Pis'ma v ZhETF, vol. 100, iss. 2, pp. 134-140 © 2014 July 25
Multigap superconductivity in doped p-type cuprates
Ya. G. Ponomarev+1\ V. A. Alyoshin*, E. V.Antipov*, T.E. Oskina*, A.KrapF, S. V. Kulbachinskii+0, M. G. Mikheev+, M. V. Sudakova+, S. N. Tchesnokov+, L. M. Fisher°
+ Faculty of Physics, Lornonosov MSU, 119991 Moscow, Russia * Department of Chemistry, Lornonosov MSU, 119991 Moscow, Russia xInstitut fur Physik, Humboldt-Universität zu Berlin, D-12489 Berlin, Germany ° Lenin Russian Electrotechnical Institute, 111250 Moscow, Russia Submitted 29 May 2014
Andreev and tunneling spectroscopy studies ol Bi2Sr2Can_iCunC>2n+4+,5, HgBa2Can_iCunC>2n+2+,5 and Tl2Ba2Can_iCun02n+4+ä have shown that superconductivity in single-layer (n = 1) and two-layer (n = 2) phases has a single-gap character. Qualitatively different results were obtained for three-layer phases. In doped p-type Hg-1223, Bi-2223, and Tl-2223 samples two (or three) superconducting gaps were observed. The existence ol multigap superconductivity in superconducting cuprates with n > 3 is explained by a difference in doping levels ol outer (OP) and internal (IP) CuC>2-planes.
DOI: 10.7868/S0370274X14140112
1. Introduction. It has been found that the layered cuprate superconductors Bi2Sr2Ca n— 1 Cu„02„+4+<s, HgBa2Ca n—1 Cu and Tl2Ba2Can_iCu„02n+4+<s are natural superlattices of the type SISI... where S - superconducting blocks containing one or more of CuC>2-planes intercalated with calcium, I - insulating blocks (spacers) having a standard structure for a given cuprate family. Introduction of the excess oxygen Oa in the central part of spacers plays a key role in the formation of superconducting properties of CuC>2-blocks [1]. Within one superconducting phase with a predetermined number n of CuCVplanes the maximum superconducting transition temperature Tcmax can be reached by selecting a proper concentration of excess oxygen O^. Note that when (5 = 0 the above-mentioned compounds are Mott insulators with antiferromagnetic ordering of spins in CuC>2-planes. Excess oxygen binds electrons from CuC>2-layers, generating in them p-type charge carriers. A weak oxygen doping destroys the long-range antiferromagnetic order, causing a dielectric-to-metal transition. As a result an open hole Fermi surface is formed [1] with the Fermi level in the vicinity of an extended van Hove singularity (EVHS) with giant peaks in the quasiparticle density of states [2].
It is very important that the oxygen Oa does not create strong scattering centers in CuC>2-blocks, as it is located at a considerable distance from them. At the
-^e-mail: ponomarevy@mail.ru
same time, the excess oxygen forms charge traps in the center of spacers, creating favorable conditions for resonant tunneling in the c-direction [1]. At T <TC doped HTSC crystals behave like a stack of strongly coupled Josephson junctions and the superconducting current in the c-direction, therefore, has a Josephson nature (weak superconductivity).
High-temperature superconductivity is realized in the CuC>2-planes within a relatively narrow range of concentrations of impurity holes p. According to the photoemission spectroscopy a superconducting gap is maximal in T-M direction and minimal in T-Y direction [1, 2]. Anisotropy of the gap decreases significantly with increasing p [3].
Note that there is a possibility of the Fermi level pinning at the EVHS in a certain range of concentrations of impurity holes p. The critical temperature Tc changes with p by a parabolic law [4] and there is a scaling of the critical temperature Tc and superconducting gap A on doping [5].
According to Abrikosov a high critical temperature Tc in HTSC is realized mainly due to the presence of EVHS near the Fermi level [1, 2]. EVHS has been observed experimentally in cuprate superconductors by photoemission and tunneling spectroscopies [2, 5, 6]. In Abrikosov model a major role in the formation of the pairing potential is played by virtual optical phonons with small wave vectors k. Due to these phonons pairing carriers are kept in the vicinity of the EVHS ("forward" scattering). Abrikosov has shown that near op-
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timal doping a pre-exponential factor in the expression for Tc loses the Debye frequency causing a disappearance of the isotope effect. With departure from the optimal doping the Debye frequency reappears in the pre-exponential factor, and the isotope effect is restored. The latter corresponds to the experimental data.
In superconducting cuprates HgBa2Ca„_iCun02n+2+,s (HBCCO), Bi2Sr2Ca„_iCu„02„+4+«5 (BSCCO), and Tl2Ba2Ca„_iCu„02„+4+,s (TBCCO) phases Hg-1201, Bi-2201, and Tl-2201 contain a single Cu02-plane, phases Hg-1212, Bi-2212, and Tl-2212 contain two Cu02-planes and phases Hg-1223, Bi-2223, and Tl-2223 - three Cu02-planes. In HBCCO, BSCCO, and TBCCO superconducting Cu02-blocks are separated by insulating structural blocks (spacers) BaO-HgO^-BaO, SrO-BiO-BiO-SrO, and BaO-TlO-TlO-BaO respectively.
