Pis'ma v ZhETF, vol. 97, iss. 7, pp. 485-490 © 2013 April 10
Role of anion ordering in the coexistence of spin-density-wave and superconductivity in (TMTSF)2ClO4
Ya. A . Gerasimenko+1), V. A . Prudkoglyad+, A . V. Kornilov+, S. V. Sanduleanu+*, J. S. Quallsx, V. M. Pudalov+*
+ Lebedev Physical Institute of the RAS, 119991 Moscow, Russia * Moscow Institute of Physics and Technology, 141700 Dolgoprudny, Russia x Sonoma State University, Rohnert Park, CA 94928, USA Submitted 13 March 2013
Using various transport and magnetotransport probes we study the coexistence of spin-density wave and superconductor states in (TMTSF)2 CIO4 at various degrees of CIO4 anions ordering. In the two-phase complex state when both superconductivity and spin-density wave are observed in transport, we find prehistory effects, enhancement of the superconducting critical field, and strong spatial anisotropy of the superconducting state. These features are inconsistent with the conventional model of structural inhomogeneities produced by anion ordering transition. We reveal instead that superconductor and spin-density wave regions overlap on the temperature - dimerization gap V phase diagram, where V is varied by anion ordering. The effect of anion ordering on (TMTSF)2 ClO4 properties is thus analogous to that of pressure on (TMTSF)2X (X=PF6 or AsF6), thereby unifying general picture of the coexistence of superconductivity and spin-density wave in these compounds.
DOI: 10.7868/S0370274X13070096
1. Introduction. For many unconventional superconductors including recently discovered Fe-based pnic-tides [1] the P-T phase diagram contains overlapping regions of the superconductor (SC) and antifer-romagnetic insulator phase (here P is either external pressure, or internal "chemical" pressure, induced by chemical substitution or doping). The coexistence of the two phases causes unusual superconducting properties in the overlap region. Of a particular interest are quasi one-dimensional (Q1D) organic compounds of the (TMTSF)2X-family [2]. The (TMTSF)2PF6 (hereafter PF6) was found to demonstrate rich physics near the spin-density wave (SDW) endpoint [2]. In the vicinity of the SDW transition the PF6 bulk homogeneous state is spontaneously split into SDW and metal/SC areas [3-5] with substantially enhanced superconducting critical field [6] and of strongly anisotropic shape [7]. The unusual SC behavior is believed to be due to curious properties of the SDW phase [8-10]. The latter is governed by nesting of the Fermi surface (FS), which easily occurs in Q1D compounds with two slightly corrugated FS sheets (cf. Fig. 1a). Application of pressure increases the dimensionality of electron dispersion (via increasing the t'b = t2/ta transfer integral), which spoils nest-
a) (b)
Fig. 1. (Color online) (a) - Fermi surface of PF6 or ClO4 in the absence of anion ordering. The wave vector Q0 nests opposing sheets of the Fermi surface. (b) - Fermi surface of ClO4 in presence of anion ordering. Dashed line indicates the new Brillouin zone boundary due to doubled periodicity along 6-axis. Now there are four sheets, which can be nested with Qo only when the splitting V is small. (c) -Sketch of the ky bandwidth measured by AMRO with or without anion ordering. (d) - One sheet of the Fermi surface with the orbit (red line), that corresponds to the magnetic field aligned in the direction of zeroth order AMRO peak. The direction depends on ky bandwidth
e-mail: gyaroslav@sci.lebedev.ru Письма в ЖЭТФ том 97 вып. 7-8 2013
ing and shifts formation of SDW to progressively lower temperatures.
In contrast, for apparently similar (TMTSF)2ClO4 (hereafter ClO4) compound, the SC/SDW coexistence is completely different. ClO4 anions lack inversion symmetry, possess a dipole momentum, and experience ordering transition at TAO = 24 K. The transition doubles the lattice along 5-axis and splits the Fermi surface in four open sheets (see Fig. 1b). The latter spoils nesting and suppresses SDW phase giving rise to the onset of superconductor state [2]. Due to finite time kinetics [11] of the anion ordering transition, the degree of anion ordering can be varied by adjusting the cooling rate in the vicinity of TAo. This novel parameter enables to reveal a rich phase diagram, where SDW phase is favored for strong disorders, SC and SDW coexist at intermediate and SC sets in for weak disorders [12]. Earlier measurements with ClO4 did not reveal any features similar to those for PF6 and suggested that the SC/SDW coexistence is due to superconducting anion-ordered inclusions in the SDW insulating disordered background [12]. This was further supported by extensive X-ray measurements, where anion-ordered inclusions were indeed observed [11].
