Construction of a thermoacoustic Stirling cooler
Yuki Uedaa, Tetsushi Biwaa, Taichi Yazakib, Uichiro Mizutania
aDepartment of Crystalline Materials Science, Nagoya University, Ngoya, 464,Japan
bDepartment of Physics, Aichi University of Education, Kariya, 448, Japan
In 1999, an efficient thermoacoustic engine was built by Backhaus and Swift, where an acoustic traveling wave executes a Stirling cycle [S. Backhaus and G. W. Swift, Nature 399, 335(1999)]. They reported that this engine produced the acoustic output power with the efficiency reaching 40% of the ideal Carnot efficiency in spite of the absence of any moving parts. In this work, we exprimentally revealed how the traveling wave performs the energy conversion. On the basis of the results, we inserted a regenerator inside the thermoacoustic engine in order to enable acoustic heat pumping through a Stirling cycle. Our prototype cooler reached -25 °C by using output power resulting from the input heat power of 200 W.
Construction of a 3He cryostat using a charcoal sorption pump
Yasuhisa Ushidaa, Hiroyuki Nakanea, Kunihiro Kimuraa, Takashi Tsuzukia, Takashi Nishiokaa, Syoji Inoueb, Koji Matsushitab, Noriaki K. Satoa
aDepartment of Physics, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan.
bTechnical Division, School of Science, Nagoya University, Nagoya 464-8602, Japan.
Recently, pressure induced superconductors were discovered in f-electron materials. Since a superconducting transition temperature of these materials is lower than 1 K, it is desirable to make measurements under high pressure at a temperature as low as possible. For this purpose, we have constructed a highly simple 3He cryostat with large cooling power using a charcoal sorption pump. The temperature is controlled by changing a position of the charcoal pump, instead of using a heater. We found that a sample cools down to 0.31 K within 30 minutes after transferring liquid 4He. We will show specific heat measurement results obtained by using this cryostat.
Vibration characteristics of dilution refrigerator with cryogenic cycle
Junyun Li, Oleg Kirichek, Larry Linfitt, Alvin Adams, Vladimir Mikheev
Superconductivity, Oxford Instruments, Tubney Woods, Abingdon, Oxon, OX13 5QX, UK
One of the main advantages of the dilution refrigerators with a cryogenic cycle of circulation is a relatively low level of mechanical vibrations. It can be explained by absence of pumping lines, which transmits vibrations from mechanical pumps to the cryostat. However, there is no experimental data so far to support this opinion. We present results of mixing chamber vibration measurements at base temperature 15 mK of the dilution refrigerator with two adsorption pumps. It was shown that the cryo-valve switching off/on causes short time vibration disturbance, which decays within 0.2 s. During operation of the refrigerator between cryo-valve switchings the amplitude of vibrations is lower than 10 Å. The analysis of vibration spectrum allows us to suggest measures, which can further reduce the amplitude of disturbances.
Copper Nuclear Demagnetization Stage for Studies of Quantum Fluids below 100 mK
Hisashi Nakagawa, Hideo Yano, Osamu Ishikawa, Tohru Hata
Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan
For the study of superfluid 3He and 3He-4He mixture below 100mK,we have constructed a massive copper nuclear stage(90.6mole of Cu in 8T ). Our stage has been machined from bulk oxigen free copper with purity 99.99%.Heat treatment increased the RRR up to 4000. With this nuclear stage we have managed to cool a Pt-NMR thermometer located in the sample space to 45mK. The total heat leak was found to be 1nW on the 10th day after reaching 4.2K and to be constant during 2months. The measured heat capacities at several fields were consistent with the calculated values between 60mK and 4mK. This indicates that our stage has no extra heat capacity. Due to time-independent and small heat leaks,we can reach the lowest temperature quite soon and maintain temperatures below 100mK for several weeks.
