My research activity may be subdivided in two general parts: (classical) electronic properties of metals and high-temperature superconductivity. The latter involves a large variety of properties which I have been attempting to investigate: the normal state transport properties (the thermal conductivity, thermoelectric power (the Seebeck coefficient), in-plane and out-of-plane resistivities), magnetic properties in the superconducting state (mainly - the non-equilibrium magnetization), and the out-of-plane transport properties of high-Tc materials both in the normal and superconducting states. The out-of-plane (or c-axis) transport measurements turned to be very rich in physics and new effects, like intrinsic Josephson effect. I consider these to represent my main achievements in the field and likely be the subject of my near future studies, see p.4).

In the chronological order:

1) In 1982-87 I worked at Kapitza Institute, Moscow, first as a trainee, and then – as a PhD-student on a problem of detecting the van Hove singularities in the electronic spectra of common metals through measurements of their thermoelectric power S under the action of pressure P (or doping), which may change the Fermi-level EF of electronic liquid in metals, thus pushing it towards the possible singularity in the electronic spectra. For instance, when the Fermi-level "touches" the bottom of the some otherwise empty zone from below, a new electronic-like part (void) of the Fermi-surface (FS) appears, see the picture. The elastic scattering of electrons by impurities can kick an electron from the main (large) part of the Fermi surface over to that small (new) one, where electrons have a negligibly small group velocity v(E).

This leads to a singularity in the energy-dependent mean free path l(E,P) = l0(E) + d l(E,P) of electrons; dl = 0 for P<Pc, and dl ¹0 for P>Pc, Pc is the "critical" value of the pressure, at which FS acquires a new part. Since the size of the new part is very small, the probability of scattering on to it is also very small. This means that dl(E)<< l0(E), and therefore there is no chances to detect it by measuring the electrical resistivity. However, the Seebeck coefficient of metals is proportional to l’(E),

which makes it to be sensitive to topological changes of FS. It has been theoretically shown1, that the Seebeck coefficient as a function of pressure (or doping) should have a singularity ~ ±Re(P-Pc)-1/2, see the picture above. The particular sign depends on whether the electron- or hole- like parts of FS appear or disappear. The measurements of S may thus provide a valuable information on the details of the electronic structure of some metals.

Experimentally, however, such measurements are not simple to perform due to smallness of S at low temperatures (for pure metals S~10-8 V K-1). This dictates the use of SQUID-based voltmeters with extremely high sensitivity of about 10-12-10-14 V, which moreover should be combined with the use of a special high-pressure cell, where the quasi hydrostatic pressure of up to 12-25 kbar at low temperatures can be achieved.

After scrambling through all the experimental difficulties, I finally managed to unambiguously demonstrate the existence of the effect in a number of common metals (In, Tl) and alloys (Mo-Re, Tl-Cd, Tl-In).2-4,7 (See also a review paper by A. Varlamov, Adv. Phys. 38(5), 469 (1989))  Moreover, I have discovered that the sign of the effect for high purity indium changes with temperature. This was totally unexpected and therefore stimulated further theoretical investigations. It turned out that the inelastic processes (scattering on phonons) have, in fact, an opposite contribution to the effect.7

All this was summarized in my thesis "Thermo emf and electronic topological transitions in pure metals and alloys", which was successfully defended in 1987.

[1] See a recent review paper by Ya.M. Blanter, M.I. Kaganov, A.V. Pantsulaya, and A.A.Varlamov The theory of electronic topological transitions,  Physics Reports245(4), 159 (1994)
[2] N. V. Zavaritsky, V. I. Makarov, A. A. Yurgens, Thermo-emf and the critical temperature of the superconducting transition of thallium and rhenium accompanying a topological transition, Pis'ma v ZhETP 42, 148 (1985); (JETP Lett., 42, 182, (1985)).

