Nuclear spin relaxation at ultralow temperatures

Abstract

Nuclear spin relaxation induced by hyperfine coupling is studied theoretically at positive and negative submicrokelvin temperatures. By avoiding the assumption of the high-temperature limit, adopted in conventional theories, we derive a formula in which the relaxation rate is expressed in terms of thermal averages of nuclear spin energies. The exchange interaction induces an asymmetry in the energy spectrum, which leads to relaxation rates that depend on whether the nuclear spin temperature is positive or negative. High-temperature expansion methods and Monte Carlo simulations are applied to explain the anomalous results by Hakonen et al. in rhodium qualitatively. ͓S0163-1829͑99͒12313-0͔

Pressure evolution of the structure of (NH 3 )K 3 C 60

“…The nature of the electronic state of (NH 3 )K 3 C 60 at low temperatures has been investigated extensively and established as that of an antiferromagnetic (AF) insulator with a Néel temperature T N ≈ 40 K [3][4][5][6][7]. In addition, a structural phase transition has been observed below 150 K to an orthorhombic structure (space group F ddd) derived by doubling the lattice constants of the high-temperature phase along all three axes [8]. The K + -NH 3 pairs orient along the 110 direction in an antiferroelectric fashion, while the C 60 units adopt two distinct orientations related by 90 • about c and are antiferrorotatively ordered along a and b (fig.…”