Thermodynamic evidence for the Bose glass transition in twinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ crystals

We used a micromechanical torsional oscillator to measure the magnetic response of a twinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ single crystal disk near the Bose glass transition. We observe an anomaly in the temperature dependence of the magnetization consistent with the appearance of a magnetic shielding perpendicular to the correlated pinning of the twin boundaries. This effect is related to the thermodynamic transition from the vortex liquid phase to a Bose glass state.

Figure 1

(a) Scanning electron microscope image of the polysilicon mechanical torsional oscillator with the superconducting ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (YBCO) sample mounted on it. (b) Cross section of the micromechanical silicon oscillator with the indication of the relevant geometrical dimensions. (c) Polarized light microscopy picture of the measured disk glued to the oscillator. The normal of the twin planes were at ${75}^{\circ }$ from the rotational axis of the paddle.

Figure 2

(a) Angular displacement of the silicon torsional oscillator, $\alpha$, as a function of temperature with an applied external magnetic field of 1 T for different orientations, ${\theta }_H$, between the substrate and the magnetic field. All data are take from field cooling experiments, and show reversibility upon warming. For angles $|{\theta }_H|<1.5^{\circ }$ the curves become nonmonotonic presenting a maximal value at a given temperature. The inset sketch is a vectorial representation of the sample magnetization indicating the different angles involved in this experiment. In (b) we plot representative data for small ${\theta }_H$ showing the symmetric response around the $c$-axis direction.

Figure 3

(a) Magnetization of the superconducting YBCO disk as a function of temperature for different applied magnetic fields. Continuous lines are data obtained from our experiments and discrete points values extracted from Welp et al. [17]. (b) Angular displacements of the oscillator as a function of temperatures for an applied field of 2 T for values of ${\theta }_H$ close to zero. A clear shift to lower values for $T^{*}$ as $|{\theta }_H|$ increases is visible. The inset plots the dependence of $T^{*}$ as a function of the orientation, ${\theta }_H$, for an applied magnetic field of 2 T.

Figure 4

(a) Displacement angle of the oscillator as a function of temperature for different magnetic fields applied at an angle ${\theta }_H=-0.4^{\circ }$. Inset: Magnetic field versus temperature phase diagram for the measured $dM/dT$ discontinuity in comparison with Bose glass critical temperatures obtained from transport measurements reported by Grigera et al. [6] and Maiorov et al. [5].