Pressure Induced Semiconductor to Metal Transition in TmTe

Abstract

The pressure induced semiconductor to metal transition in the rare earth compound TmTe has been investigated with electrical resistivity measurements under high pressure. At room temperature, the resistivity showed an exponential decrease up to 2 GPa, indicating a linear closing of the energy gap, followed by an almost pressure independent metallic regime. The resistivity in the metallic regime showed a logarithmic temperature dependence reminiscent of a Kondo effect and a TmSe-like anomaly appeared at low temperature and above 5 GPa. At 5.7 GPa the resistivity showed an abrupt decrease that corresponded to the structural phase transition. [S0031-9007(97)02350-8] PACS numbers: 75.20.Hr, 75.30.MbElectrons move in solids interacting with each other by the Coulomb force. To know the electronic state in a solid is fundamentally a many body problem. Band theory, a mean field theory to treat this problem, has been successful for many kinds of materials, but it is not able to explain the properties of d or f electron systems in which the interactions among the electrons are strong. A wide variety of magnetic and transport properties arise from the correlations among d or f electrons: metal-insulator transition, itinerant magnetism, valence fluctuation, Kondo effect, heavy fermions and superconductivity, and so on. These are now called strongly correlated electron systems and many new concepts on solid state physics have been constructed. However, there are still many problems to be solved and the understanding of these systems is not complete.The Tm monochalcogenide series, which crystallize in the NaCl structure, is one of these mysterious strongly correlated electron systems. TmSe has been studied from the standpoint of the valence fluctuation between 4f 12 and 4f 13 , Kondo effect, antiferromagnetic order, and metal-insulator transition. This compound has the peculiarity that two magnetic valence states Tm 21 and Tm 31 are concerned in the valence fluctuation. However, the situation is so complicated that there is no clear explanation of the physical properties of this compound.TmTe is a divalent semiconductor with thirteen 4f electrons (with one 4f hole). The lattice constant and the Curie constant show that the Tm ions in TmTe are divalent at ambient pressure [1]. The localized 4f 13 level is situated within the energy gap between a filled Te-5p valence band and an empty Tm-5d conduction band [2,3]. The gap D for the electronic excitation of one 4f electron from the localized 4f level to the 5d conduction band is estimated to be 350 meV [4].In TmSe, due to the smaller ion radius of Se than that of Te, the closer distance between Tm ions leads to an increase in the strength of the crystalline field that splits the 5d band into 5d-e g and 5d-t 2g bands [2]. Consequently the energy gap D between the 4f level and the bottom of the conduction band is narrowed. Then the valence fluctuating state appears due to the strong mixing between the 4f electrons and the 5d conduction electrons. In TmSe, both the lattice constant a...

Electronic structure and magneto-optical Kerr effect of Tm monochalcogenides

“…This transition was ascribed to the onset of long-range ordering among the Tm quadrupolar moments, but the exact mechanism is still controversial. 24 The pressure effects on transport properties of TmTe were investigated by Matsumura et al 25 It was found that at room temperature, the resistivity of TmTe with increasing pressure showed an exponential decrease up to 2 GPa, indicating a linear closing of the energy gap ͑with d⌬/dP ϭϪ207 meV/GPa), followed by an almost pressureindependent metallic regime. The resistivity in the metallic regime showed a logarithmic temperature dependence reminiscent of the Kondo effect, and a TmSe-like anomaly appeared at low temperature and above 5 GPa.…”

Magnetic Behavior of TmTe at High Pressure

“…It was shown that the behavior of the electrical resistance was not that of a typical metal, but displaced a Kondo lattice-like behavior. The Te analog, TmTe, exhibits anomalous high pressure electrical resistance behavior above 2 GPa [2].similar to the Kondo-like behavior seen in TmSe. To provide a more complete understanding of this behavior, we have performed single crystal magnetization measurements to 6.1 GPa…”

“…Ferromagnetic behavior was not observed at 6.1 GPa. Electrical resistivity measurements have been performed under high pressure [2]. At 5.7 GPa the resistivity showed an abrupt decrease which was believed to be due to a structural phase transition.…”

Pressure induced heavy electron states in TmTe

“…However, according to electric resistivity (q) under high pressure [2] and volume compression experiments [3], it exhibits an interesting electronic phase transition to trivalent metallic state from divalent insulating states without any crystal structure change. Up to today, no optical study has been reported concerned with the electronic states by such an electronic transition under pressure because of the difficulty in experiments in a THz region by thermal light sources.…”