Research Overview
In order to construct and keep our society with highly advanced technology, it is necessary to find and solve new physical properties for a variety of functional materials. Condensed matter physics is the fundamental science of materials based on the laws of Newton mechanics, electromagnetism, thermodynamics, statistical mechanics, quantum mechanics, and so on. It has the greatest impact on our daily lives by providing foundations for technology developments. For example, the inventions of transistors and semiconductor chips have led to the widespread use of a variety of data storage, telecommunication, and electric devices such as cellular phones, CD, DVD, personal computers, and so on.
The interest in condensed matter physics is often explained by referring the famous sentence of “More is different” by Dr. Philip W. Anderson (Nobel prize in Physics, 1977), wherein complex assemblies (Avogadro order) of particles behave in ways dramatically different from their individual constituents (e.g. electrons). For example, a range of phenomena related to unconventional superconductivity in strongly correlated electron systems are not well understood, although the microscopic physics of individual electrons and lattices is well known. Thus, condensed matter physics plays a vital role for understanding and utilizing the nature of many functional materials, and therefore will contribute to the unprecedented growth of modern technology which benefits us in everyday life!
Our effort has been devoted to the research into low dimensional electron systems such as a transition metal chalcogenide and organic conductors. Let me introduce briefly our research achievements and recent activities that are listed below.
Topics
1. Superconductivity in transition metal trichalcogenide NbSe3
NbSe3 is known as a typical quasi-one-dimensional (Q1D) conductor. The crystal structure of NbSe3 consists of three pairs of inequivalent chains, referred as I, II, III and the charge-density-wave (CDW) develops on the chain III at T1 = 142 K and on the chain I at T2 = 58 K. The both CDW phases are suppressed with increasing pressure and superconducting (SC) phase appears at Tc ~ 3 K in a pressure range above ~ 0.8 GPa. Our study refined the T-P phase diagram of NbSe3 to investigate the correlation between the CDW phase and the SC phase [S. Yasuzuka et al., J. Phys. Soc. Jpn. 74, 1782 (2005)].
To start our studies, we prepared single crystals of NbSe3 by chemical vapor transportation method. Using cubic anvil apparatus, we extensively measured the resistance near critical pressure of P1 (= 3.2 GPa) for the T1-CDW phase. I revealed that the T1-CDW phase coexists with the SC phase on the chain III at 2.0 < P < 3.2 GPa. Moreover, we found that the SC transition near P1 on the T1-CDW side is very broad and becomes much sharpened with increasing pressure. Based on these findings, we proposed that the coexistence is attributed to the quantum tunneling of the Cooper pairs between the SC domains separated by the CDW domains [S. Yasuzuka et al., J. Phys. Soc. Jpn. 74, 1782 (2005)].
In addition, we measured the resistance and the magnetoresistance near P2 (= 0.75 GPa), where the T2-CDW phase competes with the SC phase on the chain I. Just above P2, we found remarkable charge fluctuations associated with the T2-CDW phase. The experimental results demonstrate that the quantum fluctuations, which have been neglected in a reliable mean field theory, play a crucial role for the competition between the T2-CDW and the SC phases at around P2 [S. Yasuzuka et al, Phys. Rev. B 60, 4406 (1999); S. Yasuzuka et al., J. Phys. Soc. Jpn. 74, 1787 (2005)].
2. Fermi surface studies of organic conductors
A key for the better understanding of inorganic metals as well as organic conductors showing exotic physical properties is the knowledge of the electronic structure, i.e. the Fermi surface (FS). For various quasi-two-dimensional (Q2D) electronic systems, such as organic conductors, the Fermi surfaces have been studied by means of an angular-dependent magnetoresistance oscillation (AMRO) and the quantum oscillations such as the Shubnikov-de Haas (SdH) and de Haas-van Alphen (dHvA) effects. From the period of the AMRO, Fermi wave number for a certain direction in the 2D conducting plane can be obtained so as to map the Fermi surface contour. In addition, magnetoresistance shows sharp increase when the field orientation is parallel to the conducting layer. This is called the peak effect. The observation of the peak effect is interpreted as evidence of the coherent interlayer transport in Q2D systems. Measurements of angular-dependent magnetoresistance and quantum oscillations in Q2D conductors have been recognized as powerful tools to probe their FS's. For examples, see S. Yasuzuka et al., J. Phys. Soc. Jpn. 74, 679 (2005); S. Yasuzuka et al., J. Phys. Soc. Jpn. 81, 035006 (2012).
