Effects of pressure on the ferromagnetic state of the charge density wave compound SmNiC2

Abstract

We report the pressure response of charge-density-wave (CDW) and ferromagnetic (FM) phases of the rare-earth intermetallic SmNiC2 up to 5.5 GPa. The CDW transition temperature (TCDW ), which is reflected as a sharp inflection in the electrical resistivity, is almost independent of pressure up to 2.18 GPa but is strongly enhanced at higher pressures, increasing from 155.7 K at 2.2 GPa to 279.3 K at 5.5 GPa. Commensurate with the sharp increase in TCDW , the first-order FM phase transition, which decreases with applied pressure, bifurcates into the upper (TM1) and lower (Tc) phase transitions and the lower transition changes its nature to second order above 2.18 GPa. Enhancement both in the residual resistivity and the Fermi-liquid T 2 coefficient A near 3.8 GPa suggests abundant magnetic quantum fluctuations that arise from the possible presence of a FM quantum critical point. Low dimensional metallic systems have attracted much interest because of their propensity towards an ordered phase. Density waves are prominent examples in quasione-dimensional compounds, where a large anisotropy in the Fermi surface leads to a structural instability accompanied by a periodic lattice distortion 1 . Sensitivity to the Fermi surface topology makes it relatively easy to tune the ordered phases via such external parameters as chemical doping, pressure, and magnetic fields. For the transition-metal dichalcogenide TiSe 2 , a CDW transition temperature is suppressed with increasing Cu intercalation and is intercepted by a dome of superconductivity centered around a projected critical concentration where the extrapolated T CDW becomes zero.2 External pressure acts similarly to suppress the CDW phase of TiSe 2 , inducing superconductivity in the vicinity of a projected CDW critical point.3 These results both by Cu intercalation and external pressure suggest that correlated electrons spontaneously adjust to a new emergent phase in the vicinity of a quantum critical point.Rare-earth intermetallic compounds ReNiC 2 (Re =La, Ce, Nd, Sm, Gd, Tb, Er) show various ground states of CDWs and magnetism.4,5 Among the intermetallics, SmNiC 2 is unique in that it becomes ferromagnetic, while other members are prone to an antiferromagnetic instability. X-ray scattering studies of SmNiC 2 reveal satellite peaks corresponding to an incommensurate wave vector (0.5, 0.52, 0) below 148 K, signaling formation of a charge-density wave.6 The abrupt disappearance of the satellite peak at the ferromagnetic transition temperature (T c = 17.4 K) indicates a destruction of the CDW phase and a strong correlation between the FM and CDW phases. First-principles electronic structure calculations find that Fermi-surface nesting is important for the CDW state and weaker nesting in the ferromagnetic phase leads to the destruction of the CDW below T c (ref. 7). Kim et al. recently estimated that hydrostatic pressure will enhance the CDW because the lattice constant of the Ni chain along the a-axis decreases faster than other axes, thus enhancing the Fermi surface nes...

Chemical and hydrostatic pressure effect on charge density waves of SmNiC2

“…According to a recent report of the transport properties of SmNiC 2 , T CDW increases with pressure, unlike typical CDW materials. In addition, another CDW transition occurs at lower temperature (T CDW2 ), which decreases with pressure [8]. This is also observed by the chemical pressure with other RNiC 2 systems (NdNiC 2 and GdNiC 2 ) [3].…”

“…This compound shows the competition between the charge density wave (CDW) and ferromagnetism (FM) [1][2][3][4][5][6][7][8]. Its CDW phase is developed below T CDW = 148 K and it completely disappears at T C = 17.5 K with the FM transition.…”

“…However, there has been little understanding of - Pressure is an important parameter for the control of not only the CDW phase but also magnetism or superconductivity. In particular, there have been many pressure experiments for transition metal chalcogenides [15][16][17] and some rare earth compounds [18][19][20], including SmNiC 2 [8], in order to investigate the competition between the CDW and magnetism or superconductivity. However, there has been little theoretical analysis to verify the competition mechanism and pressure dependent order parameter changes.…”

Tuning the ferromagnetic phase in the CDW compound SmNiC2 via chemical alloying

“…6(d), is close to R ln4 at about 150 K, comparable to the CDW transition temperature for SmNiC 2 . The magnetic entropy recovered at T C ( = 17.1 K) accounts for 80% of R ln2, the entropy expected for the doublet ground state of the J  = 5/2 multiplet for Sm 3+ (4f 6). Incomplete recovery of the entropy at T C could be ascribed to either the fluctuating valence between Sm 3+ and Sm 2+ or the Kondo screening effects of Sm 4 f spins by the itinerant electrons.…”

“…Study on the effects of hydrostatic pressure in SmNiC 2 has suggested that singular quantum fluctuations may create novel quantum phases in a vicinity of the projected FM quantum critical point (QCP) near 3.8 GPa (ref. 6). These novel phases, however, hide the presence of the QCP, requiring an alternative approach to probe the nature of the candidate FM QCP.…”