Spin accumulation in thin Cs salts on contact with optically polarized Cs vapor

The spin angular momentum accumulates in the Cs nuclei of salt on contact with optically pumped Cs vapor. The spin polarization in stable chloride as well as dissociative hydride indicates that nuclear dipole interaction works in spin transferring with a lesser role of atom exchange. In the solid film, not only the spin buildup but also the decay of enhanced polarization is faster than the thermal recovery rate for the bulk salt. Eliminating the signal of thick salt, we find that the nuclear spin polarization in the chloride film reaches over 100 times the thermal equilibrium.

Figure 1

Setup for optical NMR enhancement. A salt-vapor cell was placed at 0.56 T of the permanent magnet. The field homogeneity (15 ppm) was better than the NMR linewidth of salt (300 ppm). The magnet was thermally isolated from the NMR probe with an Al block cooled by water. The NMR frequency varied due to temperature drift of magnet, but it was negligible for evaluating the spin polarization.

Figure 2

NMR signals in CsCl (28 mg) with 2.1 kPa N${}_2$ were averaged 12 times for optical pumping and 144 times for thermal equilibrium with periodic intervals of 30 min. The enhancements were $9.6$ and $-13.2$, respectively, at the laser frequencies 351 720.0 and 351 731.5 GHz. The linewidth is narrower than the hydrides, but a small broadening due to the drift of magnetic field was observed.

Figure 3

Laser-frequency dependence of the NMR area for CsD ($▴$, $▾$) and CsCl ($•$ first run) for the $\sigma$-polarized incident light. The signals were averaged several times with the repetition time 100 s for CsD and 200 s for CsCl.

Figure 4

Nuclear spin polarization was positively built up in CsD by optical pumping at 351 720 GHz. Horizontal axis is the delay time of RF $\pi$/2 pulse from saturation RF pulses. The buildup time at each temperature is indicated along with a single exponential curve best fitted free at the origin ($t=0$).

Figure 5

Thermal recovery rate for Cs nuclei in the hydrides ($\square$, $▵$, $◊$) increases more rapidly than exponential without optical pumping, though the rate for deuteron ($\circ$) decreases by heating. The spin buildup rates by pumping ($▴$ 2.7 T, $•$ 0.56 T) became larger than the corresponding rates in the dark.

Figure 6

Buildup rate in CsCl for optical pumping at 0.56 T (first $•$, third $\blacksquare$). The bulk degassed polycrystal was used as reference at 0.56 T ($\circ$), and thick salt on the sidewall of pumping cell at 2.7 T ($▵$).

Figure 7

The spin buildup for CsCl in the first run by optical pumping at 351 720 GHz ($•$ the left vertical axis) and the thermal recovery ($\circ$ the right axis). Dotted curves are the best fit by a single exponential function fixed at the origin.

Figure 8

Buildup rate (black curve) at the rising edge, signal amplitude (dotted red), and enhancement (dashed orange) for $t\to \infty$ are calculated from Eq. (A3). The buildup rate is scaled by the units of $\gamma$ at the left vertical axis. The amplitude is normalized by $J_{s}A/\gamma$, and the enhancement is by $J_s/\gamma N{I}_0{\lambda }_d$. The thickness ${\xi }_d=0.01$ corresponds to a few atomic layers since the spin diffusion length $\lesssim 100$ nm.

Figure 9

Spin rise and decay for CsCl in the third run. The inset shows the saturation RF pulses and the duration of optical pumping. A large fitting error in parenthesis means that the rising and falling edges were too fast to be described by a single exponential function (dotted curve).

Figure 10

Temperature dependence of the NMR area for CsD ($▴$ from Fig. 4) and CsCl ($•$ first run, $\blacksquare$ third run). The arrows indicate the time sequence of measurements. Negative areas are also plotted for the third run. For a wide range of temperatures, the thermal signal helps calibrate the sensitivity.

Figure 11

Single-shot detection of NMR signal in CsCl. The enhancement in the third run at 125${}^{\circ }$C was larger than in the first run at 100${}^{\circ }$C. Thermal signals were averaged many times. Frequency shift was due to the change of applied magnetic field according to the cell temperature.

Figure 12

Positive enhancement ($•$) was measured at 125${}^{\circ }$C for the cured CsCl cell. The curves are $E_R$ calculated for the true enhancement $E=50$, 100, 200, and $\infty$ by use of $E\delta =9.4$ in Eq. (1).

Figure 13

Spin transfer from polarized atomic vapor to a salt film. The initial condition $I(x,0)=0$ is due to saturation RF pulses, and the diffusion spin current transmits to the substrate at $x=d$. $J_a$ and $J_s$ are the spin current density at $x=0$, respectively, in the vapor and in the salt.