Super High Conductivity Effect in Metal - Polymer - Metal Structures
A. N. Ionova, V. A. Zakrevskiia, V. M. Svetlichnyb, R. Rentzschc
aA.F. Ioffe Physico-Technical Institute, RAS , 194021, St. Petersburg, Russia
bInstitute of Macromolecular Compounds, RAS, 199004, St. Petersburg, Russia
cInstitut fur Experimentalphysik, Freie Univesitat Berlin, D-14195 - Berlin, Germany
We have observed that films of poly[4,4'-bis(4''-N-phenoxy)diphenyl-sulfone]amic acid of 1,3-bis(3',4-dicarboxyphenoxy)benzene - polyimide precursor or co-poly[4,4'-bis(4''-N-phenoxy) diphenyl-sulfone-a,b-bis (g- amino propyl)oligodimethylsiloxane] imide of 1,3-bis(3',4- dicarboxyphenoxy)benzene with thickness about 1m placed between two metallic electrodes become highly conducting in a relatively small electric field (E < 1 V/cm). If the metallic electrodes (Sn, Nb) in sandwich structures were in the superconducting state an effective resistance of zero was recorded. A typical current - voltage characteristic of an S-P-S structure looks like a Josephson type.
Quantum wire networks for superconducting quantum-dot superlattices
T. Kimuraa, H. Tamuraa, K. Kurokib, K. Shiraishic, H. Takayanagia, R. Aritad
aNTT Basic Research Laboratories, Atsugi 243-0198, Japan
bDept. of Appl. Phys. and Chem., The Univ. of Electro-Communications, Chofu 182-8585, Japan
cDepartment of Physics, Tsukuba University, Tsukuba 305-8271, Japan, and Research Consortium for SYNAF Project, National Inst. of Adv. Indust. Science and Tech., Tsukuba 305-8568, Japan
dDepartment of Physics, University of Tokyo, Hongo 113-0033, Japan
Quantum wire networks have been proposed for fabricating quantum-dot superlattices with the square and the plaquette lattice structures. These artificial lattices are well represented by Hubbard models with parameters determined by the local density approximation. The superconducting transition temperature Tc (=90 mK) for the plaquette lattice is more than two times Tc (=40 mK) for the square lattice and is sufficiently high for achieving superconductivity in experiments.
Infrared observation of the CDW amplitude decreasing due to hole doping in Ba1-xKxBiO3
Taichiro Nishio, Hiromoto Uwe
Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
Optical reflectivity mesurements in a frequency range from 50 -1000cm-1 have been done for semiconducting BKBO single crystals. The amplitude of the CDW estimated from the oscillator strength of the transverse optical phonon is substantially large in BBO. The amplitude value decreases with increasing x and tends to approach zero at a critical composition of the metal-semiconductor transition. We found that the dynamical charge transfer with the vibration of the Bi-O bonds is quite large in this system and a decrease of the amplitude leads to the superconducting as well as metallic states.
Three channels non-force magnetic SQUID microscope
S. I. Bondarenkoa, A. A. Shabloa, P. P. Pavlova, N. Nakagawab
aInstitute of Low Temperature Physics and Engineering,47 Lenin ave. 61103 Kharkov, Ukraine
bCenter for Nondestructive Evaluation, Iowa State University, 1915 School Rd.,Bldg.2, Ames, Iowa 50011, US
Scanning magnetic microscope (SMM)without force and appreciable magnetic influence on research object was developed. SMM contains three measuring channels. First of them is special fluxgate with relatively low magnetic sensitivity for a preliminary magnetic study of the object surface. The second channel has been developed on a base HTc SQUID with ferromagnetic concentrator and it is intended for detailed (with 0.1-1 micron resolution) study of "warm" objects at T=300K. The third channel also has as the detector HTc SQUID with ferromagnetic concentrator of a magnetic flux, but it is intend for detailed study of the "cold" objects at T=77K.
Development of High-Tc SQUID Microscope
Kuniaki Sata, Takekazu Ishida
Department of Physics and Electronics, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
We have developed a SQUID microscope using a high-Tc SQUID sensor (Sumitomo Electric Hightechs). The SQUID sensor has a sensitivity of 1pT and the effective sensor area is 0.2 mm2. An X-Y stage (Suruga K201-30LMS) moves 30× 30 mm area and its resolution is 0.05 mm. The SQUID sensor is cooled by liquid nitrogen, and can be held at 77 K for 1 h. The distance between the sensor and a sapphire window is less than 100 mm. The sample temperature is limited to room temperature. The device is controlled by LabVIEW-6i and the motion controller (National Instruments UMI-7764). The performance of the SQUID microscope is examined by a regular array of Ni dots.
