Charge density wave quantum critical point with strong enhancement of superconductivity

Abstract

led to a great interest in QCPs are the very unusual properties that are observed near a QCP, such as unconventional superconductivity (SC), non-Fermi-liquid or anomalous critical behaviour 3-6 . Quantum fluctuations are therefore suspected to cause these anomalous properties, but most aspects are far from being understood and are still the subject of active discussions. An issue that has attracted great interest is the appearance of unconventional SC observed at magnetic QCPs, that is where a magnetic ordered state is suppressed. Such unconventional SC states have been reported for very different kinds of compounds, for example heavy-fermion systems 3,7 , cuprates 8,9 and iron pnictides 10,11 . There is some evidence that backs up the hypothesis that the binding of electrons into the superconducting Cooper pairs is not mediated by phononic excitations as in classical superconductors, but by magnetic excitations connected to the disappearing magnetic order 12,13 . The fact that SC is observed only in the vicinity of the QCP and presents a strong dependence on the tuning parameter supports this hypothesis. It often results in a dome-like shape of the phase diagram of tuning parameter versus temperature.However, QCPs are not restricted to magnetic systems; they can be associated with any continuous phase transition. While searching for appropriate non-magnetic systems that show a QCP with associated SC, we looked at compounds that show a charge density wave (CDW) type of structural transition. CDW systems present an instability of the electronic states close to the Fermi level F , which results in a modulation of the electronic charges below a transition temperature T CDW (ref. 14). The modulation periodicity is usually associated with a nesting vector of the Fermi surface. Therefore, a gap opens in the electronic density of states (DOS) at F , N ( F ), below T CDW . In one-dimensional (1D) systems, for which such a transition was initially proposed, this effect changes the character of the system from metallic for T > T CDW to insulating for T < T CDW . In 2D or 3D systems, the gap opens only on a part of the Fermi surface and, therefore, the conductivity remains metallic below T CDW . The opening of the gap does, however, lead to a very characteristic upturn of the resistivity ρ(T ) at T CDW .Some CDW transitions can be tuned to a T = 0 critical point as well [15][16][17][18][19][20][21] . The appearance or a strengthening of a superconducting state is frequently found near the vanishing point of the CDW state, similar to magnetic QCPs. Yet, in this case, it can easily be explained within the standard BCS-based theories: the gap associated with the CDW closes at the critical point, resulting in an increase of N ( F ), which, in turn, leads to an increase of the SC transition temperature T c . As a consequence, the dependence of T c on the tuning parameter is usually rather weak in the non-CDW regime beyond the critical point 16,17,[20][21][22][23] . There is currently no clear evidence that quantum critical fluc...

Evolution of charge density wave order and superconductivity under pressure in LaPt2Si2

“…In addition, it has been proposed that the accumulation of entropy near a quantum critical point (QCP), due to quantum critical fluctuations, may give rise to novel phases such as supercon-ductivity [15]. However, one difference between a magnetic QCP in strongly correlated electronic systems and that of a QCP in CDW compounds, is that non-Fermi liquid behaviors seem to be absent in the vicinity of a CDW QCP [1][2][3]. Furthermore, the observation of CDW domain walls above the superconducting dome and the separation of the CDW QCP and superconductivity in pressurized 1T -TiSe 2 seems to challenge the view that the appearance of a superconducting dome is associated with the CDW QCP [16].…”

Unconventional superconductivity in CuxBi2Se3 from magnetic susceptibility and electrical transport

“…As seen in figure 4, the χ(T) shows an upward behavior below 50 K, which can be attributed to a tiny amount of paramagnetic defects in non-magnetic or weakly-correlated magnetic systems [8,30]. Such upturn in χ(T) is also observed in other non-magnetic materials, for instance, Lu(Pt 1−x Pd x ) 2 In [8] and LaPtGe 3 (T c = 1.2 K) superconductors [31]. Furthermore, we find the χ(T) curve between 5 K and 50 K could be well fitted by a modified Curie-Weiss law [32][33][34], reading…”

“…The drop of χ across the kink temperature (T k ) implies a decrease of electronic DOS at Fermi energy (E F ). The quasilinear behavior of χ above the T k and its weak temperature dependence have been widely observed in some CDW materials like Cu x TiSe 2 [7], Lu(Pt 1−x Pd x ) 2 In [8], and (Sr, Ca) 3 Ir 4 Sn 13 [9]. In Cu 0.09 Bi 2 Se 3 , the superconductivity is induced only by Cu intercalation, which is in favor of Cu 1+ non-magnetic state (as is in the case of interstitial site, see reference [23]).…”

“…The observed kink in χ(T) is likely to be attributed to the charge density anomaly, or possibly the CDW transition, below which the Fermi surface (FS) is partially or fully gapped resulting in the reduction of DOS at E F . For this reason, the resistivity will sometimes display an upturn feature when the temperature crosses the T k [7][8][9]. However, other factors, like changes in the effective carrier mass and the scattering rate [9], shall also be taken into account to interpret this upturn.…”

Extent of Fermi-surface reconstruction in the high-temperature superconductor HgBa 2 CuO 4+ δ

“…The common characteristic temperatures suggest the features in ρ(T ) are related to the sign-change of the Hall effect, and thus Fermi-surface reconstruction. The behavior of ρ(T ) is reminiscent to that associated with phase transitions that open a gap on the Fermi-surface such as charge-or spin-density waves (26)(27)(28)(29). In these systems, the initial increase in ρ(T ) is interpreted as a reduction in charge carriers by a gap opening.…”