac$-Josephson current act as electric and magnetic current sources, respectively. The latter synchronizes with cavity modes of the mesa with integral harmonics of the fundamental radiation.

cond-mat.supr-con↗

arXiv2009

Enhancement of ferromagnetism by p-wave Cooper pairing in superconducting ferromagnets

In superconducting ferromagnets for which the Curie temperature $T_{m}$ exceeds the superconducting transition temperature $T_{c}$, it was suggested that ferromagnetic spin fluctuations could lead to superconductivity with p-wave spin triplet Cooper pairing. Using the Stoner model of itinerant ferromagnetism, we study the feedback effect of the p-wave superconductivity on the ferromagnetism. Below $T_{c}$, the ferromagnetism is enhanced by the p-wave superconductivity. At zero temperature, the critical Stoner value for itinerant ferromagnetism is reduced by the strength of the p-wave pairing potential, and the magnetization increases correspondingly. More important, our results suggest that once Stoner ferromagnetism is established, $T_m$ is unlikely to ever be below $T_c$. For strong and weak ferromagnetism, three and two peaks in the temperature dependence of the specific heat are respectively predicted, the upper peak in the latter case corresponding to a first-order transition.

cond-mat.supr-con↗

arXiv2009

Radiation from a Josephson STAR-emitter

We calculate the angular dependence of the radiation-zone output power and electric polarization of stimulated terahertz amplified radiation (STAR) emitted from a $dc$ voltage applied across cylindrical and rectangular stacks of intrinsic Josephson junctions. During coherent emission, a spatially uniform $ac$ Josephson current density in the stack acts as a surface electric current density antenna source, leading to an harmonic radiation frequency spectrum, as in experiment, but absent in all cavity modesl of cylindrical mesas. Spatial fluctuations of the $ac$ Josephson current cause its fundamental mode to lock onto the lowest finite energy cylindrical cavity mode, causing it to resonate, leading to a non-uniform magnetic surface current density radiation source, and a non-trivial combined fundamental frequency output power with linear polarization We also present a model of the superconducting substrate, and present results for rectangular mesas.

cond-mat.supr-con↗

arXiv2009

Angular dependence of the radiation power of a Josephson STAR-emitter

We calculate the angular dependence of the power of stimulated terahertz amplified radiation (STAR) emitted from a $dc$ voltage applied across a stack of intrinsic Josephson junctions. During coherent emission, we assume a spatially uniform $ac$ Josephson current density in the stack acts as a surface electric current density antenna source, and the cavity features of the stack are contained in a magnetic surface current density source. A superconducting substrate acts as a perfect magnetic conductor with $H_{||,ac}=0$ on its surface. The combined results agree very well with recent experimental observations. Existing Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ crystals atop perfect electric conductors could have Josephson STAR-emitter power in excess of 5 mW, acceptable for many device applications.

cond-mat.supr-con↗

arXiv2008

Single-ion and exchange anisotropy effects and multiferroic behavior in high-symmetry tetramer single molecule magnets

We study single-ion and exchange anisotropy effects in equal-spin $s_1$ tetramer single molecule magnets exhibiting $T_d$, $D_{4h}$, $D_{2d}$, $C_{4h}$, $C_{4v}$, or $S_4$ ionic point group symmetry. We first write the group-invariant quadratic single-ion and symmetric anisotropic exchange Hamiltonians in the appropriate local coordinates. We then rewrite these local Hamiltonians in the molecular or laboratory representation, along with the Dzyaloshinskii-Moriay (DM) and isotropic Heisenberg, biquadratic, and three-center quartic Hamiltonians. Using our exact, compact forms for the single-ion spin matrix elements, we evaluate the eigenstate energies analytically to first order in the microscopic anisotropy interactions, corresponding to the strong exchange limit, and provide tables of simple formulas for the energies of the lowest four eigenstate manifolds of ferromagnetic (FM) and anitiferromagnetic (AFM) tetramers with arbitrary $s_1$. For AFM tetramers, we illustrate the first-order level-crossing inductions for $s_1=1/2,1,3/2$, and obtain a preliminary estimate of the microscopic parameters in a Ni$_4$ from a fit to magnetization data. Accurate analytic expressions for the thermodynamics, electron paramagnetic resonance absorption and inelastic neutron scattering cross-section are given, allowing for a determination of three of the microscopic anisotropy interactions from the second excited state manifold of FM tetramers. We also predict that tetramers with symmetries $S_4$ and $D_{2d}$ should exhibit both DM interactions and multiferroic states, and illustrate our predictions for $s_1=1/2, 1$.

cond-mat.str-el↗

arXiv2006

Single-ion and exchange anisotropy in high-symmetry tetramer single molecule magnets