[3] A. N. Velikodnyi, N. V. Zavaritsky, T. A. Ignatyeva, A. A. Yurgens, Thermal emf and topological electronic transition in the Mo1-xRex system, Pis'ma v ZhETP 43, 597 (1986); (JETP Lett. 43, 773 (1986)).

[4] N. V. Zavaritsky, V. I. Makarov, A. A. Yurgens, Electronic topological transitions in indium under pressure, Pis'ma v ZhETP 45, 306, (1987); (JETP Lett.45, 387 (1987)).

[7] N. V. Zavaritsky, A. I. Kopeliovich, V. I. Makarov, and A. A. Yurgens, Electron- phonon interaction and topological features in the thermal emf of metals, Zh. Eksp. Teor. Fiz. 94, 344 (1988), (Sov. Phys. JETP 67, 1283 (1988)).

2) At about the same time the high-Tc superconductivity has been discovered. I switched the gears and started measurements on the new superconductors. With my participation, many transport properties (thermal conductivity, the Nernst and Seebeck coefficients) have been measured both on ceramic and single crystals of Y-123 and Bi-2212.5,6,8-16

Among the most significant, there was a study of thermal conductivity of Y-123 and its dependence on the oxygen content.6,7 For one and the same specimen, the thermal conductivity first decreased with decreasing of the oxygen content, and then increased again for nearly insulating state (maximum oxygen depletion). The changes in the thermal conductivity were completely reversible, i.e. after introducing oxygen back into the material by high-temperature annealing in the oxygen atmosphere, the thermal conductivity returned to its initial value.

This observation clearly indicated that quasi particles are predominantly scattered by the oxygen atoms or oxygen-atom vacancies. In the optimally doped specimen there is not many vacancies present, which means that the thermal conductivity should have (relatively) large value. When some of the oxygen atoms are removed, their vacancies play role of the scattering centers, and the thermal conductivity decreases. On the other hand, if one removes all the available oxygen atoms, one gets again the perfect crystal structure (although insulating) without scattering centers, and the thermal conductivity should increase again, in perfect correspondence with experiment.6,8 See also a review paper by C. Uher, in Physical properties of high temperature superconductors III, ed. by D. M. Ginsberg, World Sci. Publ. Co., Singapore 1992.

[5] N. V. Zavaritsky, V. N. Zavaritsky, S. V. Petrov, A. A. Yurgens, Characteristics of superconducting La2-xSrxCuO4 (x=0.05– 0.2) and MBa2Cu3O7 (M = Lu,Y) oxide ceramics, Pis'ma v ZhETP 46, 23 (1987); (JETP Lett. 46(Suppl), S18 (1987)).

[6] N. V. Zavaritsky, A. V. Samoilov, A. A. Yurgens, Kinetic properties of ceramic YBa2Cu3Ox at various oxygen concentrations (6<x<7), Pis'ma v ZhETP 48, 221 (1988); (JETP Lett. 48, 242 (1988)).

[8] A. V. Samoilov, A. A. Yurgens, N. V. Zavaritsky, Effect of oxygen concentration on transport properties of YBa2Cu3O7-x (6<x<7), Progress in High Temperature Superconductivity, Editors: A. I. Larkin, N. V. Zavaritsky. (World Scientific Publishing Co. Pte. Ltd., Singapore, 1989)- vol. 11:High Temperature Superconductivity From Russia., pp.238–249.

[10] A. A. Yurgens, Acoustic properties and thermal conductivity of high-Tc superconductors. in Investigation of High Temperature Superconductors. Modern Spectroscopic and Microscopic Methods. Proc. of the USSR - FRG Bilaterial Seminar. 30 Oct - 5 Nov 1989. Tallinn. Acad. of Sci. of the USSR, "Hyperox", M90, p. 261

[11] N. V. Zavaritsky, A. V. Samoilov, A. A. Yurgens, Transport properties of Bi2Sr2CaCu2Ox single crystals with Tc=95 K, Physica C 169, 174 (1990).

