LOW TEMPERATURE PHYSICS VOLUME 30, NUMBERS 7–8 JULY–AUGUST 2004 Superconducting and mesoscopic structures „Preface… ͓DOI: 10.1063/1.1789091͔ ͑ ͒ It is remarkable that for 40 years the Josephson effect massive banks S1 and S2 . The coupling allows the ex- has maintained its position at the center of condensed matter change of electrons between the banks and establishes super- physics. The reason probably lies in the very concept of conducting phase coherence in the system as a whole. The weak coherent coupling between macroscopic quantum sys- weakness of the coupling means that the superconducting tems. It allows one to separate the effects of interaction, order parameters of the banks are essentially the same as for which creates the long-range order, from the correlation disconnected superconductors, and they are characterized by ␹ ␹ themselves, corresponding to this order. It provided the pos- the phases of the order parameters 1 and 2 . The Josephson sibility to investigate the overlap of mutually exclusive ͑in weak link can be considered as a ‘‘mixer’’ of the two super- the bulk͒ long-range orders. It gives the opportunity to look conducting macroscopic quantum states in the banks. The at the effects of finite size of the system. Josephson effect result of the mixing is a phase-dependent current-carrying also gives a framework for the discussion and realization of state with current flowing from one bank to another. This macroscopic quantum phenomena ͑beyond the almost trivial current is determined ͑parameterized͒ by the phase differ- ͒ ␸ϭ␹ Ϫ␹ fact that superconductors are macroscopic quantum objects . ence 2 1 across the weak link. The specific form of The last five years have seen the demonstration of macro- the current–phase relation I(␸) depends on the type of weak scopic quantum resonant tunneling, quantum coherence, and link. quantum entanglement in Josephson structures. Josephson A number of papers consider the coherent transport in physics repaid the physics of bulk superconductivity by pro- Josephson weak links with coupling more complicated than viding a means of investigation of unusual superconductors just a tunneling barrier. In the paper by Kulik different types ͑ ͒ e.g., demonstrating d-wave symmetry in high-Tc cuprates . of superconducting weak links are reviewed, focusing on the Brian D. Josephson discovered his remarkable effect in origin of jumps in the current–phase relations. The author 1962. Josephson predicted that a zero voltage supercurrent also discusses persistent currents in mesoscopic and nano- could flow in a junction between two superconductors sepa- scopic Aharonov–Bohm structures. Novel effects in super- rated by a tunnel barrier. The magnitude of the Josephson conducting nanojunctions are studied theoretically in the pa- current is related to the difference of the phases of the mac- per by Zaikin. It is shown that interplay between quantum roscopic wave functions ͑complex order parameters͒ of the interference effects and Andreev reflection in S–N–S junc- superconductors forming the junction. P. W. Anderson and J. tions with insulating barriers may qualitatively modify the M. Rowell first observed this dc Josephson effect in 1963. If Josephson current. Several papers included deal with spin a dc voltage V is applied to the junction, an ac supercurrent effects in mesoscopic Josephson junctions. Shnirman et al. with the frequency 2eV/ប appears between the supercon- study the dynamics of a single spin embedded in the tunnel- ductors. The first direct observation of the ac Josephson ef- ing barrier between two superconductors. A new effect, ‘‘Jo- fect was made 40 years ago in Kharkov, Ukraine ͑I. K. Yan- sephson nutation,’’ is predicted. The paper by Krive et al. son, V. M. Svistunov, and I. M. Dmitrenko, Zh. E´ ksp. Teor. reviews the charge and spin effects in S–Luttinger liquid–S Fiz. 47, 2091 ͑1964͒; 48, 976 ͑1965͓͒Sov. Phys. JETP 20, and S–quantum wire–S junctions. 1404 ͑1965͒; 21, 650 ͑1965͔͒͒. Soon after Josephson’s pre- The properties of the current carrying states in a weak dictions for the tunnel junctions, it became clear that the link depend not only on the manner of coupling but also on effects are much more general and occur whenever two su- the properties of the superconducting banks. The modern perconductors are weakly coupled. The physics of weak su- physics of superconductivity is essentially the physics of un- perconductivity ͑a term introduced by P. W. Anderson͒ be- conventional superconductivity. came an area of a great interest for experimental and The discovery of d-wave symmetry of the order param- theoretical investigations. More than forty years after its dis- eter in high-temperature superconductors and of triplet su- covery, the Josephson effect still attracts considerable atten- perconductivity in compound Sr2RuO4 has caused a stream tion and keeps providing us with exciting new physics and of theoretical and experimental research on unconventional applications. superconductors. The sensitivity of the Josephson effect to This issue is devoted to aspects of