A brief review of some of the arguments pointing towards the possibility of organic conductors of the form {alpha}-(BEDT-TTF)&amp;J3g(SCN)4 (where M=K, T1 and Rb) being candidates for Frohlich superconductivity is given

Ardavan, A.

Balicas, Luis

Brooks, John S.

Goddard, Paul

Harrison, Neil

Mielke, Charles H.

Montgomery, Lawrence

Narduzzo, A.

Singleton, John

Tokumoto, M.

UNT Digital Library

LA-UR- Approved for public release: distribution is unlimited. Title: Author(s). Submitted fo 3ossible F6hlich Superconductivity in Strong Magnetic -ields / / Shades H. Mielke, Neil Harris‘on, A. Ardavan, Paul Goddard John Singleton: A. Nardu’zzo, Lawrence Montgohery Luis Balicas, John S. €3rooks, M. Tokumoto / Yamada Conference LVI , the Fourth International Symposium on Crystalline Organic Metals, Superconductors and Ferromagnets Los Alamos NATIONAL LABORATORY Los Alamos National Laboratory, an affirmative actiodequal opportunity employer, is operated by the University of California for the US. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes thatthe US. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for US. Government purposes. Los Alarnos National Laboratory requests that the publisher identify this article as work performed under the auspices of tho US. Department of Energy. Los Alarnos National Laboratory strongly supports academic freedom anda researcher’s right to publish: as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness. Form 836 (8/00) Yamada Conference LVI, the Fourth International Symposium on Crystalline Organic Metals, Superconductors and Ferromagnets ISCOM 2001 - Abstract Number Harrison Possible Frohlich superconductivity in strong magnetic fields CH Mielke”*, N Harrisona , A Ardavanb, P Goddardb, J Singletonb, A Narduzzob; L K  Montgomery‘, L Balkad, JS Brooksd and M Tokumotoe i ’  a National High Magnetic Field Laboratory, LANL, Los Alamos, New Mexico 87544 The Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK Department of Chemistry, Indiana Univ., Bloomington, Indiana 47405 National High Magnetic Field Laboratory, Florida State Univ., Tallahassee, Florida 32306 Electrotechnical Laboratory, tsukuba, Ibaraki 305, Japan b d * Tel; +1 505 6651500; Fax: +1 505 66543 11; E-mail: crnielke@,lanl.gov Abstract A brief review of some of the arguments pointing towards the possibility of organic conductors of the form a-(BEDT-TTF)&J3g(SCN)4 (where M=K, T1 and Rb) being candidates for Frohlich superconductivity is given. Keywords: superconductivity, high magnetic fields, one. dimensional The presence of a magnetic field within a sample generally inhibits superconductivity [l]. In rare cases where superconductivity is observed to be induced by an external magnetic field, this occurs only because the external magnetic field compensates an internal antiferromagnetic exchange field, as recently realized in h-(BETS)2FeC14 [2]. The superconductivity is therefore not field-induced with respect to the intrinsic magnetic field seen by the Cooper pairs. Charge transfer salts of the form a-(BEDT-TTF)&IHg(SCN)4 (where M=K, TI and Rb) could potentially be the first examples of true field-induced superconductors [3]. A number of experimental observations are shown to be consistent with a superconducting state existing at fields exceeding Bk - 23 T for M =  K or 32 T [ M I  for M= Rb [6], while experiments employing pSR and NMR techniques find little evidence for an intrinsic magnetic field [7,8]. More unusually, a-(BEDT-TTF)&fHg(SCN)4 salts are thought to possess charge-density wave (CDW) ground states at low magnetic fields [9-131. According to mean field theory, the phase that exists at fields above Bk (where properties akin to superconductivity are observed) is expected to be an exotic form of CDW where the up-spin and down-spin components ofthe quasi-one-dimensional Fermi surface sheets nest independently [9,14]. This invokes the question as to whether a CDW under these previously unprecedented conditions can develop some of the characteristics of a superconductor in the manner originally predicted by Frohlich ~ 5 1 .  