NMR powder spectra in case of strong quadrupole interaction
A. V. Egorova,b, I. R. Mukhamedshina,b, H. Suzukib
aMRS Laboratory, Kazan State University, Kazan, 420008, Russia
bUltralow Temperature Physics Laboratory, Kanazawa University, Kanazawa, 920-1192, Japan
Many new materials are available only in form of powders and in case of large quadrupole splittings NMR spectra become extremely broad. An example is copper NMR in high-Tc superconductors. In this case nuclear Zeeman term of Hamiltonian is in speaking terms with quadrupole one and perturbation theory is not valid for the spectra calculation. We suggest the straightforward numerical simulation of the spectra alghorithm which exhibits a good fit of the experimental data even in case of moderate computing resources. It is shown that asymmetry parameter of the nuclear quadrupole interaction, quadrupole frequency and Knight shift can be evaluated from the powder spectrum. The distribution of spin-lattice relaxation rates over the spectra in case of quadrupole and magnetic relaxation is discussed.
Transport Properties of In2Bi and InBi Single Crystals
Katsuhiko Nishimura, Takayuki Yasukawa, Katsunori Mori
Faculty of Engineering, Toyama University, Toyama 930-8555, Japan
We have made measurements of specific heat, resistivity, thermopower and magnetization of a superconductor, In2Bi, and a normal metal, InBi, using single crystals at temperatures from 2 to 300 K and fields up to 7 T. Applying an external field at the specific heat measurement of In2Bi enabled us to estimate an energy gap between the normal and superconducting states, which can be compared with that from the BCS theory. Magnetization of In2Bi at 2 K showed the de Haas-van Alphen oscillations at the c-axis, but no such oscillation was observed at the a-axis. An origin for the different transport properties of these compounds was considered focusing on the coefficients of electronic specific heat and the Debye temperatures.
Negative resistance in I-V characteristics and 2D vortex dynamics in amorphous W/Si multilayer superconductors with periodic antidot arrays
Yuka Kurosua, Makiko Yokoyamaa, Yoshinori Kuwasawaa, Satoshi Matsudab, Tsutomu Nojimac
aDepartment of Physics, Faculty of Science, Chiba University, Yayoi-cho, Chiba 263-8522, Japan
bDepartment of Physics, Faculty of Education, Chiba University, Yayoi-cho, Chiba 263-8522, Japan
cCenter for Low Temperature Science, Tohoku University, Katahira, Sendai 980-8577, Japan
We have examined the vortex dynamics in W/Si multilayers, where the arrays of antidots with 1 mm diameter are fabricated in the form of square and triangular lattices with the distance of 2 mm. In the measurements of I-V characteristics as a function of temperature T and magnetic field H, we find a specific regime at high H, where the I-V curves coincide irrespective of T. Especially the I-V curves show a negative slope in the multilayer with triangular arrays. The elastic flows of the vortex lattices in the narrow paths between the arrays and their interaction with the vortices around antidots are discussed.
Superconductivity of Metals in Porous Matrices
Elena V. Charnayaa, Cheng Tienb, Chin-Shuei Wurb
aInstitute of Physics, St.Petersburg State University, St.Peterburg 198504 Russia
bDepartment of Physics, National Cheng Kung University, Tainan 70101 Taiwan
Studies of superconductivity for gallium, mercury, tin, and indium embedded into porous glasses and artificial opals are presented. Magnetic properties and resistance of porous matrices filled with metals were measured from 1.7 to 10 K. Size effects on the superconducting phase transitions were found. Double-step resistivity and magnetization were obtained in some samples dependently on filling factors and pore geometry. Most superconducting properties were treated within the framework of the model of granular superconductors. Properties, which were also found in high-temperature superconductors, like flux jumps and the paramagnetic Meissner effect were observed for porous glasses filed with tin.
