We propose, implement, and test experimentally long Josephson 0-pi junctions fabricated using conventional Nb-AlOx-Nb technology. We show that by using a pair of current injectors one can create an arbitrary discontinuity of the Josephson phase and, in particular, a pi discontinuity, just as in d-wave/s-wave or in d-wave/d-wave junctions, and study fractional Josephson vortices which spontaneously appear. Moreover, using such junctions, we can investigate the dynamics of the fractional vortices-a domain which is not yet available for natural 0-pi junctions due to their inherently high damping. We observe half-integer zero-field steps which appear on the current-voltage characteristics due to the hopping of semifluxons.
W e propose a new type of Josephson vortex ratchet. In thi s system a Josephson vortex m oves i n a peri odi c asym m etri c potenti alunder the acti on of a determ i ni sti c or random force w i th zero ti m e average. For som e i m pl em entati ons the am pl i tude of the potenti alcan be control l ed duri ng the experi m ent,thus,al l ow i ng to tune the perform ance ofthe system and bui l d rocki ng as w el las ashi ng ratchets. W e present a m odeldescri bi ng the dynam i cs ofthe uxon i n such a system ,show num eri calsi m ul ati on resul ts,and di scussthe di erencesbetw een conventi onaland Josephson vortex ratchets. T he i nvesti gati on ofthi s system m ay l ead to the devel opm ent ofthe uxon recti er | a devi ce w hi ch produces a quanti zed dc vol tage from col ored noi se (non-equi l i bri um uctuati ons). I. IN T R O D U C T IO NTo extractusefulwork from random m oti on wasa dream ofm anki nd si nce the daysw hen the B row ni an m oti on was recogni zed. U nfortunatel y,the second l aw oftherm odynam i cs forbi ds to extract energy \for free" from equi l i bri um therm al uctuati ons (w hi te noi se),w hi ch was di dacti cal l y dem onstrated by R .Feynm an i n hi s Lectures [ 1] . N everthel ess,one can extractusefulwork from non-equil ibrium or tim e-correl ated (col ored)noi se \notpayi ng" for i t,usi ng so-cal l ed ratchets,i.e.,system s w i th an asym m etri c peri odi c potenti al [ 2] . R ecentl y there was a boost ofacti vi ty i n thi s el d rel ated to the experi m entali nvesti gati on ofdi rected m oti on i n bi ol ogi calsystem s,so-cal l ed B row ni an m otors w hi ch,e.g.,m ove m uscl es ortransportvesi cl esi n a cel l [ 3] . In the l atter case the probabl e m echani sm ofoperati on i s the m oti on ofki nesi n m ol ecul es al ong the surface ofm i crotubul es,w hi ch can be m apped to the m oti on ofB row ni an parti cl esal ong a one-di m ensi onalratchetpotenti alw i th the peri od 8: 2nm [ 4] . T he non-equi l i bri um energy i ssuppl i ed by chem i calreacti on ofspl i tti ng ofadenosi ne tri phosphate (AT P) w hi ch takes pl ace cl ose to the ki nesi n m ol ecul e.In addi ti on to the appl i cati on ofratchetsasnoi se recti ers,i twassuggested to use them forvery e ci entseparati on ofsm al lobjects w i th di erent m obi l i ty,e.g.,D N A m ol ecul es,vi ruses,etc. [ 5, 6] . T he parti cl e separati on i s based on so-cal l ed determ i ni sti c ratchets [ 7] ,w here the parti cl esm ove i n a certai n di recti on underthe acti on ofa determ i ni sti c force w i th zero ti m e average.M oreover,changi ng the force pro l e one can reverse the di recti on ofthe parti cl e m oti on [ 7] . T he cl assi cati on and di scussi on ofdi erent types ofratchet system s can be found i n R ef. [ 8] .In thi spaperwe focuson Josephson ratchetsw hi ch are ofparti cul ari nterestbecause (a)the di rected m oti on resul ts i n a dc vol tage accordi ng to the Josephson rel ati on and (b) these system s can operate at very hi gh frequenci es up to about 100G H z. A s a rst exam pl e we ...