[12] N. V. Zavaritsky, A. V. Samoilov, A. A. Yurgens, Transport coefficients and flux motion in Bi2Sr2CaCu2Ox single crystals, Physica C 180, 417 (1991).

[15] N. V. Zavaritsky, A. V. Samoilov, A. A. Yurgens, Anisotropy of the magneto-thermoelectric power of high-Tc, Pis’ma v ZhETP 55, 133 (1992) (JETP Lett.55, 127 (1992)).

[16] N. V. Zavaritsky, A. V. Samoilov, A. A. Yurgens, Vortex motion under the influence of temperature gradient, Phys. Rev. B 46, 6643 (1992) (Rapid Comm.).

3) After building my own SQUID-magnetometer from the scratch, I started studying the remnant non-equilibrium magnetization of Bi-2212 single crystals. Since there were many groups around the world doing the same, I have attempted to introduce a new element in this and made measurements with a sample being illuminated with visible light inside the magnetometer. The first few measurements clearly showed that light has a huge effect on the relaxation speed (creep) g  = dM/dt of the non-equilibrium magnetization M. Very small intensities of light (0.1 – 0.0001 mW cm-2) were yet strong enough to see drastic changes in g.17 In the later works by Landau (Switzerland) it has been confirmed that the discovered effect is not due to just trivial heating by light (I. L. Landau andL. Rinderer, (Proceedings of LT21) Czech. J. Phys. 46 (Suppl.S3), 1721 (1996)), but involves a more complicated physics of non-equilibrium superconductivity.

From analysis of our experiments it became clear that the excited by light quasiparticles in Bi-2212 may have a prolonged lifetime due to inhomogeneities of the superconductor, which may effectively trap quasiparticles thus impeding the recombination. The diffusion length of the quasiparticles (during the time of their recombination) was estimated to be 10-5-10-3 cm, which is comparable with thickness of commonly used single crystals of Bi-2212.

[17] A. A. Yurgens and N. V. Zavaritsky, Optically induced magnetic relaxation in the Bi2Sr2CaCu2Ox single crystals, Physica C 203, 277 (1992). 4) In 1994 I moved to Sweden, to Chalmers University of Technology (CTH) and switched the gears of my research activity once again. That time I was interested in one really wonderful effect, discovered in the group of Prof. P. Müller (Germany). It appeared that the c-axis superconducting transport involves tunneling of the Cooper pairs from one double-CuO2 plane to another within one unit cell inside the single crystal. It was obviously the Lord himself who made such intrinsic Josephson junctions (IJJ) to exist. So far, only the artificially made superconducting structures possessed this kind of effects. Naturally formed Josephson junctions (see Fig.) presumably have the highest quality in terms of their uniformity, again, due to their being formed inside a single crystalline material. Only epitaxial heterostructures fabricated of semiconductors with perfectly matched crystal-lattice parameters may compete with such tunnel junctions.
Since I have missed making the discovery myself, I’ve attempted to introduce a new element in the experimental investigation of the new effect. I have suggested to make use of the photolithographic facilities at CTH and make small mesa structures on surfaces of relatively big Bi-2212 single crystals. After my taking time to find the right recipe for making mesas and contacts to them, we succeeded in making them, thus being the first to confirm the existence of the intrinsic Josephson effect in transport measurements.18,19,22 Moreover, the mesas have certain advantages over just single crystals.

First, the height of mesas may be controlled by changing, for instance, the etching time. The smaller the height - the smaller the number of IJJ participating in the measurements. This greatly facilitates analysis of the data obtained and allows one to see the properties of an individual IJJ more easily, without great deal of heat evolvement. Later, I introduced a more sophisticated technology involving in situ control of the mesa height which allowed us to make mesas of a specified height (a specified number of IJJ).22 Even properties of only one IJJ in the mesa have been thoroughly investigated.20

Second, since the mesa volume is about 6 orders of magnitude smaller than that of the whole crystal, the probability of having defects in the former is by the same order of magnitude smaller. We consider this advantage as significant, because very often in practice far going conclusions are inadvertently made based on measurements on simply defect samples. Monitoring properties of the limited number of IJJ allows us to easily detect the defect mesas and reject such a sample.