Magnetic response of hard type-II superconductors with a semicircular indentation
Leonardo R. E. Cabral, Antônio Rodrigues, J. Albino Aguiar
Departamento de Física, Universidade Federal de Pernambuco, 50670-901, Recife, PE, Brasil
The magnetic response of superconductors with a semicircular indentation in parallel field is studied. The indentation is placed in the corner of a long supercondutor. The Meissner and vortex fields are calculated for any indentation radius in the London background. This allowed us to obtain the energy and the forces acting on a vortex in a given position inside the superconductor. The entrance magnetic field dependence on the indentation radius is calculated and compared with the entrance field for an identation free semi-infinite superconductor. The role of the indentation on the surface barrier and on the vortex dynamics is also discussed.
Fluxoid distributions in superconductive honeycomb networks
Osamu Satoa, Masaru Katob
aDepartment of Liberal Arts, Osaka Prefectural College of Technology, Neyagawa, Osaka, 572-8572, Japan
bDepartment of Mathematical Sciences, Osaka Prefecture University, Sakai, Osaka, 599-8531, Japan
We have studied fluxoid distributions and superconducting transition temperature of honeycomb superconductive networks with edges in the magnetic field making use of the de Gennes-Alexander equation. In honeycomb networks with edges, the suppression of superconducting transition temperature in the magnetic field is smaller compared with that of edgeless periodic boundary networks. In the weak magnetic field, fluxoids emerge from center of the network. As magnetic field increases, fluxoids are distributed in the shape of edge lines of the network.
Dimensional Crossover from Two to One Dimension in Small-Josephson-Junction Arrays
Yamaguchi Takahide, Hisao Miyazaki, Youiti Ootuka
Institute of Physics, University of Tsukuba, 305-8571, Japan.
We have fabricated two-dimensional (2D) small-Josephson-junction arrays with different array widths, including a 1D array, on a substrate simultaneously. For such a group of arrays with nominally the same junction parameters (EJ/EC » 1), we observe a crossover from superconducting to insulating behavior as the array width is reduced, which is explained by an enhancement of quantum fluctuations due to the dimensional reduction. I will also talk about scaling behavior in current-voltage characteristics of the arrays.
Vortex motion channeling effects in Nb with mesoscopic arrays of Ni lines
Jose L. Vicenta, Elvira M. Gonzaleza, Jose V. Anguitab
aDepartamento Fisica Materiales, Facultad CC. Fisicas, Universidad Complutense, 28040 Madrid, Spain
bInstituto Microelectronica Madrid, CNM, CSIC, Isaac Newton 8 (PTM), 28760 Madrid, Spain
Electron Beam Lithography and Ion Beam Etching allow us to fabricate ordered array of submicrometric Ni lines in sputtered Nb films. Optical lithography and etching define a cross-shape bridge which allows to apply the transport current paralle or perpendicular to the array of Ni lines. Magnetotransport (r,H) and (I,V) curves were measured close to the critical temperature. The experimental results show anisotropic vortex motion with clear channeling effects with matching features between the periodic array of pinning potentials and the driving vortex lattice.
Microwave nonlinear effects in He-cooled superconducting microstrip resonators
A. L. Karuzskii, A. E. Krapivka, A. V. Perestoronin, A. I. Golovashkin
P. N. Lebedev Physical Institute of RAS, Leninsky pr. 53, Moscow 119991, Russia
Thermally induced nonlinear phenomena in superconducting niobium microstrip resonators filled by liquid helium are studied in microwaves (10 GHz). With increasing input power a resonance curve of the resonator revealed the nonlinear behaviour and its form depended on the rate and direction of the microwave generator frequency sweep. The found nonlinear resonance is explained by thermally induced variations of the helium dielectric permittivity caused by the microwave power losses in superconductor. A number of interesting manifestations of this thermal instability have been observed including the parametric pulse generation of the monochromatic microwave signal, generation of acoustic pulses, stimulated Mandelshtam-Brillouin scattering like effect.
Work supported in part by the GNTP ANFKS "Superconductivity" (96-081).