We study experimentally the critical depinning current Ic versus applied magnetic field B in Nb thin films which contain 2D arrays of circular antidots placed on the nodes of quasiperiodic (QP) fivefold Penrose lattices. Close to the transition temperature Tc we observe matching of the vortex lattice with the QP pinning array, confirming essential features in the Ic(B) patterns as predicted by Misko et al. [Phys. Rev. Lett. 95(2005)]. We find a significant enhancement in Ic(B) for QP pinning arrays in comparison to Ic in samples with randomly distributed antidots or no antidots.PACS numbers: 74.25. Qt, 74.25.Sv, 74.70.Ad, 74.78.Na The formation of Abrikosov vortices in the mixed state of type-II superconductors [1] and their arrangement in various types of "vortex-phases", ranging from the ordered, triangular Abrikosov lattice to disordered phases [2,3,4] has a strong impact on the electric properties of superconductors. Both, in terms of device applications and with respect to the fundamental physical properties of so-called "vortex-matter", the interaction of vortices with defects, which act as pinning sites, plays an important role. Recent progress in the fabrication of nanostructures provided the possibility to realize superconducting thin films which contain artificial defects as pinning sites with well-defined size, geometry and spatial arrangement. In particular, artificially produced periodic arrays of submicron holes (antidots) [5,6,7,8] and magnetic dots [9,10,11,12] as pinning sites have been intensively investigated during the last years, to address the fundamental question how vortex pinning -and thus the critical current density j c in superconductors -can be drastically increased.In this context, it has been shown that a very stable vortex configuration, and hence an enhancement of the critical current I c occurs when the vortex lattice is commensurate with the underlying periodic pinning array. This situation occurs in particular at the so-called first matching field B 1 = Φ 0 /A, i.e., when the applied field B corresponds to one flux quantum Φ 0 = h/2e per unit-cell area A of the pinning array. In general, I c (B) may show a strongly non-monotonic behavior, with local maxima at matching fields B m = mB 1 (m: integer or a rational number), which reflects the periodicity of the array of artificial pinning sites.As pointed out by Misko et al. [13], an enhancement of I c occurs only for an applied field close to matching fields, which makes it desirable to use artificial pinning arrays with many built-in periods, in order to provide either very many peaks in I c (B) or an extremely broad peak in * Electronic address: koelle@uni-tuebingen.de I c (B). Accordingly, Misko et al. studied analytically and by numerical simulation vortex pinning by quasiperiodic chains and by 2D pinning arrays, the latter forming a fivefold Penrose lattice [14], and they predicted that a Penrose lattice of pinning sites can provide an enormous enhancement of I c , even compared to triangular and random pinning arrays.We ...
We experimentally demonstrate the occurrence of negative absolute resistance (NAR) up to about −1Ω in response to an externally applied dc current for a shunted Nb-Al/AlOx-Nb Josephson junction, exposed to a microwave current at frequencies in the GHz range. The realization (or not) of NAR depends crucially on the amplitude of the applied microwave current. Theoretically, the system is described by means of the resistively and capacitively shunted junction model in terms of a moderately damped, classical Brownian particle dynamics in a one-dimensional potential. We find excellent agreement of the experimental results with numerical simulations of the model.
We investigate 3-junction SQUIDs which show voltage rectification if biased with an ac current drive with zero mean value. The Josephson phase across the SQUID experiences an effective ratchet potential, and the device acts as an efficient rocking ratchet, as demonstrated experimentally for adiabatic and nonadiabatic drive frequencies. For high-frequency drives the rectified voltage is quantized due to synchronization of the phase dynamics with the external drive. The experimental data are in excellent agreement with numerical simulations including thermal fluctuations.PACS numbers: 74.40.+k, 85.25.Dq During the last decade, directed molecular motion in the absence of a directed net driving force or temperature gradient in biological systems has drawn much attention to Brownian motors.[1, 2] Nonequilibrium fluctuations can induce e.g. transport of particles along periodic structures which lack reflection symmetry -socalled ratchets. An important class of ratchets is given by the rocking ratchet, characterized by a time-independent potential and an external perturbation (driving force) which may be either deterministic or stochastic, or a combination of both. [3,4,5,6,7] The number of ratchet systems considered for experimental studies has steadily been growing during recent years.[2] In particular, superconducting ratchets, based on the motion of Abrikosov vortices [8,9,10,11,12], Josephson vortices [13,14,15,16,17] or the phase difference of the superconducting wavefunction (Josephson phase) in SQUID ratchets [18,19,20,21] have been investigated. Those systems offer the advantage of (i) good experimental control over externally applied driving forces (here: currents), (ii) easy detection of directed motion, which creates a dc voltage, and (iii) experimental access to studies over a wide frequency range of external perturbations (adiabatic and non-adiabatic regime), and transition from overdamped to underdamped dynamic regimes, enabling studies of inertial effects and transition to chaos.Zapata et al.[18] proposed a 3-junction (3JJ) SQUID ratchet, which consists of a superconducting loop of inductance L, intersected by one Josephson junction in one arm and by two Josephson junctions connected in series in the other arm. For vanishing L a quasi-one dimensional (1D) ratchet potential can be obtained, and rectification of an ac bias current for low-and high-frequency drive of such a rocking ratchet has been predicted. In this paper we investigate a 3JJ SQUID, similar to the one proposed in [18]. We derive the equations of motion and the ratchet potential for our type of device, we present its experimental realization, and we investigate its operation as a rocking ratchet for both adiabatic and * Electronic address: koelle@uni-tuebingen. non-adiabatic drive. We also compare experimental results with numerical simulations for our device and for the originally proposed 3JJ SQUID ratchet.We first discuss the underlying dynamic equations, using the resistively and capacitively shunted junction model [22,23]. To simp...
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