For equal-spin $s_1$ tetramer single molecule magnets with ionic point groups $g=T_d, D_{4h}, D_{2d}, C_{4h}, C_{4v}$, and $S_4$, we write the group-invariant single-ion, Dzaloshinskii-Moriya, and symmetric anisotropic near-neighbor and next-nearest-neighbor exchange Hamiltonians using the respective local axial and azimuthal vector groups. The anisotropic exchange renormalized near-neighbor and next-nearest-neighbor isotropic exchange interactions are $\tilde{J}_g$ and $\tilde{J}_g'$. Using our exact, compact forms for the single-ion matrix elements, we evaluate the eigenstates of the full Hamiltonian to first order in the various interactions. There are two types of ferromagnetic (FM) and antiferromagnetic (AFM) tetramers. In Type I, $\tilde{J}_g'-\tilde{J}_g>0$, the tetramers act as two dimers with the maximal pair quantum numbers $s_{13}=s_{24}=2s_1$ at low temperature. Type II tetramers with $\tilde{J}_g'-\tilde{J}_g<0$ are frustrated, with minimal values of the pair quantum numbers $s_{13}$ and $s_{24}$ at low temperature. For both Type I and Type II AFM tetramers, we evaluate the first-order level-crossing inductions analytically for arbitary $s_1$, and illustrate the results for $s_1=1/2, 1, 3/2$. Accurate Hartree expressions for the thermodynamics, electron paramagnetic resonance (EPR) absorption and inelastic neutron scattering cross-section are given. A procedure to extract the effective microscopic parameters for Types I and II tetramers using EPR is given.

cond-mat.mes-hall↗

arXiv2006

Spin anisotropy effects in dimer single molecule magnets

We present a model of equal spin $s_1$ dimer single molecule magnets. The spins within each dimer interact via the Heisenberg and the most general set of four quadratic anisotropic spin interactions with respective strengths $J$ and $\{J_j\}$, and with the magnetic induction ${\bf B}$. We solve the model exactly for $s_1=1/2, 1, 5/2$, and for antiferromagnetic Heisenberg couplings ($J<0$), present ${\bf M}({\bf B})$ curves at low $T$ for these cases. Low-$T$ $C_V({\bf B})$ curves for $s_1=1/2$ and electron paramagnetic susceptibility $χ({\bf B},ω)$ for $s_1=1$ are also provided. For weak anisotropy interactions, we employ a perturbative treatment, and show that the Hartree and extended Hartree approximations lead to reliable analytic results at low $T$ and large $B$ for these quantities and for the inelastic neutron scattering cross-section $S({\bf B}, {\bf q},ω)$. Our results are discussed with regard to existing ${\bf M}({\bf B})$ experiments on $s_1=5/2$ Fe$_2$ dimer single molecule magnets, and suggest that one of them contains a substantial amount of single-ion anisotropy, without any sizeable global spin anisotropy. We urge further experiments of the above types on single crystals of Fe$_2$ and on some $s_=9/2$ [Mn$_4$]$_2$ dimers, in order to elucidate the precise values of the various microscopic interactions.

cond-mat.mes-hall↗

arXiv2005

Josephson (001) tilt grain boundary junctions of high temperature superconductors

We calculate the critical current $I_c$ across in-plane (001) tilt grain boundary junctions of high temperature superconductors. We solve for the electronic states corresponding to the electron-doped cuprates, two slightly different hole-doped cuprates, and an extremely underdoped hole-doped cuprate in each half-space, and weakly connect the two half-spaces by either specular or random quasiparticle tunneling. We treat symmetric, straight, and fully asymmetric junctions with s-, extended-s-, or d$_{x^2-y^2}$-wave order parameters. For symmetric junctions with random grain boundary tunneling, our results are generally in agreement with the Sigrist-Rice form for ideal junctions that has been used to interpret ``phase-sensitive'' experiments consisting of such in-plane grain boundary junctions. For specular grain boundary tunneling across symmetric juncitons, our results depend upon the Fermi surface topology, but are usually rather consistent with the random facet model of Tsuei {\it et al.} [Phys. Rev. Lett. {\bf 73}, 593 (1994)]. Our results for asymmetric junctions of electron-doped cuparates are in agreement with the Sigrist-Rice form. However, ou resutls for asymmetric junctions of hole-doped cuprates show that the details of the Fermi surface topology and of the tunneling processes are both very important, so that the ``phase-sensitive'' experiments based upon the in-plane Josephson junctions are less definitive than has generally been thought.

cond-mat.supr-con↗

arXiv2005

Effect of dipolar interactions on the magnetization of a cubic array of nanomagnets

