Application of a small oscillating magnetic field activates vortex motion deep within the vortex-solid region of YBa2Cu3O7
aa r X i v : . [ c ond - m a t . s up r- c on ] N ov Application of a small oscillating magnetic field activates vortex motion deep insidethe vortex-solid region of YBa Cu O M. Reibelt ∗ Physics Institute, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland (Dated: October 2, 2018)We have investigated the magnetic phase diagram of a fully oxygenated detwinned YBa Cu O single crystal by means of a combination of a low-temperature thermal analysis (DTA) techniquewith a simultaneous application of a small oscillating magnetic field. We observed a dip-shapedfeature in magneto-caloric data deep inside the vortex-solid region of YBa Cu O , which was acti-vated by the oscillating magnetic field. The feature can be explained by a self-heating effect due tovortex motion. However, the origin of this new feature is unknown. PACS numbers: 74.72.-h,74.25.Dw,74.25.Uv,74.25.Wx
INTRODUCTION
The investigation of the magnetic phase diagram ofhigh- T c superconductors has been of great importancesince their discovery. The effects of oscillating magneticfields on the vortex lattice of type-II superconductorshave been investigated theoretically [1–11]. Experimen-tally, the application of a small oscillating magnetic field(so-called shaking field) h ac has turned out to be a use-ful tool for the investigation of the properties of type-IIsuperconductors [12–19]. In this work we repeated themeasurements of [20] but with an additional shaking fieldapplied. We report on the, to the best of our knowl-edge, first observation of a dip-shaped feature in themagneto-caloric data ∆ T ( H ) that appeared only in thefield sweeps with simultaneously applied shaking field,while no such feature was present in the measurementswithout shaking field. SAMPLE CHARACTERIZATION ANDEXPERIMENTAL DETAILS
The investigated sample (mass ≈ . Cu O single crystal [21]. In order to re-duce vortex pinning, the crystal was slightly overdopedto achieve full stoichiometry, and it was mechanically de-twinned [22, 23]. The crystal has a critical temperature T c ≈
86 K and in a magnetic field of 7 T the meltingof the vortex lattice was observed at T m (7 T ) ≈
80 K[24]. The crystal was part of a mosaic of crystalswhich was studied in SANS experiments by White etal. [22, 25]. We conducted measurements of the quasi-isothermal magneto-caloric effect on the crystal, whichwas aligned with its c -axis parallel to the external mag-netic field ( H k c ). The calorimetric measurementswere done in a low-temperature home-built differential-thermal analysis (DTA) setup [26]. Prior to a measure-ment, we cooled the sample to the target temperaturein a constant magnetic field µ H = 7 T. The magneto-caloric effect was then monitored while decreasing H to zero at a constant rate µ dH/dt ≈ − − ; the tem-perature varied less than 1 K for the whole field sweep.The setup and measurement procedure were the sameas in our previous work on this crystal [20], except thatwe simultaneously applied a shaking field h ac in additionto the main magnetic field H during the measurement.The shaking field was applied perpendicular to the mainmagnetic field ( h ac ⊥ H ). RESULTS
In Fig. 1 we plotted the magneto-caloric data∆ T ( H ) = T ref ( H ) − T sample ( H ) for various tempera-tures on ramping the magnetic field H from 7 T down tozero. We labeled each curve with the sample tempera-ture at H = 2 . T , which is approximately the magneticfield where we observed the feature in the measurementswith additionally applied shaking field. For the black( T (2 . T ) = 17 . T (2 . T ) = 26 . FIG. 1: Magneto-caloric data ∆ T ( H ) = T ref ( H ) − T sample ( H ) of an YBa Cu O single crystal for several fieldsweeps at different temperatures; with and without an appliedshaking field. amplitude h ac ≈ . f = 500 Hz. Forcomparison we also plotted a field sweep where no shak-ing field was applied (blue curve, T (2 . T ) = 18 . h ac = 0 and h ac = 0) exhibitthe already in [20] reported first-order phase transitionbelow 1 T, marked by ellipses. Strikingly, a second,broader dip-shaped feature sets in at around 2 . T ( H ) curves results from a heating ofthe sample, which is most likely caused by a self-heatingeffect of the sample due to shaking field activatedvortex motion. The vortex shaking in combination withthe field sweep depins vortices which start to “walk”through the sample [7], thereby dissipating energy whichheats up the sample. We want to emphasize, that thisfeature occurs deep inside the vortex-solid region ofYBa Cu O . However, although unlikely we can notrule out that the feature stems from the latent heatof a first-order phase transition, since we have no datawith simultaneously applied shaking field on increasingthe magnetic field or from a temperature sweep thatshould show a cooling effect in case of a first-order phasetransition on crossing the supposed phase-transition linefrom the opposite side. Additional measurements arenecessary to clarify this issue. FIG. 2: Low-field part of the phase diagram of YBa Cu O for H k c . The position of the shaking field activated featureis drawn in as red diamonds. In Fig. 2 we replotted the low-field part of the phasediagram of YBa Cu O from Fig. 2(a) of Ref. [20], whichshows the phase-transition line for the first-order phasetransition at low magnetic fields which we reported al-ready in [20] (red dashed line in Fig. 2). We added thedata from the SANS measurements of White et al. etal. [22, 25] (blue triangle and circles in Fig. 2), whoobserved a vortex-lattice reorientation transition witha temperature independent phase-transition line (bluedashed line in Fig. 2). Finally, we added to the mag-netic phase diagram in Fig. 2 as red diamonds the dataof the onset of the shaking field activated feature, whichwas marked with arrows in Fig. 1. The data point at ≈ K stems from a field sweep which we did not showin Fig. 1. For clarity reasons we omitted the error bars forthe data from Refs. [20, 22]. The proximity of the newfeature to the vortex-lattice reorientation transition inthe magnetic phase diagram may be coincidental. White et al. [22] suggested that the vortex-lattice reorientationtransition, which they observed at about 2 T, is probablydriven by Fermi surface effects. Decreasing the magneticfield increases the distance between the vortices, whichin turn lowers the influence of the non-local effects whichcouple the vortex lattice to the Fermi surface. Vortexpinning should be affected by a symmetry change of thevortex lattice [27, 28]. Approaching the vortex-latticereorientation transition on decreasing the magnetic fieldlikely changes the vortex pinning potential already some-what before the transition is actually reached, therebyaiding the depinning process activated by the combinedaction of the shaking field and the decreasing main mag-netic field. CONCLUSIONS
To conclude, we have investigated the magnetic phasediagram of a fully oxygenated detwinned YBa Cu O single crystal by means of a combination of a low-temperature thermal analysis (DTA) technique with asimultaneous application of a shaking field. We founda shaking field activated dip-shaped feature deep insidethe vortex-solid region of YBa Cu O , which can be ex-plained by a self-heating effect due to activated vortexmotion. Merely a few preliminary measurements wereperformed with a shaking field simultaneously applied.The proximity of the new feature to the vortex-latticereorientation transition in the magnetic phase diagrammay be coincidental, but an influence of the reorienta-tion transition on the vortex pinning potential is possi-ble. However, the origin of the in this work reported newfeature is unknown. ACKNOWLEDGEMENTS
We thank to the group of A. Schilling, E. M. For-gan, J. S. White, and V. Hinkov for their supportand useful discussions, and we thank A. Erb for pro-viding the sample. This work was supported by theSchweizerische Nationalfonds zur F¨orderung der Wis-senschaftlichen Forschung, Grants No. 20-111653 andNo. 20-119793.
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