Near-Surface Structural Phase Transition of ${\mathrm{SrTiO}}_3$ Studied with Zero-Field $\beta$-Detected Nuclear Spin Relaxation and Resonance

We demonstrate that zero-field $\beta$-detected nuclear quadrupole resonance and spin relaxation of low energy ${}^8\mathrm{Li}$ can be used as a sensitive local probe of structural phase transitions near a surface. We find that the transition near the surface of a ${\mathrm{SrTiO}}_3$ single crystal occurs at $T_c\sim 150\mathrm{K}$, i.e., $\sim 45\mathrm{K}$ higher than $T_c^{\mathrm{bulk}}$, and that the tetragonal domains formed below $T_c$ are randomly oriented.

Figure 1

The polarization as a function of time, normalized to its initial value at $T=290\mathrm{K}$, at several temperatures.

Figure 2

The normalized initial polarization as a function of temperature. The solid line is a guide to the eye. The inset is the calculated ${}^8\mathrm{Li}$ stopping profile.

Figure 3

The calculated $\eta$ as a function of temperature. The inset shows the different types of ${}^8\mathrm{Li}$ sites in the ${\mathrm{SrTiO}}_3$ unit cell.

Figure 4

${}^8\mathrm{Li}$ $\beta$-NQR lines in ${\mathrm{SrTiO}}_3$ for different temperatures. The line shifts, broadens, and weakens as the sample is cooled from room temperature to 100 K. The solid lines are fits to a Lorentzian, which describes the data very accurately.

Figure 5

The resonance frequency, $3{\nu }_q$, as a function of temperature, obtained from fitting the lines to a Lorentzian shape. The solid lines are the calculated value of $3{\nu }_q$ from the relative increase of ${\nu }_q$ scaled to match the data at 250 K.