the physics of super- the symmetry of the complex order parameter in the junc- conducting and mesoscopic structures. It represents reviews tion’s banks stimulated numerous studies of Josephson weak and original articles on the subject. The issue opens with links between unconventional superconductors. The current– papers by Yanson and Dmitrenko which review the initial phase relations for unconventional Josephson weak links are steps in study of the ac Josephson effect in tunnel junctions quite different from the conventional ones. For example, in and further experimental investigation of weakly coupled su- grain-boundary junctions, depending on the angle of misori- perconductors at the Institute for Low Temperature Physics entation of d-wave order parameters in the banks, the and Engineering in Kharkov. current–phase relation is changed from a sin(␸)-like curve to The Josephson effect arises in superconducting weak a Ϫsin(2␸) dependence. links—junctions of two weakly coupled superconductors Clearly, this determines new features in the behavior of 1063-777X/2004/30(7–8)/2/$26.00513 © 2004 American Institute of Physics 514 Low Temp. Phys. 30 (7–8), July–August 2004 such Josephson junctions in applied voltage or magnetic unconventional superconductors needs further theoretical field. A considerable number of the papers included in this and experimental investigations. issue concern the study of unconventional Josephson weak Mesoscopic structures consisting of several Josephson links. One of the most striking manifestations of the uncon- junctions are now being studied intensively from the point of ventional order parameter symmetry is the appearance of view of qubit realization. A paper by Il’ichev et al. summa- spontaneous current flowing along the contact interface in rizes the results of implementation of advanced impedance addition to the Josephson current. The spontaneous current measurement technique for characterization of interferometer-type superconducting qubits. In a theoretical arises due to the breaking of the time-reversal symmetry (T) paper by Ioffe et al. a new class of Josephson arrays is in- in the system. The study of T-breaking phenomena is not troduced. These arrays have nontrivial topology and exhibit only of fundamental significance but also attracts interest novel quantum states at low temperatures. In the paper by from the standpoint of realization of qubits, basic units of Kuplevakhky, a detailed theory of Josephson vortices in lay- quantum computers. The review by Kolesnichenko et al. fo- ered superconductors is developed. The quantum dynamics cuses on spontaneous currents in junctions between d-wave of the order parameter and time-dependent BCS pairing is and triplet superconductors. It also contains the review of investigated in the framework of the Wigner distribution superconducting qubit basics with emphasis on the properties function by Amin et al. of d-wave qubits. A theoretical paper by Tanaka et al. con- A single issue cannot cover all aspects of the research. It siders the impurity scattering effect on charge transport in gives the reader a brief overview of the current state of ac- high-Tc cuprate junctions. The results of experimental inves- tivities, which, we hope, will be useful and will stimulate tigations of high-Tc grain boundary junctions and hetero- further investigations in the field of superconducting and me- structures are presented in the papers by Tafuri et al. Ko- soscopic structures. missinski et al. and Timofeev et al. The specific features of We greatly appreciate helpful discussions with all the ac Josephson effect in unconventional superconductors are contributors during the preparation of this issue. reported in the theoretical paper by Kwon et al. Note that the A. N. Omelyanchouk problem of existence of the fractional ac Josephson effect in LOW TEMPERATURE PHYSICS VOLUME 30, NUMBERS 7–8 JULY–AUGUST 2004 The ac Josephson effect: observation of electromagnetic radiation „Review… I. K. Yanson* B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, 47 Lenin Ave., Kharkov 61103, Ukraine ͑Submitted January 22, 2004͒ Fiz. Nizk. Temp. 30, 689–697 ͑July–August 2004͒ An historical review of the discovery and the early period of research on the Josephson effect is given. Experiments on the tunneling effect in superconductors done in the 1960s at the Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, Kharkov ͑ILTPE͒, which led to the observation of Josephson electromagnetic radiation are described in detail. The experimental data are illustrated by the original curves, and the dates they were taken are indicated. The physical mechanism for the generation of rf radiation in superconducting tunnel junctions is examined, and some of the more promising applications of the ac Josephson effect are briefly listed. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1789911͔ 1. INTRODUCTION of the potential difference across the barrier, which is respon- sible for some mechanism of dissipation of the oscillation In the first half of the year 1962 the young English the- energy.