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Klemm group picture
Group picture from left to right: Jingchuan Zhang, Klemm, Bianca Hall, and Christopher Loerscher. Christopher Loerscher spent four months in Grenoble in 2013, doing experiments with Jean-Pascal Brison on the p-wave superconductorUCoGe during his Chateaubriand Fellowship. Right: Ms. Candy Reid, a senior physics major working on THz radiation from mesas of the high-temperature superconductor Bi2Sr2CaCu2O8+d.

Klemm Beijing group picture
Group picture from left to right: Jingxiang Zhao, Prof. Qiang Gu, Klemm, and Jingchuan Zhang.

Layered Superconductors Volume 1 book cover

Dr. Richard A. Klemm

Associate Professor
Department of Physics
University of Central Florida
4000 Central Florida Blvd.
Orlando, FL 32816-2385

PS 402
407-882-1160, fax: 407-823-5112, email:

Outstanding Referee Plaque
Richard Klemm has worked in many fields of condensed matter physics. He began his scientific career as a synthetic organic chemist, working during the summers while an undergraduate in his father's lab at the University of Oregon. Then, while an undergrad at Stanford, he worked with Fritz Schaefer and Frank Harris on atomic hyperfine structure calculations. After graduating, he worked as a Research Technician at Synvar Research Institute in Palo Alto under the supervision of Fred Gamble. During that year, he was making transition metal dichalcogenides, in the hope of making interesting layered superconductors, and while Gamble was in Hawaii for a conference, Klemm found two interesting papers, one on the intercalation of TiS2 with long-chain organic amides, the other on the intercalation of graphite by FeCl3, and its subsequent reduction to make iron graphite. When Gamble returned, Klemm told him of his idea to try intercalation of a superconducting dichalcogenide. Although 2H-TaS2 only had a Tc value of 0.6o K, he thought to try intercalating it with pyridine, a stable ring compound. He then sealed off a glass tube with 2H-TaS2 immersed in pyridine, and heated it to 200o C, and the TaS2 grew in size visibly. Then Frank DiSalvo, then a graduate student of Ted Geballe at Stanford, measured it, and found it to be superconducting at 3.4o K, and the normal state resistivity was a anisotropic as in any of the high Tc superconductors discovered more than 18 years later. After this discovery, Klemm went to Harvard, and wrote his doctoral thesis on Layered Superconductors, which included calculations of the upper critical field both parallel and perpendicular to the layers, and of the fluctuation diamagnetism and conductivity.

tas2py structure
Top: Sketch of a potential structure of 2H-TaS2(pyridine)1/2.

Klemm picture
Bottom, from left to right: Frank J. DiSalvo, Fred R. Gamble, Richard A. Klemm, and Ted H. Geballe (1970).
Klemm picture
Theoretical upper critical field versus temperature for a layered superconductor with the field parallel to the layers. [R. A. Klemm, A. Luther, and M. R. Beasley, Phys. Rev. B 12, 877 (1975)].

After graduate school, Klemm worked on the theory of quasi-one-dimensional conductors, using the bosonization technique pioneered by his thesis advisor, Alan Luther, and with Vic Emery. He also collaborated on renormalization group calculations on coupled one-dimensional conductors with Patrick Lee and Maurice Rice at Bell Labs, and studied the two-chain model for TTF-TCNQ with a particle-like and a hole-like chain in the same compound. Then, in collaboration with John Hertz (U. Chicago), he studied the dynamics of spin glasses while at Iowa State. While at ISU and at Exxon, he worked with Kurt Scharnberg (Hamburg) on heavion fermion superconductors, which were thought by some to be potential p-wave superconductors. Twenty years later, the Scharnberg-Klemm theory of the upper critical field of p-wave superconductors with broken symmetry was found by Hardy and Huxley to fit the temperature dependence of the upper critical field of URhGe in all three axis directions, pictured below. Hardy=Huxley Hc2 in URhGe
The upper critical field Hc2 of URhGe from the 2005 PRL of Hardy and Huxley, including quantitative fits to the predictions in the 1985 PRL by Scharnberg and Klemm for the p-wave state with completely broken symmetry (the solid curves, with the pairing interaction locked onto one crystal axis direction only).