One of our topics is the observation of the 3D FS in a single component molecular metal, [Ni(tmdt)2]. The realization of a molecular metal based on the crystallization of single-component, neutral molecules was a long-standing quest. In 2001, Tanaka et al successfully synthesized a single-component molecular metal [Ni(tmdt)2]. Ab initio electronic structure calculations predicted that [Ni(tmdt)2] has both electron and hole 3D FS's, where the volume of each pocket occupies 3.5 % of the first Brillouin zone. Results of the band calculations were experimentally confirmed by our dHvA study. The rigorous confirmation of its metallic nature by the observation of dHvA oscillations marked a milestone in the design of the conducting properties for molecular systems [H. Tanaka and S. Yasuzuka et al., J. Am. Chem. Soc. 126, 10518 (2004)].
3. Metallization of organic conductors under high pressure
Since the discovery of an organic superconductor with the highest Tc at ~ 8 GPa, the pressure region up to 10 GPa has been regarded as a new pressure range of interests in organic conductors. For a review, see S. Yasuzuka et al., Sci. Technol. Adv. Mater. 10, 024307 (2009). One of our topics is “metallization of Mott insulator (TTM-TTP)I3 with highly one-dimensional (1D) half-filled band” [S. Yasuzuka et al., J. Phys. Soc. Jpn. 75, 053701 (2006)]. It is theoretically predicted that the ground state of the 1D half filled system is the Mott insulator at any positive on-site Coulomb interaction. So pressure-induced metallization of a real material with the highly 1D half-filled band is one of the most challenging studies as related to high-Tc curates. I found that the metal-insulator transition temperature, TMI, of (TTM-TTP)I3 decreases linearly with increasing pressure and is directing towards 0 K near 10 GPa. Moreover, we showed that the origin of high-temperature metallic behavior above 5.7 GPa is understood in terms of the Tomonaga-Luttinger liquid described by the Hubbard model.
Another topic is “metallization of the monumental Peierls insulator TTF-TCNQ” [S. Yasuzuka et al., J. Phys. Soc. Jpn. 76, 033701 (2007)., Papers of Editors’ Choice]. The birth of organic conductor TTF-TCNQ opened a vast area of organic conductors in 1973. TTF-TCNQ was first organic conductor to show metallic behavior down to 60 K, below which a metal-insulator (Peierls) transition occurs. We succeeded in metallizing TTF-TCNQ under high pressure, which has been anticipated since 1973. Before this work, the pressure study of this material was limited below 3 GPa, where the Peierls transition was enhanced with pressure. We performed the pressure experiments beyond 3 GPa and found that the Peierls transition is suppressed at 8 GPa. Our study showed that the quest for superconductivity in TTF-TCNQ is now within the real scope of research with particular interest in its interplay with the Peierls transition.
4. Anisotropic vortex dynamics in d-wave organic superconductors
Superconductivity in strongly correlated electron systems has been a fascinating subject of investigation in the past few decades, since the realization of unconventional non-s-wave parings can be expected due to strong electron-electron repulsion. The gap functions of these unconventional superconductors mostly have zeros (have nodes) along certain directions in the momentum space. The existence of nodes (point or line) can be inferred from power-law dependences in physical quantities such as the specific heat or the nuclear spin relaxation rate. To identify the gap structure is, however, a more formidable task. For instance, the gap symmetry dxy differs from dx2-y2 only in the position of the line nodes (45 degrees rotation of the latter becomes identical with the former). Direction-sensitive experiments are therefore needed to discriminate these gap structures. The dependence of the specific heat and the thermal conductivity on magnetic field orientation was demonstrated to be powerful tools for probing the nodal directions in an anisotropic superconducting gap structure. In a semiclassical way, a magnetic field can be taken into account by introducing a“Doppler shift”in the energy spectrum of the quasiparticles (QPs) due to superfluid flow around the vortices.
In addition to the specific heat and thermal conductivity, gap nodes are expected to affect the flux-flow transport since the flux-flow resistivity is a measure of QP dissipation in the vortex dynamics. To investigate the correlation between the superconducting gap structure and vortex dynamics, we measured the in-plane anisotropy of the flux-flow resistivity for k-(ET)2Cu(NCS)2. As a result, we observed a clear in-plane fourfold-symmetric anisotropy, reflecting the d-wave superconducting gap symmetry, in the flux-flow resistivity. The experimental results highlighted the need for theoretical work to understand the role of dissipation in anisotropic vortex dynamics in layered superconductors with d-wave pairing symmetry [S. Yasuzuka et al., J. Phys. Soc. Jpn. 82, 064716 (2012)].