Y. Uwatoko*, I. Umehara†, M. Ohashi‡, T. Nakano§, G. Oomi¶||

* The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Japan

† Department of Physics, Faculty of Engineering, Yokohama National University, Yokohama, Japan

‡ Faculty of Environmental Design, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa, Japan

§ Department of Materials Science and Technology, Faculty of Engineering, Niigata University, Ikarashi, Niigata, Japan

¶ Department of Physics, Kyushu University, Hakozaki, Fukuoka, Japan

|| Department of Education and Creation Engineering, Kurume Institute of technology, Kamitsu-machi, Kurume, Fukuoka, Japan

Abstract

Electronic properties of rare earth metals, alloys and intermetallic compounds have attracted a lot of attention not only from the viewpoint of fundamental aspects but also from the application. These materials show a wide variety of electronic and magnetic properties, such as magnetic order, superconductivity, intermediate valence, Kondo effect and so forth. In this chapter we introduce a lot of experimental studies about the effect of pressure on the thermal, structural and electronic properties of rare earth compounds, in which the electronic states are marginal due to unstable 4f electrons. We describe the present status in this research area. In section 2, the relations between crystal structure and electronic and magnetic state by using X-ray and neutron diffraction under pressure are reported. Effect of pressure on the thermal properties mainly for heavy fermions are reported in section 3. Novel pressure-induced electronic phase transitions such as the crossover in the electronic states and superconductivity are introduced for several heavy fermion materials in section 4. In the last section 5, we show miscellaneous examples which are found recently for amorphous state rare earth compounds and rare earth magnetic multilayers.

Abbreviations

α thermal expansion coefficient

αmag the magnetic contribution to α

γ electronic specific heat coefficient

εF Fermi energy

κ the compressibility

κi linear compressibilities along i-axis (i = a, b, or c)

ρ resistivity

τ wave vector

ωs angular frequency

Δ(0) superconducting gap at T = 0

Γ′ Grüneisen parameter

Δl/l linear thermal expansion

Δmag energy gap of antiferromagnetic magnon

ΘD Debye temperature

A coefficient of T2 term in resistivity

AFM antiferromagnetism

B0 bulk modulus

B0′ pressure derivative of bulk modulus

BCS Bardeen–Cooper–Schrieffer

BT the isothermal bulk modulus

C heat capacity

Cac alternating current specific heat

CEF crystalline electric field

CK concentrated Kondo

Cs heat capacity of superconductivity

CV the specific heat at constant volume

D(E) density of state at energy E

E energy

FRP fiber-reinforced plastic

FWHM full width at half maximum

g0 the degeneracy factor of ground state

g1 the degeneracy factor of exited sate

H magnetic field

reduced Planck constant or Dirac constant

HF heavy fermion

hpp high-pressure phase

HRC Rockwell hardness in C-scale

IC incommensurate

kB Boltzmann constant

lpp low-pressure phase

MPMS magnetic property measurement system

n electronic density

ND neutron diffraction

NMR nuclear magnetic resonance

P pressure

Pc critical pressure

Q wave vector

q1 propagation vector

R gas constant

SC superconductivity

T temperature

T0 a characteristic temperature

TC Curie temperature

Tc superconducting transition temperature

TF the temperature where electron becomes in Fermi liquid state

TK Kondo temperature

Tmax temperature showing resistance maximum

TN Néel temperature

TPT topological phase transition

Tsf spin fluctuation temperature

U internal energy

V volume of unit cell

V0 volume at ambient pressure

1. INTRODUCTION

The physical properties of condensed matter are dominated by interactions between particles or quasiparticles. Among them, the electron correlations are the most important interactions in determining the electronic and magnetic properties of condensed matter. In other words, the understanding of the mechanisms of electron correlations in condensed matter is one of the most basic problems in solid-state physics. High-Tc superconductivity (SC) and novel physical properties of rare earth compounds are thought to originate from strong electron correlations in f- and d-electron systems. In alloys and intermetallic compounds, including rare earth elements, many anomalous physical properties such as Kondo effect, magnetic ordering, SC, and other have been observed. These phenomena are closely connected to the electron correlations between localized 4f electrons and conduction electrons.

It has been well known that physical properties of materials on the border of magnetic instability are strongly dependent on external parameters such as external pressure, magnetic fields, and chemical composition. Novel electronic properties of rare earth compounds are due to the existence of localized 4f electrons, which generally speaking, have strong electron correlations and unstable electronic states. These properties are also expected from the anomalous pressure–temperature phase diagram of rare earth elements, in which numerous phase transitions are observed at low temperature and at high pressure. The heavy fermion (HF) compounds, which are a group of Kondo compounds that have extremely large specific heat coefficients, are typical examples having unstable electronic states, and it is well known that their physical properties are affected strongly by a change of magnetic field and external pressure.

In this chapter, we present the structural and thermal properties of rare earth compounds under high pressure in connection with their electronic and magnetic properties. Pressure is an excellent tool facilitating many types of phase transitions, including quantum phase transitions (QPTs) in condensed matter. There is a large body of research describing physical properties of condensed matter under high pressure in relationship not only to magnetic and electronic properties but also to structural properties. Because of size constrains, we confine our discussion to the present status of research in thermal and electronic aspects of physical properties of rare earth compounds, in which almost all examples are or may potentially be highly correlated compounds including rare...