Using the mesas we started to perform different kinds of measurements on them. These include:

a) Vortex matter. Abrikosov vortices (pancakes).

Thermodynamics and transport properties of type II superconductors in magnetic field are essentially the properties of the vortex system or "vortex matter". High-Tc superconductors have a very high amplitude of thermal fluctuations in the vortex state. This results in destruction of the vortex crystal state well below the mean field upper critical field. The fluctuations are especially important and strong in the layered materials where vortex lines consist of weakly coupled "pancakes".

The pancakes interact with each other magnetically and via interlayer Josephson coupling. These interactions encourage pancakes to align in perpendicular to planes direction.
However, when the interlayer interactions are weak, thermal fluctuations and local pinning centers cause misalignment (see picture). This leads to a loss in phase coherence between adjacent pairs of superconducting planes and hence - a reduction in the critical current.24,30 If the c-axis critical current is totally suppressed by the magnetic field, the measured c-axis resistance is nothing else but the sum of the sub-gap resistances of all the IJJ in series.24 Although this directly follows from the Lawrence-Doniach model of highly anisotropic high-Tc compounds, it has never been shown experimentally.

We have demonstrated that indeed, the measured c-axis resistance in the magnetic field corresponds to the sub-gap resistance of an individual IJJ times the number of layers in a given stack of IJJ.24 Moreover, the magnitude of the magnetoresistance peak effect may be predicted from the zero-field current-voltage characteristics measured in the whole temperature range.24

In Ref. 30 our measurements of the field-dependent c-axis tunneling characteristics across the intrinsic inter-planar Josephson junctions in 2212-BSCCO mesa structures probe the inter-planar pancake correlation on an atomic interlayer length scale, complementing earlier measurements of vortex correlation on the much longer length scales involved in low angle neutron diffraction, mSR, magnetic, and bulk transport measurements. We have interpreted our measurements in terms of the interplane alignment of pancake vortices, which provided information on the rich magnetic phase diagram of Bi-2212.30

One of the ideas for decreasing the creep of pancakes and helping them to align in the c-axis direction, was heavy-ion irradiation of the single crystals and thin films. High-energy heavy ions produce straight columns of damage tracks across the whole thickness of the crystal. It is then energy favorable for pancakes to sit at the columnar defects. However, for randomly distributed columns, the number of pancakes being aligned along the columns does not monotonically depend on the applied magnetic field. Numerical simulations showed that there is a maximum at approximately 1/3 of the matching field BF(the field at which number of vortices equals number of columnar defects). In our measurements we have shown experimentally that indeed the c-axis critical current has a pronounced maximum at 1/3 of the matching field and no feature at B=BF .*

[24] A. Yurgens, D. Winkler, N. V. Zavaritsky, and T. Claeson, Relationship between the Out-Of-Plane Resistance and the Subgap Resistance of Intrinsic Josephson Junctions in Bi2Sr2CaCu2O8+d, Phys. Rev. Lett. 79, 5122 (1997).

[30] A. Yurgens, D. Winkler, and T. Claeson, G. Yang, I. F.G. Parker, and C. E. Gough 2212-BSCCO Intrinsic Josephson junctions in a magnetic field, Phys. Rev. B59(9), xxxx (1999).

* A. Yurgens, N. Mros, D. Winkler, T. Claeson, and M. Konczykowski, Partial filling of columnar defects by vortices as seen from direct measurements of the c-axis critical current of Bi-2212, Phys. Rev. B 60, 12480 (1999).

b) Vortex dynamics. Josephson vortices.