Spin diffusion in a superconductor through mesoscopic ferromagnet/superconductor interfaces
Yun-Sok Shina, Hu-Jong Leea, Jinhee Kimb
aDepartment of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea
bElectronic Device Group, Korea Research Institute of Standards and Science, Taejon 305-600, Korea
We observed the suppression of the superconductivity in a mesoscopic Al wire (74 nm thick and 400 nm wide) in contact with an overlaid ferromagnetic Co wire (66 nm thick and 270 nm wide). A specimen was fabricated by using a combination of e-beam mircropatterning and e-gun evaporation. A spin-polarized current was injected to the Al wire from the Co wire through the interface and the transport properties of a few mm-long portion of Al wire from the interface were measured by the conventional lock-in technique. The estimated spin diffusion length in Al wire increased as T approached Tc from below and showed a strong correlation with the temperature dependence of the G-L superconducting coherence length.
The Gradient Expansion Theory of Inhomogeneous Superconducting Thin Films
Mamoru Uchinami
Laboratory of Mathematical Science, Meiji Pharmaceutical University, Kiyose, Tokyo 204-8588, Japan
We pay attention to inhomogeneous superconducting thin films in external magnetic field. As inhomogeneities introduced by the impurity Coulomb potential and the external magnetic field can be considered small over a distance comparable to an electron de Broglie wavelength, the observables should possess an expansion in powers of the gradient operator which measures the inhomogeneities. Based on formulation of the finite-temperature normal and superconducting Green's functions, we evolve the gradient expansion theory for the inhomogeneous superconducting thin films.
Dynamic response of Josephson junction arrays on a dice lattice
Piero Martinoli, Mauro Tesei, Christian Leemann, Piero Martinoli
Institut de Physique, Université de Neuchâtel, CH-2000 Neuchâtel, Switzerland
We report the results of ac conductance (G) measurements performed on Josephson junction arrays with a dice lattice structure. In these systems the subtle interplay between lattice geometry and magnetic field B leads to localization phenomena (Aharonov-Bohm cage) when B corresponds to half a flux quantum per unit tile, i.e., when the array is fully frustrated (f=1/2). At f=1/2, the superfluid component wImG(B) exhibits a deep minimum, whereas the dissipative one Re[1/G(B)] shows a pronounced maximum, thereby providing compelling evidence for magnetic localization. In swept-frequency measurements performed over a broad spectral range the dynamic response at f=1/2 was compared to that at other levels of frustration to search for the glass-like features seen in numerical simulations. Glass-like behavior was indeed observed, but can not be unambiguously attributed to magnetic localization.
Evidence for finite-temperature glass transition in two dimensions
Qing-Hu Chen, Xiao Hu
National Institute for Materials Science, Tsukuba 305-0047, Japan
Large-scale simulations have been performed in the current-driven two-dimensional XY gauge glass model with resistively-shunted junction dynamics, by meanse of a very effecient algorithm proposed before [1]. It is found that the linear resistivity at low temperatures tends to zero, which shows a finite temperature glass transition. Dynamical scaling analysis demonstrates that a nearly pefect collapse of current-voltage data can be achieved if the transition temperature Tc=0.22 (in units of the Josephson coupling strength), dynamical critical exponent z=2.0, and the static exponent n = 1.2, which agrees quite well with recent findings by an equilibrium simulation [2].
(1) Q H Chen, L H Tang, P Q Tong, Phys. Rev. Lett. 87 , 067001 (2001)
(2) B J Kim, Phys. Rev. B 62, 644(2000)
Superconductor-normal metal quantum phase transition in ultrathin cylinders
M.M. Rosario, Yu. Zadorozhny, B.Y. Rock, P.T. Carrigan, H. Wang, Y. Liu
Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA
We have carried out experiments on ultrathin superconducting cylinders which revealed the existence of a destructive regime, predicted by de Gennes to occur around half-integer flux quanta in doubly connected samples with a diameter smaller than the zero-temperature superconducting coherence length [Y. Liu, et al., Science 294, 2332 (2001)]. More recently we have done measurements on the superconductor-normal metal (S-N) quantum phase transition tuned by magnetic flux in these cylinders, and studied the effects of thermal and quantum phase slips near the S-N transition.