The above question results after approximately ten years of research on these materials by several different groups. The first order phase transition that takes place at Bk is thought to be caused by the low magnetic field, low temperature CDW phase reaching its Pauli paramagnetic limit [9,11,12]. At magnetic fields higher than Bk, the free energy of a conventional CDW state becomes higher than that ofthe normal metal or a CDW phase in which the separate spin Fermi surface sheets nest independently [4,9,14]. The existence of magnetic hysteresis in the form of an offset between rising and falling fields, at all fields exceeding Bk, has been known for some time [16]. Only recently, however, has this hysteresis been shown to be connected with the eddy currents that are observed in pulsed magnetic fields [5,6,17]. The presence of currents becomes especially clear when magnetic hyteresis measurements made using magnetic torque [6] are compared with magnetic measurements made on type I1 superconductors [18] in Fig. 1. Although the field is cycled over a short interval at very high magnetic fields in Fig. lb, the basic form of the hysteresis loop remains the same as that in Fig. la. Notably, the susceptibility p0dMldB is largest and diamagnetic immediately after the sweep direction of the external magnetic field is reversed. On extracting this slope dMldB from many hysteresis loops over a wide range of magnetic fields [6], the susceptibility (thus obtained in Fig.2a [6]) is found to agree with the results of ac susceptibility measurements in Fig 2b [ 5 ] .  Small differences in the magnitude of the 2 9 . 2  29.3 2 9  . 4  P p  ( T I  Fig: 1. Comparison of measurements of the irreversible magnetization made on lead at low magnetic fields ahd- -4.2 K (a) [ 181 with those on I~-(BEDT-TTF)~KH~(SCN)~ at high magnetic fields and -80 mK @) [ 6 ] .  susceptibility results from the fact that each of the samples in Figs. 2a and 2b are from different batches. Nevertheless, the existence of a hysteretic magnetization exhibiting precisely the same behaviour as those observed in type I1 superconductors combined with a strong diamagnetic response in ac susceptibility measurements firmly establishes the existence of persistent currents of a critical magnitudej, [6 ] .  The persistence of the currents at all magnetic fields above Bk to fields in excess of 60 T rules out explanations involving the quantum Hall effect or phase transitions involving changes in Landau diamagnetism. The B c o s e  ( T )  2 5  3 0  0 . o  -0 .I h 8 - 0 . 2  -0.3 v x -0.4 0 g -5 -1  0 ............... ac susceptibility, 10 kHz I-# I 2 5  3 0  Bcose yr) Fig. 2. Comparison of the real x' and imaginary x" components of the susceptibility extracted fiom measurement of the slope of the magnetic torque at -80 mK (a) [ 6 ]  and ac susceptibility at -0.5 K (b) [5]. 8 is the angle between the applied magnetic field and the normal to the conducting planes. 3 - magnitude of the ac susceptibility is also several orders of magnitude too large to be explained in terms of Landau diamagnetism. Given that this type of behaviow is only observed in type I1 superconductors with pinning [I. 81, superconductivity, of some form, appears to be the only reasonable explanation. Oscillations in the density of states of a closed Fermi surface pocket (often referred to a- pocket) appear to modulate the magnitude of the ac susceptibility in Fig. 2a in a manner that has yet to be understood. The ac suscpetibility is largest at integer filling factors, when the chemical potential is situated in a Landau gap, though present at all filling factors at fields above B k .  \ '  Were superconductivity, of some form, involved, we should expect to observe a dramatic increase in conductivity (or reduction in resistivity) at the same low temperatures where persistence currents are observed inductively. Quantum oscillations in the magnetoresistance have been known to be unusual for some time [19,20]. However, when the resistivity is plotted versus temperature Tat  a fixed magnetic field [3] in Figs. 3a and 3b, a dramatic drop in resistivity can be seen to occur (by as much as 2 orders of magnitude) at the same fields where the ac susceptibility is maximum, and for all current directions [3]. A lesser drop occurs at all other fields. The solid lines represent a model in which a sharp superconducting transition is broadened by guassian. Fits to this model appear to be surprisingly good, and suggests that the form of the resistivity at temperatures below approximately 3 K has the form of a broadened superconducting transition. This could be consistent with an inhomogeneous superconducting phase. Figure 4a shows the results of fitted values of the midpoint of the transition T, and the width of the transition AT, plotted versus B: good fits are only obtained at fields above Bk. One can conclude from this plot that the width of the transition (in addition to the magnitude of the ac susceptibility) is modulated by the density of states of the a-pocket. The thermodynamic midpoint of the transition T,, however, appears not to be modulated at all. A T, that is independent of the density of states of the a-pocket implies that the order parameter is only associated with the quasi-one-dimensional Fermi surface sheets: the same Fermi surface sheets that nest when the low magnetic field, low temperature CDW phase is formed. There is therefore no doubt that the thermodynamic ground state at high magnetic fields is primarily 4 t l " " '  G- I , I , ,  i ...,. . 1 a' I a Fig. 3.  Temperature dependence of the in-plane resistivity (a) of a-(BEDT-TTF)2KHg(SCN)4 extracted from skin depth measurements and inter-plane resistivity (b) made using conventionai 4-wire techniques [3]. At integer filling (where the resistivity drop and ac susceptibility are greatest), the chemical potential is situated in a gap between Landau levels of the a-pocket. 10 T, (2nd order) 1 1  I Fig. 4. (a) The transition T, (solid line) extracted from the fits (using the method described in the text and in more detail in Ref. [3]), together with dashed lines corresponding to T, + ATJ2 and T, - ATc/2. @) The phase diagram after including T, from the fits. associated with the quasi-one-dimensional sheets. This observation is consistent with the mean field theory of CDW’s [9,14]. The physical properties of this high magnetic field phase, however, closely resemble those of a superconductor, and are not at all like those of a CDW. It is for these reasons that we propose a-(BEDT-TTF)&fHg(SCN)4 as a strong candidate for Frohlich [ 151 superconductivity. On combining the T, data (solid line) with existing data on the transition Tp into the CDW phase [4,13] (solid line) and the first order transition field Bk (dotted line), all three lines can be seen to meet at a tri-critical point. This work is supported by the National Science Fouridation, Florida State and the Department of Energy. \ .  [ 11 M. Tinkham, Introduction to Superconductivity; 2nd ed., McGraw-Hill, New York, 1996. Mater. Sci. Eng. A 262 (1999) 9. [2] S .  Uji et al, Nature 410 (2001) 908 [3] N. Harrison et al, J. Phys. Condens, Mat. 13 (2001) L389. [4] N. Harrison et al, Phys. Rev. B 62 (2000) 14212. [5] N. Harrison et al, Phys. Rev. Lett. 86 (2001) 1586. [6] A. Ardavan et al (to be published). [7] F. L.,Pratt et al, Phys. Rev. Lett, 74 (1995) 3892. [8] K. Miyagawa, A, Kawamoto, K. Kanoda, Phys, Rev. B 56 (1997) R8487. [9] R. H. McKenzie, cond-mat/9706235 (unpublished). [lo] N. Biskup et al, Solid State Commun. 107 (1998) 503. [ l l ]  N. Harrison, Phys. Rev. Lett. 83 (1999) 1395, E121 J. S. Qualls et al, Phys. Rev. B 62 (2000) 10008, [13] M. V. Kartsovnik et al, Synth. Met.120 (2001) 687. [14] D. Zanchi, A. Bjklis, G. Montambaux, Phys. Rev. B 53 (1996) 1240. [15] H. Frohlich, Proc. R. SOC. London, Ser. A 223 (1954) 296. [16] P. Christ et al, Physica B 204 (1995) 153. [17] N. Harrison et al, Phys. Rev. Lett. 77 (1996) 15’76. [18] C. P. Bean, Rev. Mod. Phys. 36 (1964) 31. [19] S. Hill et al, Phys. Rev. B 55 (1997) R4891. E201 M. M. Honold et al, J. Phys. Condens. Mat. 9 (1997) L533. 6 

Possible Frohlich superconductivity in strong magnetic fields.

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Possible Frohlich superconductivity in strong magnetic fields.

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