The highly anisotropic layered high-Tc compounds exhibit intrinsic Josephson effects. In the simplified picture, layered compounds may be viewed as stacks of Josephson tunnel junctions. To employ possible advantages of high- quality Josephson junctions made by the Nature inside the single-cristalline material, one has to study the main features of such junctions. One of the basic properties is the response to the external magnetic field, which in the case of ordinary low-temperature junctions causes the critical current of the junction to oscillate.

In stacks of Josephson junctions, the coupling between adjacent junctions via magnetic interaction gets important. If sufficiently strong, this coupling may provide synchronization of many junctions in the stack (phase locking), so that the collective behavior of all junctions is equivalent to a single one with a much higher IcRn. This is very important for possible practical applications.

It has been shown both experimentally and theoretically (Krasnov) that in stacks of IJJ the Josephson vortices may have a number of modes with almost equal Gibbs free energy. This cause the measured critical current to have mutiple most probable values at a given temperature. The probability may change with temperature due to change of the magnetic coupling in between adjacent layers. This, in turn, may lead to a sudden change of the critical current in perfect correspondence with experiemnt.27,28

[27] V. M. Krasnov, N. Mros, A. Yurgens, and D. Winkler, Magnetic field dependence of the critical current in stacked Josephson junctions. Evidence for fluxon modes in Bi2Sr2CaCu2O8+x mesas,- Physica C 304, 172 (1998).

[28] N. Mros, V. M. Krasnov, A. Yurgens, D. Winkler, and T. Claeson, Multiple-valued c-axis critical current and phase locking in Bi2Sr2CaCu2O8+x single crystals, Phys. Rev. B 57, R8135 (1998).

c) Technology of the in-situ controlled fabrication of stacks and superconducting properties of intrinsic Josephson junctions. The c-axis energy gap.

Usual experimental methods of the energy gap spectroscopy in high- Tc materials are essentially surface probes, like STM or ARPES (angle resolved photo emission spectroscopy). The intrinsic tunneling effects allow one to get information on superconducting properties far away from the possibly deteriorated surface (moisture, loss of oxygen, etc.). In contrast to other techniques, we argue that tunnel junctions formed inside the single crystal provide genuine information of the properties of bismuth compounds.

From the c-axis current-voltage characteristics we can obtain the value of the c-axis energy gap parameter, which turned out to be about 10 –15 meV, about one half of the expected value (from earlier STM and similar measurements). We argued that the reason for that may be a proximity induced superconductivity in the intermediate BiO layers. This assumption allowed us to explain both the reduced energy gap in the c-direction and its unusually strong temperature dependence.22

[22] A. Yurgens, D. Winkler, N. V. Zavaritsky, and T Claeson, Strong temperature dependence of the gap voltage of Bi2Sr2CaCu2Ox intrinsic Josephson junctions, Phys. Rev. B, 53, R8887 (1996).

[23] A. Yurgens, D. Winkler, N. V. Zavaritsky, and T. Claeson, In situ controlled fabrication of stacks of high-Tc intrinsic Josephson junctions, Appl. Phys. Lett. 70, 1760 (1997).

d) Experimental test of interlayer tunneling model of HTSC.

The interlayer tunneling model (ILT), developed by Anderson with collaborators is one of the leading candidates for high-temperature superconductivity (P. W. Anderson et al., The theory of Superconductivity in the High-Tc Cuprate Superconductors (Princeton Univ. Press, Princeton, 1997). In this model the energy gain that drives the formation of the pairs is associated with the decrease of the kinetic energy due to the easy motion of the pairs accompanied by the impeded single-particle tunneling along the c-axis. The model  predicts an increase of the condensation energy Ec proportionally to the interlayer Josephson coupling, or to the inverse square of the c-axis penetration depth lc. A conclusive experimental check of this prediction for different single-layer high-Tc materials would provide a critical test of ILT model.