Indirect RKKY Exchange and Magnetic Phase Transitions in Ferromagnetic Insulator/Superconductor Multilayers
Mansur Khusainova, Bulat Garipovb, Yurii Proshinb
aMax-Plank-Institut fur Physik komplexer Systeme,D-01187 Dresden, Germany
bKazan State University, 420008 Kazan, Russia
An exchange model of the proximity effect for layered ferromagnetic insulator/superconductor (FI/S) structures is proposed. This model, along with direct exchange inside FI layers, includes long-range antiferromagneic RKKY exchange between localized spins on FI/S boundaries via S layer conduction electrons. Within this framework, possible mutual accommodation scenarios (layered antiferromagnetism as well as cryptoferromagnetism) for the superconducting and magnetic order parameters are found, the corresponding phase diagrams are plotted, and experimental results explained.
Dimensional crossover and hidden incommensurability in Josephson junction arrays of periodically repeated Sierpinski gaskets
Piero Martinolia, René Meyera, Sergey E. Korshunovb, Christian Leemanna, Piero Martinolia
aInstitut de Physique, Université de Neuchâtel, CH-2000 Neuchâtel, Switzerland
bL.D. Landau Institute for Theoretical Physics, Kosygina 2, 117940 Moscow, Russia
Josephson junction arrays of pediodically repeated Sierpinski gaskets are model superconductors sharing essential geometrical features with truly percolative systems. When exposed to a magnetic field B, their euclidian (E) or fractal (F) behavior depends on the level of frustration imposed by B and has been explored with high-resolution measurements of the sample magnetoinductance L(B). The EF crossover is reflected in a radical change of the fine structure of L(B), which is well described by theory. Anomalies in the symmetry of L(B) are attributed to a hidden incommensurability, which arises from the asymmetric diamagnetic response of the superconducting islands and perturbs the self-similarity of the gaskets.
Controlling distribution functions through supercurrent
Tero T. Heikkiläa, Tommy Vänskäa, Frank K. Wilhelmb
aMaterials Physics Laboratory, Helsinki University of Technology, P.O. Box 2200, FIN-02015 HUT, Finland
bSektion Physik and CeNS, Ludwig-Maximilians-Universität, Theresienstr. 37, D-80333 München, Germany
Josephson supercurrent in a normal-metal weak link can be accurately controlled by varying the quasiparticle distribution in the weak link through a voltage between the superconductors and extra normal-metal probes. This has been recently realized experimentally. We show that the opposite is also possible: the control of the quasiparticle distribution function via the supercurrent driven through the weak link. We suggest an experimental setup for probing this phenomenon.
Positive cross-correlations induced by ferromagnetic contacts in hybrid NS structures
Fabio Taddeia, Rosario Faziob
aNEST-INFM & ISI Foundation, Viale Settimio Severo, 65, I-10133 Torino, Italy
bNEST-INFM & Scuola Normale Superiore, I-56126 Pisa, Italy
Due to the Fermionic nature of carriers, correlations between electric currents flowing through two different contacts attached to a conductor present a negative sign. Possibility for positive cross-correlations has been demonstrated in hybrid NS structures under certain conditions. In this paper we show that positive cross-correlations can be induced, if not already present, in such structures by employing ferromagnetic leads with magnetizations aligned anti-parallel to each other. We consider three-terminal hybrid structures and calculate the mean-square correlations of current fluctuations as a function of the bias voltage at finite temperature.
Anomalous Resistive Oscillations in Mesoscopic Al Superconucting Wire
Xueqiang Zhanga, Jun Changa, Zhengchao Yina, Yusheng Hea, Tao Yangb, Yuandong Daib
aNational Laboratory for Superconductivity & Institute of Physics, CAS, Beijing 100080, China
bPhysics Department, Peking University, Beijing 100871, China
Differential resistance oscillations were observed in quasi-one dimensional mesoscopic aluminum wires under weak AC currents. The amplitude of the oscillation decreases with increase of the current and eventually changes into an abnormal peak near Tc in R-T curves. The oscillations vanish when magnetic field larger than 1 mT. With a small DC bias current the oscillations can be depressed. However, with increasing bias, oscillations become extremely large and negative differential resistance appears. The above anomaly was also confirmed by our I-V characteristics measurements. Preliminary interpretation of the anomalous oscillations suggests that the anomalies be related to the moving of the normal-superconducting boundaries between the voltage probes as temperatures close to Tc.