Presently, he is working with four graduate students, the ``p-wave group'' and the ``Beijing p-wave group'', shown at the top of this webpage in the Fall of 2012 and in June of 2013. Loerscher and Zhang have used the Klemm-Clem transformations to incorporate single-electron effective mass anisotropy into the expressions for the upper critical field at an arbitrary field direction. This they are using to make further predictions for the angular dependence of the upper critical field for the polar state with completely broken symmetry, as in URhGe, and for the SK and/or ABM states possibly relevant for Sr2RuO4, thought by many to be a chiral p-wave state with order parameters A(px+ipy) and B(px-ipy), which would be in the non-chiral polar state with completely broken symmetry obtained from |A|=|B| at and just below the upper critical field for the field parallel to the layers, and in the chiral SK state with a non-equal mix of |A| and |B| for the field perpendicular to the layers. These fits will require the inclusion of Pauli pairbreaking, never before calculated for an antiparallel-spin pair state of any p-wave pairing orbital symmetry, as the recent measurements of the upper critical field parallel to the layers by Deguchi et al. and by Kittaka et al. show dramatic Pauli limiting effects. This Pauli limiting was first noted by Machida and Ichioka, and was discussed further in Layered Superconductors Volume 1. Although many workers had favored the O, Ru, and Sr Knight shift measurement results in Sr2RuO4, all of which seemed to favor a parallel-spin state for fields both parallel and normal to the layers, such measurements are indirect probes of the superconducting Cooper pairs, as they directly probe the nuclear spins, which only interact directly with the local atomic s-orbitals, and hence the interactions with the conduction electrons and/or the Cooper pairs, is at best a third-order effect. Such measurements can be affected by the presence of normal Ru inclusions, known to occur in that material, and by magnetic vortices since strong magnetic fields are necessary to perform most Knight shift measurements, whereas the upper critical field is a direct (and can be thermodynamic) measurement of the destruction of the superconducting Cooper pairs. Bianca Hall is studying these and other examples of the failure of Knight shift measurements. Loerscher and Zhang will also model the temperature and angular dependence of Hc2 in UCoGe, which has a complicated ferrimagnetic structure in the presence of the magnetic field, and the possibility of magnetic Fermi surface breakdown effects (such as a Lifshitz transition), using an ellipsoidal Fermi surface model. Hall, who recently became an official doctoral student at UCF, is also interested in the critical fields of (superconducting) doped topological insulators such as CuxBi2Se3, which has an Hc2 not inconsistent with a p-wave polar state for fields both parallel and normal to the layers, as shown in a PRL in 2012 by Bay et al. This might indicate that the orbital symmetry of the superconducting order parameter arose from an isotropic p-wave pairing interaction, as predicted by Scharnberg and Klemm in PRB in 1980. Why this would happen in a layered superconductor is presently a mystery in need of detailed study. Zhao, the newest member of the p-wave group, is working on microscopic models of parallel-spin pairing, such as the Appel-Fay model of ferromagnetic spin fluctuations.