Having been working quite a lot with high-pressure techniques (see p.1) I came to an idea of checking the theory by using the hydrostatic pressure as a tool to change the interlayer coupling in the layered compounds. The c-axis critical current (which we can nicely measure) directly reflects the interlayer coupling, while the superconducting critical temperature may serve as a measure of the condensation energy.

Quickly after the emergence of the idea we made the corresponding experiements on Bi- based single layer compound Bi-2201, and its double-layer ally, Bi-2212. We have observed that while the interlayer coupling Ic increases dramatically with pressure (up to 270 % GPa-1), the critical temperature only slightly increases at a rate of 2-6 %GPa-1. See these Figures. These incommensurate effects of pressure suggest that the CuO-interlayer coupling has a little effect on Tc of the Bi-compounds, in contrast to the ILT model.

· A. Yurgens, D. Winkler, T. Claeson, T. Murayama and Y. Ando, Interlayer coupling and superconducting critical temperature of Bi-2201 and Bi-2212: Incommensurate effects of pressure, Phys. Rev. Lett., 79(25), 5122 (1997). e) Pseudo-gap and sub-gap spectroscopy with intrinsic Josephson junctions

The existence of a pseudo-gap in electronic excitation spectra of high-Tcsuperconductors, which appears below certain temperature T*> Tc is considered to be amongst the most important features of high-Tc cuprates.29 Many experiments have provided evidence for a gap-like structure in electronic excitation spectra, like NMR, ARPES, specific heat, electron-tunneling spectroscopy, and STM. The latter two methods have the additional advantage as being able to measure the density of states locally. However, they also have a drawback of being essentially surface probes. Surface is often poorly characterized and is subject of chemical influence from moisture etc.

Intrinsic Josephson effect allows one to monitor the intrinsically inherent tunneling properties inside the single crystal far away from the surface. This is the main advantage of using IJJ for electronic spectroscopy. However, it has its drawbacks, too. Due to relatively high value of the c-axis superconducting energy gap parameter Dc (12-15 meV), and relatively high critical current density Jc (200-1000 A cm-2), the heating and non-equilibrium effects become important. They may obscure results. This means that one has to work very close to Tc (where both Jc and Dc get much smaller).

Another approach, which we tried to use, was decreasing the interlayer coupling via intercalation of big molecules of (chemically inert) HgBr2 in between BiO layers in Bi-2212. Experiments show that doing this does not change Tc much, but allow one to decrease the c-axis critical current more than two orders of magnitude, thus allowing us to avoid heating and non-equilibrium problems and to widen the temperature range where the pseudo-gap features in tunneling spectra could be observed.

[21] A. Yurgens, D. Winkler, N. Zavaritsky, and T. Claeson, Gap and sub-gap structures of intrinsic Josephson tunnel junctions in Bi2Sr2CaCu2Ox single crystals, Proc. of the Oxide Superconductors: Physics and Nanoengineering II, 27 Jan. - 2 Feb. 1996, San Jose, CA, (SPIE, Bellingham, 1996), p.433-451.

[29] C. Gough, I. Parker, M. Endres, P. J. Thomas, G. Yang, and A. Yurgens, Observation of normal and superconducting state energy gap features from intrinsic c-axis interlayer tunneling in 2212-BSCCO crystals (Invited Paper), Proceedings of SPIE Vol. 3480 Superconducting Superlattices II: Native and Artificial, San-Jose, CA (1998).

· I. F. G. Parker, M. Endres, P. J. Thomas, G. Yang, A. Yurgens, C. E. Gough, Emergence of superconducting gap features in Bi2Sr2CaCu2O8+x single crystals from intrinsic c-axis interlayer tunneling,- cond-mat/#9809328,  unpublished.

· A. Yurgens, D. Winkler, T. Claeson, Seong-Ju Hwang and Jin-Ho Choy, Intrinsic tunneling spectroscopy of (HgBr2)-Bi2212 single crystals,  Int. J. Mod. Phys. B 13, 3758 (1999), cond-mat/9907159