Size Effect on Vortex States in Superconducting Mesoscopic Aluminum Disks
Yoshikazu Terai, Taro Yakabe, Chieko Terakura, Taichi Terashima, Syuma Yasuzuka, Tadashi Takamasu, Shinya Uji
National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki, 305-0003, Japan
We have measured the non-periodic resistance peaks in superconducting Al disks under the magnetic field, whose sizes are much smaller than the superconducting coherence length (x0) of Al bulk. In circular and square disks with the size of 600 nm, there is no remarkable difference in the field intervals (DH) of the resistance peaks. However, in the disks of 500 nm, we found significant difference in DH. These results suggest that new vortex states depending on the sample topology appear when the size is sufficiently smaller than x0.
Paramagnetic supercurrent in a mesoscopic superconducting disk
Akinobu Kanda, Youiti Ootuka
Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
We report an experimental evidence for the paramagnetic supercurrent in a mesoscopic superconducting disk. The sample is an Al superconducting disk with a thin (0.1 mm width) drain lead. The radius of the disk is 0.75 mm and the thickness is 33 nm. Four Cu leads are connected to different parts of the ring periphery through highly resistive tunnel junctions. From voltage drop across a tunnel junction, we study the change in local superconducting energy gap as a function of perpendicular magnetic field. We find that the energy gap at each point decreases simultaneously with decreasing the magnetic field, showing that the circulating supercurrent is paramagnetic. The condition for the observation is the same as that for the paramagnetic Meissner effect [A. K. Geim et al., Nature 396, 144 (1998)], strongly indicating that the origin of the paramagnetic Meissner effect is the paramagnetic supercurrent.
Pauli Paramagnetic Effect and Spin-Orbit Scattering Time in Nb/Al2O3 Superconducting Multilayers
Yoshihisa Obia, Manabu Ikebeb, Hiroyuki Fujishirob
aInstitute for Materials Research, Tohoku University, Sendai 980-8577, Japan
bFaculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
The parallel critical field Hc2// of a thin film superconductor is inversely proportional to the film thickness d and Hc2// can be made arbitrary large for small enough d. In case of such a large orbital critical field, the Pauli paramagnetic effect controlled by the spin-orbit scattering becomes the main origin to limit Hc2// of the film. Then the analyses of Hc2// can provide an effective method to evaluate the spin-orbit scattering time ts0. Hc2// was calculated for various ts0 values and the thickness dependence of Hc2// of sputtered Nb/Al2O3 multilayers was analyzed. The effective values of ts0 were determined for Nb sublayers, which accounted for about 1/30 of the total electron scattering.
Suppression of Ferromagnetism due to Superconducting Proximity Effects
Hidenori Goto
The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako-shi, Saitama, 351-0198 Japan
Superconducting proximity effects in a ferromagnetic metal are directly observed by means of 59Co NMR in Co/Al bilayers. Below the superconducting transition temperature, the resonant frequency decreases and the nuclear spin-lattice relaxation rate divided by temperature, 1/T1T, increases. These results indicate the reduction of magnetic moment in the Co layer. We discuss the spatial variation of the local magnetic moment and its temperature dependence by analyzing the resonant line shapes.
Superconductivity at 20 mK in Compacted Submicrometer Platinum Powders
Alexander Schindlera, Reinhard Königa, Thomas Herrmannsdörferb, Hans F Brauna, Georg Eskaa, Detlef Güntherc, Michael Meissnerc, Michael Mertigd, Reiner Wahld, Wolfgang Pomped
aPhysikalisches Institut, Universität Bayreuth, D-95440 Bayreuth, Germany
bForschungszentrum Rossendorf, D-01314 Dresden, Germany
cHahn-Meitner-Institut, D-14109 Berlin, Germany
dTechnische Universität Dresden, D-01062 Dresden, Germany
The superconducting transition temperature and critical magnetic field observed in compacted Pt powder samples with submicrometer grain size are as high as 20 mK and 18 mT and thus more than one order of magnitude larger than for those with larger (micrometer) grain size. The submicrometer samples exhibit strong lattice strain and significantly smaller Debye temperatures compared to bulk platinum. Moreover, the ferromagnetic spin fluctuations of the conduction electrons are partially quenched in these compacts.