The problem of obtaining optimally doped cuprate samples with n > 3 by a standard method becomes complicated. For example, NMR-spectroscopy studies [7] showed that a copper nuclear magnetic resonance in HgBa2Ca„_iCu„02n+2+5 with n > 3 transforms into a doublet which was explained by different levels of oxygen doping in internal (IP) and outer (OP) Cu02-planes. This effect explains a non-trivial dependence of the critical temperature on the number n of Cu02-planes [8]. It is obvious that the appearance of defects in superconducting Cu02-planes will primarily cause smearing of the EVHS and, consequently, suppression of superconductivity. To maximize the critical temperature TCmax it is necessary to fulfill two conditions: i) the Fermi level should be aligned with the EVHS (using doping), ii) the structural perfection of Cu02-planes must be provided. Both of these conditions are fulfilled automatically when HTSC are doped with excess oxygen, which changes the concentration of holes in Cu02-plane being out of the plane (in the central part of the insulating blocks). It is extremely important that the excess oxygen has practically no effect on the mobility of impurity holes in superconducting Cu02-planes.
The results obtained in the present investigation are as follows: 1) the Andreev and tunneling spectroscopy studies showed that superconductivity in the optimally doped samples of Bi-2201 (Tc = 25 ± 3K), Hg-1201 (Tc = 93 ± 2K), Bi-2212 (Tc = 92 ± 2K), Tl-2212 (Tc = 105 ± 2K), and Hg-1212 (Tc = 120 ± 5K) has a single-gap character; 2) superconductivity in Bi-2223 (Tc = 110 ± 5K), Tl-2223 (Tc = 118 ± 5K), and Hg-1223 (Tc = 124 ± 5K) has a multigap character due to the difference in doping levels of internal (IP) and outer (OP) Cu02-planes in superconducting blocks.
2. The experimental procedure and the samples studied. The measuring system is assembled on the basis of multifunctional input-output board AT-MIO-16X (National Instruments) and a personal computer. Temperature dependences of a sample resistance R(T), current-voltage characteristics I(V), and differential conductivity dI(V)/dV of break-junctions were measured using a four-probe technique. Current-voltage characteristics (CVCs) were recorded by a fixed current method. Temperature control was carried out using a calibrated Ge sensor. dI(V)/dV-characteristics were recorded using a high speed high precision automatic digital AC bridge (modulation method).
Polycrystalline HgBa2Ca„_iCun02n+2+,s tablets with n = 1, 2, and 3 were synthesized at the Department of Chemistry, Moscow State University [9, 10]. Single crystals and polycrystalline samples of Bi2Sr2Ca„_iCu„02n+4+5 were synthesized at the Humboldt University (Berlin) and Moscow State University [11, 12]. Tablets were cut into samples of rectangular form (0.3 x 0.7 x 2.0 mm3) using the MTI diamond wire saw. Samples were then mounted on a substrate made of copper foil coated laminate. The copper foil was cut into four rectangular plates serving as electrical contact pads. A short groove was made in the foil substrate relatively deep and served as a stress concentrator. Low-resistance contacts between the sample and the current and potential leads were prepared using indium-gallium solder. At room temperature the solder is in a liquid phase which prevents samples from being damaged due to inevitable deformations of the substrate during installation. After cooling the In-Ga solder firmly fixes sample in a proper position. The substrate for the sample was glued to beryllium copper spring of thickness 0.3 mm. When gently pressing on the back side of the spring with a micrometer screw caliper, the sample was broken just above the stress concentrator. Generation of a crack in the sample (along a5-planes mainly) and subsequent adjustment of the contact was made in liquid helium.
3. Experiment results. The obvious advantages of the technique used for junction preparation in HTSC samples (break junction technique) include a high surface quality of cryogenic cleavages (both in single crystals and polycrystalline samples) and a possibility of tuning of point contacts with a micrometer screw. It should be also noted that the defects are usually ejected to the grain boundaries in the process of synthesis. For this reason it is impossible to obtain ballistic (Sharvin) point contacts utilizing cracks between the grains. In the present investigation all point contacts in the ballis-
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tic regime were formed using cryogenic intragranular cleavages.
The point-contact (Andreev) spectroscopy gives, in principle, more precise values of the superconducting gap A than the tunneling spectroscopy. First, the sub-harmonic gap structure (SGS) in the CVCs of the contacts due to multiple Andreev reflections (MAR) becomes detectable only in the case of submicron size of these contacts and, as a consequence, the heterogeneity of the samples appears to be less pronounc
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