Gapless excitations in the ground state of 1T−TaS2

Abstract

1T-TaS2 is a layered transition metal dichalgeonide with a very rich phase diagram. At T=180K it undergoes a metal to Mott insulator transition. Mott insulators usually display anti-ferromagnetic ordering in the insulating phase but 1T-TaS2 was never shown to order magnetically. In this letter we show that 1T-TaS2 has a large paramagnetic contribution to the magnetic susceptibility but it does not show any sign of magnetic ordering or freezing down to 20mK, as probed by µSR, possibly indicating a quantum spin liquid ground state. Although 1T-TaS2 exhibits a strong resistive behavior both in and out-of plane at low temperatures we find a linear term in the heat capacity suggesting the existence of a Fermi-surface, which has an anomalously strong magnetic field dependence.The elusive quantum spin liquid (QSL) state has been the subject of numerous papers since Anderson suggested the resonating valence bond (RVB) as the ground state of the S = 1/2 quantum Heisenberg antiferromagnet on a triangular lattice [1]. A QSL is an exotic state of matter with an insulating ground state that does not break the crystal symmetry. The spins are highly entangled, and quantum fluctuations prevent magnetic ordering down to absolute zero T=0K. Theoretically, it has been suggested that a QSL can support exotic spinon excitations [1-3].The experimental search for QSL materials has been focused on S = 1/2 quantum spins on frustrated lattices with triangular motifs. Despite extensive research, very few candidates exist: the 2D kagomé hebertsmithite and vesignieite [4,5], the 2D triangular lattices κ-(BEDT-TTF) 2 Cu 2 (CN) 3 , EtMe 3 Sb[Pd(dmit) 2 ] 2 and YbMgGaO 4 [6-12], and the 3D hyperkagomé Na 4 Ir 3 O 8 [13,14].Recently, 1T-TaS 2 has been suggested to have a QSL ground state [15]. 1T-TaS 2 has been a major subject of interest for over 40 years owing to its very rich phase diagram, arising from strong electron-electron and electron-phonon couplings [16,17]. At high temperatures (> 550K) the system is metallic, and it undergoes a series of charge density wave (CDW) transitions as the temperature is lowered. It can even become superconducting when subjected to external pressure or chemical disorder [18,19].Despite the seemingly complicated electronic properties, 1T-TaS 2 has a simple crystal structure composed of weakly bound van der Waals layers; each layer contains a single sheet of Tantalum (Ta) atoms, sandwiched in between two sheets of Sulfur (S) atoms in an octahedral coordination. The Ta atoms within each layer form a 2D hexagonal lattice.The basic CDW instability is formed within the Ta layers by the arrangement of 13 Ta atoms into a "Star-of-David" shaped cluster, where 12 Ta atoms move slightly inwards towards the 13th central Ta atom. In the temperature range of ∼350-550K the system is in the incommensurate (IC) CDW phase. When cooled below ∼350K, the system is in the nearly commensurate (NC) state, where the CDW clusters lock-in to form commensurate domains, and the domains in turn form a triangular lattice. The size of the...

Mottness collapse without metallization in the domain wall of the triangular-lattice Mott insulator1T−TaS2

“…where i,j are site indices, σ =↑, ↓ is spin, c † iσ are electron creation operators, n iσ = c † iσ c iσ are the local electronoccupancy operators, t ij are the hopping constants between nearest-neighbors, i are the on-site potentials, and U i are the Hubbard repulsion parameters. Down to the lowest temperatures, TaS 2 does not order magnetically [6,9]. A simple approximation to describe this experimentally observed paramagnetic Mott insulating state, theoretically predicted to exist in a range of U between the paramagnetic metal and the 120 • Néel ordered state [38,39], is the DMFT [29,40].…”

“…Remarkably, their combination can be found even in simple material systems, such as transition metal dichalcogenide (TMD) van der Waals compound 1T-TaS 2 . The ground state is a Mott insulator [1][2][3] with CDW [4] and unconventional quantum spin liquid behavior [5][6][7][8][9][10]. Upon Se substitution [11], Fe intercalation [12], or by applying pressure [13] it becomes superconducting, with both CDW and correlated behavior still present.…”

Effect of quenched disorder on the quantum spin liquid state of the triangular-lattice antiferromagnet 1T−TaS2

“…The former includes both localized and itinerant excitations, while the latter sensitively probes elemen- tary itinerant excitations and is totally insensitive to localized ones such as those responsible for Schottky contributions, which contaminates the heat capacity measurements at low temperatures. The T -linear term in the heat capacity γT has been reported, indicating the presence of gapless excitations [33]. The measurements of the thermal conductivity, on the other hand, has reported no discernible residual linear term in zero field, κ 0 /T = κ/T (T → 0) = 0, indicating the absence of itinerant magnetic excitations [34].…”

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

“…In this layer compound a well known first-order, hysteretic transition of joint structural and electronic nature takes place between a low-temperature commensurate charge-density-wave (CCDW) 13 13 phase, [6,7] believed to be Mott insulating [8,9], and a nearly commensurate charge-density-wave (NCCDW) phase, metallic and even superconducting under pressure [7] and alloying with 1T-TaSe 2 [10]. Notably, 1T-TaS 2 has been subject to very intense studies over the last decade, in connection with transient or hidden metastable phases under high excitation, [11][12][13] with the unusual substrate, thickness, and disorder dependence of its transitions [14][15][16][17], and with a spin liquid in the CCDW phase [18][19][20]. While insensitive to these electronic excitations, the present frictional study sheds fresh light on the thermodynamic nature of the important CCDW-NCCDW transformation.…”