The low-temperature highly correlated quantum phase in the charge-density-wave 1T-TaS2 compound

Abstract

A prototypical quasi-2D metallic compound, 1T-TaS 2 has been extensively studied due to an intricate interplay between a Mottinsulating ground state and a charge-density-wave order. In the low-temperature phase, 12 out of 13 Ta 4+ 5d-electrons form molecular orbitals in hexagonal star-of-David patterns, leaving one 5d-electron with S = ½ spin free. This orphan quantum spin with a large spin-orbit interaction is expected to form a highly correlated phase of its own. And it is most likely that they will form some kind of a short-range order out of a strongly spin-orbit coupled Hilbert space. In order to investigate the low-temperature magnetic properties, we performed a series of measurements including neutron scattering and muon experiments. The obtained data clearly indicate the presence of the short-ranged phase and put the upper bound on~0.4 µ B for the size of the magnetic moment, consistent with the orphan-spin scenario.npj Quantum Materials (2017)2:42 ; doi:10.1038/s41535-017-0048-1 INTRODUCTIONThe instability of charge density waves (CDW) found in lowdimensional electron systems of layered materials has attracted enormous attention recently. 1T-TaS 2 is a prototypical quasi-2D metallic compound with a strong electron-phonon coupling responsible often for CDW instabilities. Upon cooling, it undergoes a series of first-order phase transitions to CDW, Mott and superconducting phases (see e.g., ref. 1 and references therein).1T-TaS 2 bulk crystal has a lamellar structure, with each layer composed of a triangular lattice of Ta atoms which is then sandwiched by S atoms in an octahedral TaS 6 coordination with a weak Van der Waals bonding between the layers. Above~540 K, the structure is unmodulated trigonal with P-3m1 symmetry; below which the triangular lattice exhibits a series of structural modulations.2, 3 First, an incommensurate CDW phase sets in at T 540 K. Upon further cooling, the structure changes to a nearly commensurate phase at T nCDW~3 50 K. Finally, the material turns into a Mott insulating phase with in-plane √13 × √13 superlattice distortion, which coexists with a commensurate CDW phase below T CDW~1 80 K. Superconductivity emerges in 1T-TaS 2 below~2 K by introducing disorders.4 Recent angle-resolved photoemission spectroscopy experiments suggest that the melted Mott state and the superconductivity coexist in real space 5 providing better understanding of the interplay between electron correlation, charge order, and superconductivity. Unlike other Mott insulators, the CDW superlattices play the role of localization centers in the ground state of 1T-TaS 2 .According to the current understanding, the CDW phase is composed of molecular orbitals of 13 Ta atoms, forming a hexagonal pattern of so-called David-star clusters. As proposed more than three decades ago, 6 each Ta 4+ of 1T-TaS 2 provides one 5d electron, and thus there are 13 5d-electrons per each David star. Out of these 13 electrons, 12 electrons form 6 covalent bonds

Friction anomalies at first-order transition spinodals: 1T-TaS2

“…That very reproducible hysteresis pattern, partly reproduced from [7] in figure 5, suggests that T NC and T CT are, to a good approximation, spinodal points of 1T-TaS 2 . The spinodal nature is strongly confirmed by very recent heat capacity data by Kratochvilova et al, [23] showing no anomaly at T NC and T CT , despite the large electrical and structural bulk transformations. The absence of a heat capacity anomaly at both T NC and T CT is natural because the spinodal transformation occurs, upon temperature cycling ±ΔT, only once, thus averaging all internal energy effects to zero after the first passage.…”

Vanishing Wilson ratio as the hallmark of quantum spin-liquid models

“…These quantities are mostly extracted from experiments on KL systems, the prominent example being herbertsmithite [27][28][29][30][31], but also related compounds in the same class [32][33][34][35][36][37]. Another example are organic compounds where the relevant lattice is triangular [38][39][40][41] as well as charge-density-wave system 1T-TaS 2 , recently established as SL with composite S = 1/2 spins on TL [42][43][44][45]. The basic spin exchange scale in most of these systems is modest and, as a consequence, the whole T range is experimentally accessible which allows for the test of the whole range of spin excitations.…”

Similarity of thermodynamic properties of the Heisenberg model on triangular and kagome lattices

“…Another class are organic compounds, as κ-(ET) 2 Cu 2 (CN) 3 [16][17][18][19], where the spins reside on a triangular lattice. Recently, the charge-density-wave system 1T-TaS 2 , which is a Mott insulator without magnetic LRO and shows spin fluctuations at T > 0 [20][21][22][23], has been added into this family.…”