Brage Golding

Superconductivity and heavy fermions

B BATLOGG, D J BISHOP, E BUCHER, B GOLDING, Jr, A P RAMIREZ AT& T Bell Laboratorles, Murray Hill, NJ 07974, USA Z FISK, J L SMITH Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos NM 87545 USA and H R OTT Laboratonum fur FestkorperphysIk, ETH-Honggerberg 8093, Zurich, Switzerland Experimental studies are discussed which shed hght on the nature of the heavy fermlon normal state, the occurrence of heavy fermion superconductivity and anomalous properties of the superconducting state l) Magnetization and mag- netoreslstance of the Kondo lattice UBel~ demonstrate the breakdown of a description m terms of a single energy scale TK Here T K becomes temperature dependent below ~ 10 K, and the canomcal relationship between R(H) and M(H) does not hold n) Superconductivity in UPta occurs at the borderline to magnetism spin-density wave-hke instabihtJes are induced by a few percent of Th or Au, whereas mass enhancement and susceptlbihty are rapidly reduced by Ir subsntution This suggests that spin fluctuations, rather than phonons, are ~mportant in medmtmg superconducting pairing of heavy fermions in UPt~ lU) The posstbillhes of noncubic strain induced by amsotropic superconductivity in UBe~ ~ is investigated by ultrasound experiments The suggestion of tetragonal strata in (U Th)Be~ is found to be not applicable, and observed frequency and amplitude dependent absorption anomalies at T,~ T, might reflect domain wall motion

Echo Phenomena in Disordered Solids

It is characteristic of structurally disordered solids that many of their properties at, and below, liquid helium temperatures are dominated by localized excitations generally known as tunneling systems. A successful description1, 2 of many of the thermal3 and heat transport3, 4 properties of glassy insulators (and even some metals)5, 6 has emerged from the realization that the tunneling systems are effective scatterers of phonons. The tunneling centers possess two energy levels and transitions between levels may be caused by the absorption and emission of resonant phonons. At sufficiently low temperatures, typically below 0.1 K the tunneling systems may possess long phase memory times and this is reflected by the observation of coherent resonance phenomena such as phonon echoes.7 As we shall emphasize in this review, a systematic study of echo behavior can reveal many previously obscured details of the interaction of phonons with tunneling centers. Tunneling systems also possess electric dipole moments, thereby providing an additional means, electric field excitation, for generating echoes.8–10 A combination of phonon and electric echo techniques constitutes a very powerful approach to studying the low temperature properties of disordered solids.

Amorphous semiconductors at very low temperatures

Abstract The achievement of temperatures well below 1K has provided a unique opportunity for examining, and isolating, the low-lying excitations in amorphous semiconductors. It is a characteristic of disordered solids that their properties in this temperature region are dominated by localized tunneling systems, which are believed to originate in the tunneling of atomic groups between two nearly equivalent configurations. These systems possess a wide spectrum of two-level energy splittings and transitions between levels can be induced by the absorption of resonant acoustic or electromagnetic radiation. It has been found that coherent emission, in the form of echo radiation, can be obtained below 0.1K by coherently pumping the tunneling systems in the microwave region. This paper will attempt to review our current understanding of disordered solids at low temperatures with particular emphasis placed on amorphous SiO 2 and As 2 S 3 . Finally, we shall discuss the relationship between tunneling systems and those defects thought to be responsible for chalcogenide gap states.

Gigahertz ultrasound in single crystal superconducting YBa/sub 2/Cu/sub 3/O/sub 7/

Anisotropic sound propagation at 1 GHz in single-crystal YBa/sub 2/Cu/sub 3/O/sub 7/ is described. Longitudinal phonons have been studied from 0.01 K to 300 K, with emphasis on the temperature region near the superconducting transition T/sub c/ at 88 K. A discontinuous sound velocity is observed at T/sub c/ for propagation parallel and perpendicular to the c axis. Strain derivatives of T/sub c/ are evaluated using mean field theory. Below 1 K the temperature-dependent sound velocity has positive slope, which indicates tunneling of defects.<<ETX>>

Sound propagation in fused silica below 1K

Experiments on the propagation of sound waves in fused silica glass (Suprasil W) below 1K have been carried out to test theories of the glassy state. The tunneling state model1,2 postulates the the existence of a broad distribution of two‐level systems in all glasses which are capable of resonantly scattering phonons, thus accounting for the low phonon thermal conductivities of amorphous dielectrics. Additionally, this model predicts the temperature and frequency dependence of the reciprocal decay length, or absorption, of a phonon:1,3

Critical Sound Absorption and Dispersion in EuO Near Tc

We have investigated the temperature and magnetic field dependence of the ω/2π = .23 MHz sound velocity and attenuation in EuO near its ferromagnetic transition at 69.3 K. A temperature‐dependent excess attenuation, which is first observed about 1K above Tc, increases monotonically through Tc (within our resolution) and exhibits a broad maximum near 68.4K. Analysis of the critical velocity dispersion and attenuation above Tc yields the estimate ωτc ≈ 2–5 at (T−Tc)/Tc ≈ 10−3, where τc−1 is the critical relaxation rate for sound, presumably governed by spin‐lattice relaxation. A magnetic field of 200 Oe “suppresses” the critical part of the sound velocity.

INVITED PAPER: SUPERCONDUCTIVITY AND HEAVY FERMIONS

Experimental studies are discussed which shed light on the nature of the heavy fermion normal state, the occurrence of heavy fermion superconductivity and anomalous properties of the superconducting state: i) Magnetization and magnetoresistance of the Kondo lattice UBe13 demonstrate the breakdown of a description in terms of a single energy scale TK. Here TK becomes temperature dependent below ≈ 10 K, and the canonical relationship between R(H) and M(H) does not hold, ii) Superconductivity in UPt3 occurs at the borderline to magnetism: spin-density wave-like instabilities are induced by a few percent of Th or Au, whereas mass enhancement and susceptibility are rapidly reduced by Ir substitution. This suggests that spin fluctuations, rather than phonons, are important in mediating superconducting pairing of heavy fermions in UPt3. iii) The possibilities of noncubic strain induced by anisotropic superconductivity in UBe13 is investigated by ultrasound experiments. The suggestion of tetragonal strain in (U,Th)Be13 is found to be not applicable, and observed frequency and amplitude dependent absorption anomalies at T ≪ Tc might reflect domain wall motion.

Rotary Phonon Echoes in Silica Glass

Acoustic resonance experiments have played a central role in developing a quantitative picture of the low-temperature properties of glasses [1,2]. The strong coupling between phonons and two-level tunneling systems, the two intrinsic low energy excitations of disordered insulators, has stimulated the development of novel acoustic propagation methods. Among these, coherent phonon echo studies [2,3] have provided an abundance of detailed information on phonon — two-level system couplings and on relaxation times. At temperatures below 0.1 K, where coherence persists for relatively long times (≳10−6 sec), two and three pulse phonon echoes, the acoustic analogs of magnetic spin echoes, have been used to study tunneling systems in silica glass [2,3], and neutron-irradiated quartz [4].