It was shown later, that the lower the SDW onset temperature, TgDw, is, the stronger it is affected by magnetic field H||c [13-15], consistent with the conventional mean-field description of SDW [16]. This, however, implies that some parameter, that spoils FS nesting, increases when TgDW decreases. The latter occurs with lowering disorder, when only the FS splitting V is expected to change. Indeed, V plays the role similar to t'b in PF6. For the strongest disorder the Fermi surface for ClO4 is similar to that for PF6 and can, thus, be nested (cf. Fig. 1a). Gradual ordering of anions splits the FS, and V represents the deviation from perfect nesting (cf. Fig. 1b). Imperfect nesting decreases TgDW, until, at some disorder, the deviation reaches a critical value and SDW vanishes.
This simple picture is nicely supported by the mean-field calculations of the TgDW(V) phase diagram [17, 18]. The similarity of SDW phase diagram to that of PF6 suggests, that the remarkable features of the SC and SDW coexistence should be observed also in ClO4, in contrast to a previously suggested picture of structurally inhomogeneous granular SC state [12]. Therefore, to resolve these conflicting views, it is necessary to study in detail the SC and SDW phase diagram as a function of V. Since V is expected to be spatially inhomogeneous due to structural disorder [11], it cannot be characterized by the residual resistivity or cooling rate at TAO and should be measured directly.
In this paper we use several transport probes to characterize in real and momentum space the SDW and metal/SC coexistence for variable disorder. We use angular magnetoresistance oscillations for field rotated in the a-c-plane to probe the FS and trace changes in V. We indeed observe that V is reduced as Tsdw rises with increasing disorder, in accord with the theoretical phase diagram [17, 18]. We find that the SC phase is strongly spatially anisotropic: while transport along c-axis manifests survival of the SC state, the a-axis conduction turns out "insulating" with increasing disorder. For strong disorders we observed hysteresis in R(T) behavior and enhancement of superconducting critical field, similar to that observed in PF6. These results clearly demonstrate that the metal/SC and SDW regions do overlap on the T-V phase diagram and their coexistence is driven by the changes of splitting V varied with the degree of anion ordering, rather than by structural inhomogeneities produced at anion-ordering transition.
2. Experimental. The single crystals of typical dimensions of 1 x 0.15 x 0.05 mm3 were synthesized using a conventional electrochemical technique. Eight 10 ¡m annealed Pt wires were glued with conducting graphite paint on two opposite crystal faces normal to c-axis. Rxx and Rzz resistances were measured along a and c-axes correspondingly using the conventional 4-wire AC technique. Samples were mounted on the 3He one-axis rotator and cooled down to 40 K at the rate of 0.3K/min to avoid cracks. Initially, to achieve the strongest disorder (d1) a heat pulse was applied to the continuously cooled 3He bath at 1.3 K. This resulted in the bath heating well above TAo and consequent rapid cooling at the rate of «100K/min in the vicinity of Ta0 . All the subsequent weaker disorders were obtained by annealing a sample at temperatures lower than TAO [12]. The annealing procedure consisted in warming the sample to a certain temperature at the rate of 0.2 K/min and then cooling it back at the same rate. The highest annealing temperature corresponding to disorder d5 was 22 K. To achieve the anion-ordered state a sample was cooled from 40 to 20 K at the rate of 10mK/min.
Samples were aligned in magnetic field by virtue of the upper critical field, Hc2, anisotropy; in the a-c crystal plane Hc2 is the highest for H||a and the lowest for H||c. Therefore, a-direction was determined using the resistance drop due to the onset of superconductivity on the rotation curve at a field value slightly lower than Hc2||a, and c-direction was set 90 degrees away.
3. Results and discussion. Figure 2 shows the intralayer Rxx temperature dependence for various disorders. For strong disorders the sample undergoes SDW
nacbMa b «BT® tom 97 Bbm.7-8 2013
transition (Tsdw = 6.6 K for d1), followed by insulating temperature dependence of Rxx for lower temperatures. Upon decreasing disorder, SDW is suppressed, as seen from TSDW decrease. For disorders d3, d4 transition becomes even more smeared, however the low-temperature behavior remains insulating and indicates the presence of SDW.
For strong disorders d1, d2, the Rxx(T) behavior is insulating for T < TSDW, although there is a kink at « 1.1 K which is followed by a slower growth of resistance. For weaker disorders d3, d4 the kink is followed by resistance drop. Rxx behavior below T < 1.1 K changes from yet insulating (d3) to already metallic (d4), however resistance stays finite down to the base temperature 0.4 K. Application of low magnetic field suppresses this feature and restores insulating behavior at low temperatures, as shown in the inset to Fig. 2. This indicates that
100; 10; U
a
B 0.1 !
0.01;
-3 ;
10
10
-A ■
_L
0.5 1.0 1.5 2.0 __I_
4
T (K)
Fig. 2. (Co
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.