We investigated the effect of intermolecular dipolar interactions on a cubic 3D ensemble of 5X5X4=100 nanomagnets, each with spin $S = 5$. We employed the Landau-Lifshitz-Gilbert equation to solve for the magnetization $M(B)$ curves for several values of the damping constant $α$, the induction sweep rate, the lattice constant $a$, the temperature $T$, and the magnetic anisotropy field $H_A$. We find that the smaller the $α$, the stronger the maximum induction required to produce hysteresis. The shape of the hysteresis loops also depends on the damping constant. We find further that the system magnetizes and demagnetizes at decreasing magnetic field strengths with decreasing sweep rates, resulting in smaller hysteresis loops. Variations of $a$ within realistic values (1.5 nm - 2.5 nm) show that the dipolar interaction plays an important role in the magnetic hysteresis by controlling the relaxation process. The $T$ dependencies of $α$ and of $M$ are presented and discussed with regard to recent experimental data on nanomagnets. $H_A$ enhances the size of the hysteresis loops for external fields parallel to the anisotropy axis, but decreases it for perpendicular external fields. Finally, we reproduce and test an $M(B)$ curve for a 2D-system [M. Kayali and W. Saslow, Phys. Rev. B {\bf 70}, 174404 (2004)]. We show that its hysteretic behavior is only weakly dependent on the shape anisotropy field and the sweep rate, but depends sensitively upon the dipolar interactions. Although in 3D systems, dipole-dipole interactions generally diminish the hysteresis, in 2D systems, they strongly enhance it. For both square 2D and rectangular 3D lattices with ${\bm B}||(\hat{\bm x}+\hat{\bm y})$, dipole-dipole interactions can cause large jumps in the magnetization.

cond-mat.mes-hall↗

arXiv2005

Local spin anisotropy effects upon the magnetization and specific heat of dimer single molecule magnets

We present an exactly solvable model of equal spin $s_1$ dimer single molecule magnets. The spins within each dimer interact via the Heisenberg and the most general quadratic global and local (single-ion) anisotropic spin interactions, and with the magnetic induction ${\bf B}$. For antiferromagnetic couplings and $s_1>1/2$, the low temperature $T$ magnetization ${\bm M}({\bm B})$ exhibits $2s_1$ steps of universal height and midpoint slope, the $s$th step of which occurs at the non-universal level-crossing magnetic induction $B_{s,s_1}^{\rm lc}(θ,ϕ)$, where $θ,ϕ$ define the direction of ${\bm B}$. The specific heat $C_V$ exhibits zeroes as $T\to0$ at these $B_{s,s_1}^{\rm lc}(θ,ϕ)$ values, which are equally surrounded by universal peak pairs as $T\to0$. The non-universal $B_{s,s_1}^{\rm lc}(θ,ϕ)$ values lead to a rich variety of magnetization plateau behavior, the structure and anisotropy of which depend upon the various global and local anisotropic spin interaction energies. We solve the model exactly for $s_1=1/2$, 1, and 5/2, and present ${\bm M}({\bm B})$ and $C_V({\bm B})$ curves at low $T$ for these cases. For weakly anisotropic dimers, rather simple analytic formulas for ${\bm M}({\bm B})$ and $C_V({\bm B})$ at arbitrary $s_1$ accurately fit the exact solutions at sufficiently low $T$ or large $B$. An expression for $B_{s,s_1}^{\rm lc}(θ,ϕ)$ accurate to second order in the four independent anisotropy energies is derived. Our results are discussed with regard to existing experiments on $s_1=5/2$ Fe$_2$ dimers, suggesting further experiments on single crystals of these and some $s_1=9/2$ Mn$_4$]$_2$ dimers are warranted.

cond-mat.mes-hall↗

arXiv2005

Classification of integrable clusters of classical Heisenberg spins

In this letter we present a general classification of integrable models of identical classical spins coupled via the isotropic Heisenberg Hamiltonian. Our constructive proof of integrability provides a solution scheme for the equations describing the time evolution of each spin. We show that for a broad class of Heisenberg couplings, it is possible to solve the equations of motion iteratively by building a hierarchy of spins, each of which precesses about the composite spin of the previous level. This leads to the identification of the frequencies of precession at each level.

cond-mat.other↗

arXiv2005

First-principles calculations of the magnetism of Fe$_2$O$_2$H$_2$

By expanding the wave function in plane waves, we use the pseudopotential method of density functional theory within the generalized gradient approximation to calculate the effective magnetic coupling energies of the S=5/2 spins in the Fe2 dimer, approximated as Fe$_2$O$_2$H$_2$. Setting the Fe-O bond length at the value corresponding to the minimum total energy, we find the difference in antiferromagnetic and ferromagnetic exchange energies as a function of the Fe-O-Fe bond angle $θ$. The effective interaction is antiferromagnetic for 63$^{\circ}<θ<105^{\circ}$, and is ferromagnetic otherwise. Full potential augmented plane wave calculations were also performed at $θ=100,105^{\circ}$, confirming these results, and providing information relevant to the local anisotropy of the spin interactions.

cond-mat.mes-hall↗