Interference of Nonequilibrium Quasiparticles Injected into a Superconducting Loop
K. Yu. Arutyunov, T. T. Hongisto, J. P. Pekola
University of Jyväskylä, Department of Physics, P.B. 35, 40351, Jyväskylä, Finland
Various nanostructures consisting of an aluminium loop overlapped by a copper electrode through a tunnel barrier were fabricated. At temperatures well below 1 K the tunnel current at fixed voltage bias is periodically modulated by external magnetic field. The period of the oscillations corresponds to one flux quantum enclosed by the area of the loop. The amplitude of the current modulation reaches maximum at bias voltages corresponding to the gap energy, and decreases with increase of the voltage bias. For a given voltage bias the amplitude of the current oscillations decreases with increase of the temperature. Oscillations of the tunnel current in magnetic field are hysteretic.
Temperature distribution in a superconductor during pulsed magnetization
S. Bræck, D. V. Shantsev, T. H. Johansen, Y. M. Galperin
Department of Physics, University of Oslo, P. O. Box 1048 Blindern, N 0316 Oslo, Norway
We calculate the temperature and field distributions in a bulk superconductor during a pulsed-field magnetization process. Heat is released inside the superconductor due to magnetic flux motion and radiates from the surface. The temperature profile is found by solving the heat diffusion equation with proper boundary conditions. Due to a temperature dependence of the critical current density, the trapped flux is less for larger heating. We calculate the trapped flux for different magnetization rates and different values of the maximum applied field, BM. Optimal values of BM corresponding to maximum trapped flux are found for different sets of parameters typical for YBaCuO trapped-field magnets. Evolution of field and temperature distributions during the complete magnetizing process is presented.
Magneto-optical imaging of magnetic pinning of individual vortices to a Bloch wall
P. E. Goa, L. E. Helseth, H. Hauglin, D. V. Shantsev, M. Baziljevich, T. H. Johansen
Department of Physics, University of Oslo, P. O. Box 1048 Blindern, N 0316 Oslo, Norway
Recently, a system for magneto-optical imaging (MOI) of individual vortices was developed in our laboratory. Reported here is a direct observation of the interaction between vortices and a Bloch wall in the indicator ferrite garnet. The Bloch wall separates two domains of opposite in-plane magnetization, and acts as a miniature bar magnet. We observe that the individual vortices can be pinned magnetically to the wall and follow the wall as it moves in a controlled way. The force acting on a single vortex in a NbSe2 single crystal from the domain wall has been determined. The results are compared with model calculations, and show a good agreement. Our direct demonstration of a fast and controlled vortex manipulation and monitoring has an interesting potential for practical applications.
Spectral Properties of the Flux-Flow Noise at the Peak Effect
G. Junga,b, Y. Paltiela, Y. Myasoedova, M. L. Rappaporta, E. Zeldova, M. J. Higginsc, S. Bhattacharyac,d
aDepartment of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel
bDepartment of Physics, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
cNEC Research Institute, 4 Independence Way, Princeton, New Jersey 08540
dTata Institute of Fundamental Research, Mumbai-400005, India
Large voltage noise commonly observed in the vicinity of the peak-effect in superconductors is ascribed to a novel mechanism consisting in a dynamic admixture of two thermodynamically distinct metastable vortex phases. In the frequency domain the peak effect noise exhibits 1/fa-like spectra. The response function of dynamically coexisting metastable vortex phases is shown to be similar to that of a first-order filter. The roll-off frequency at which the power exponent a changes is controlled by the time-of-flight of vortices across the region occupied by the disordered vortex phase.
New Evaluation Method for Optical Conductivity of Superconducting Thin Films
Etsuo Kawate
National Institute of Advanced Industrial Science and Technology, 1-1-1, Central 2, Umezono, Tsukuba, Ibaraki 305-8588, JAPAN
A symmetry X optical setup has been developing for the absolute measurements of reflectance and transmittance of specular samples in the infrared region. Both the transmittance spectra and the geometric mean of the reflectance spectra were measured within the uncertainty of 0.4%, respectively. The solution of two simultaneous equations with the measured reflectance and transmittance determines the optical conductivities of the sample. These conductivities by the new method are compared with those by the the ordinary method using Kramers-Kronig relations with a measurement of either reflectance or transmittance.