Klemm group picture
He also started work with Bob Schrieffer on the theory of charge-density wave conduction within a Lee-Rice domain, and persuaded an experimental group at Exxon to study the physics of charge-density wave conduction in NbSe3 and o-TaS3, for which Klemm synthesized the crystals exhibiting the lowest threshold electric field for conduction available at the time.
During most of his career, Klemm has worked on the theory of anisotropic superconductors, working with John Clem at ISU on the anisotropic Ginzburg-Landau model for the lower critical field at an arbitrary field direction, and made many theoretical studies of the high temperature superconductors YBCO and Bi2212. In particular, he was impressed by the extremely careful work of Qiang Li at Brookhaven National Laboratory, who made c-axis twist junctions of Bi2212. Klemm convinced Li to use this technique to undertake the best experimental test of the orbital symmetry of the superconducting order parameter, by studying the c-axis critical current at many different twist junction angles, in the vicinity of the transition, where heating issues are not a problem. The results proved definitively that the order parameter in that material with nominal C2v13 point group symmetry was at least 20% s-wave for all temperatures up to Tc. This experiment proved that all theories based upon repulsive interactions or magnetic interactions were false, in contradiction to the prevailing opinion. Since then, the experiment has been reproduced using artificial cross whiskers and naturally-formed cross-whisker junctions by groups in Tsukuba, Japan and in Moscow, Russia. In addition, very recent experiments in a variety of laboratories have determined that the electron-phonon interaction does indeed play a crucial role in the pairing interaction, and it appears that the phonon responsible for the superconductivity is the half-breathing mode, which occurs at the position in the first Brillouin zone where the saddle bands of extended van Hove singularities are prominent.

Most recently, Klemm has been collaborating with Prof. Kazuo Kadowaki at the University of Tsukuba, Japan. They are working on the angular dependence of the coherent terahertz radiation emitted from mesas of Bi2212, which act as stacks of Josephson junctions. In particular, they have been interested in determining the primary microscopic source of the coherent radiation. There are presently two basic models of this source. The most popular model is to treat the mesa as a cavity that amplifies the ac Josephson frequencies formed when a dc voltage is applied across the stack of junctions. The other model is to treat the mesa as an electric dipole antenna, in which the ac Josephson supercurrent is itself the radiation source. In the existing rectangular mesa geometries, the cavity modes for wave formation across the mesa width are harmonic, as in the natural harmonic frequency spectrum of a violin string, and can be made to match the harmonic frequency spectrum of the ac Josephson current, which occurs from the non-linearities in all Josephson junctions, making a clear distinction between these two mechanisms difficult. However, Klemm then proposed to study cylindrical mesa geometries, for which the anharmonic cavity modes are analogous to the frequency spectrum of a drum, so that only one of the ac Josephson supercurrent frequencies could be amplified in the cavity model. Experiments to test this idea are currently underway. In addition, Klemm and Kadowaki, along with Tokyo Instruments, filed an international idea patent on July 17, 2008, in which they claimed that the removal of the superconducting substrate might lead to an enhancement of the output power by three orders of magnitude, potentially up to 5 mW, sufficient for many device applications. After consulting with friends and family on a name for the device, they denoted this device the Josephson STAR-emitter, where STAR is an acronym for stimulated terahertz amplified radiation. Klemm thanks Stephanie Sirgo Johnston for suggesting this name.
twist scheme
Schematic views of c-axis twist junctions designed to test for a dx2-y2-wave order parameter
twist measurement
Schematic view of the lead attachments to a bicrystal containing a c-axis twist junction. [Q. Li et al., Phys. Rev. Lett. 83, 4160 (1999).]
twist scheme
High resolution transmission electron microscope picture [Y. Zhu et al., Phys. Rev. B 57, 8601 (1998)] of three cross-sections of a 36.45o Bi2212 c-axis twist junction.
twist measurement
Off-axis electron holography picture [M. A. Schofield et al., Phys. Rev. B 67, 224512 (2003)] of four Bi2212 c-axis twist junctions.
twist scheme
C-axis critical currents measured across intrinsic and the 50o twist junction. [Q. Li et al., Phys. Rev. Lett. 83, 4160 (1999).]
twist measurement
C-axis critical current densities across the intrinsic and twist junctions of four samples at temperatures just below Tc.[Q. Li et al., Phys. Rev. Lett. 83, 4160 (1999).]
twist scheme
Ratios of the c-axis critical current densities across the twist junctions to those across the intrinsic single crystal junctions at 0.9Tc, as a function of the twist angle.[Q. Li et al., Phys. Rev. Lett. 83, 4160 (1999).]

twist measurement
Sketches of the simplest forms of the four possible pairing symmetries in a tetragonal crystal. The red and green portions have opposite signs.
twist scheme
Sketch of the equivalent sites with the allowed symmetries in a tetragonal crystal with C4v point group symmetry. Table: Group symmetry operations and the associated order parameter symmetries. [R. Klemm et al., Phys. Rev. B 61, 5913 (2000)]
twist measurement
Sketch of the orthorhombic distortion appropriate for Bi2212 with approximate C2v13 point group symmetry. Table: Associated mixed order parameter symmetries for intrinsically perfect Bi2212. [R. Klemm et al., Phys. Rev. B 61, 5913 (2000)]
twist scheme
Top: Photomicrographs of naturally-formed c-axis cross whiskers. Bottom: Relative abundance of naturally-formed cross whiskers as a function of cross angle. [Yu. Latyshev et al., Phys. Rev. B 70, 094517 (2004)].
twist measurement
Log-log plot of the resistance versus junction area for the naturally-formed cross-whisker junctions studied. Inset: Resistance versus temperature across one cross-whisker junctions. [Yu. Latyshev et al., Phys. Rev. B 70, 094517 (2004)].
twist scheme
Compilation of the log10[JcJ(A/cm2)] data of the Li et al. c-axis twist junctions (solid diamonds), the Takano et al. [Y. Takano et al., Phys. Rev. B 65, 140513 (2002)] artificial cross-whisker data (open circles), and the Latyshev et al. [Yu. Latyshev et al., Phys. Rev. B 70, 094517 (2004)] naturally-formed cross-whisker data at low T (stars, solid squares and circles). From R. A. Klemm, Phil. Mag. 85, 801 (2005).

Most recently, Klemm has taken on a new area of interest in nanomagnetism. In particular, he began by studying the synamics of small classical spins coupled via the Heisenberg interaction. More recently, he has studied equal-spin dimers, taking account of the quantum effects arising from the Heisenberg interaction. Then, he expanded that work to include the single-ion and symmetric exchange anisotropies, in collaboration with Dmitri Efremov. Most recently, they have finished a long series of calculations extending that work to equal spin tetramers, such as Ni4, which have been of great interest to experimentalists in the field. They treated the single-ion and exchange anisotropies in first-order perturbation theory, and added the effects of biquadratic and three-center quartic interactions, which can play an important role in antiferromagnetic tetramers. They also showed how to extract some of the microscopic, single-spin interactions from the second excited state manifold using electron paramagnetic resonance.
Most recently, they have been working on the Dzyaloshinskii-Moriya interactions, which can be important in single molecule magnets with sufficiently low symmetry, and they showed that this interaction can lead to new multiferroic effects, in which the magnetic interactions can generate an electric polarization.

Klemm picture
Spin-spin time correlation function for the N-spin equivalent neighbor model [R. Klemm and M. Ameduri, Phys. Rev. B 66, 012403 (2002).]
Klemm picture
Comparison the the predictions for the level-crossing inductions of a spin-5/2 dimer [D. V. Efremov and R. A. Klemm, Phys. Rev. B 74, 064408 (2006)] with data on [Fe(salen)Cl]2 [Y. Shapira et al., Phys. Rev. 59, 1046 (1999)].
Klemm picture
Magnetization at T=0 of antiferromagnetic (AFM) spin 1/2 tetramers with the Heisenberg and Dzyaloshinskii-Moriya (DM) interactions for S4 symmetry.
Klemm picture
Electric polarization arising from the DM interactions in spin 1/2 AFM tetramers with S4 symmetry and DM interactions
Klemm picture
Electric polarization at 45o azimuthal angle as a function of axial angle for AFM spin 1/2 tetramers with S4 and DM interactions.
Klemm picture
Magnetization at T=0 of spin 1/2 tetramers with the Heisenberg and DM interactions for S4 symmetry.

Klemm picture
Electric polarization arising from the DM interactions in spin 1 AFM tetramers with S4 symmetry and DM interactions

Klemm picture
Electric polarization at 45o azimuthal angle as a function of axial angle for AFM spin 1 tetramers with S4 and DM interactions.

Klemm picture