Intrinsic double-peak structure of the specific heat in low-dimensional quantum ferrimagnets

Motivated by recent magnetic measurements on A3Cu3(PO4)4 (A=Ca,Sr) and Cu(3-Clpy)2(N3)2 (3-Clpy=3-Chloropyridine), both of which behave like one-dimensional ferrimagnets, we extensively investigate the ferrimagnetic specific heat with particular emphasis on its double-peak structure. Developing a modified spin-wave theory, we reveal that ferromagnetic and antiferromagnetic dual features of ferrimagnets may potentially induce an extra low-temperature peak as well as a Schottky-type peak at mid temperatures in the specific heat.Comment: 5 pages, 6 figures embedded, Phys. Rev. B 65, 214418 (2002

Evidence for Carrier-Induced High-Tc Ferromagnetism in Mn-doped GaN film

A GaN film doped with 8.2 % Mn was grown by the molecular-beam-epitaxy technique. Magnetization measurements show that this highly Mn-doped GaN film exhibits ferromagnetism above room temperature. It is also revealed that the high-temperature ferromagnetic state is significantly suppressed below 10 K, accompanied by an increase of the electrical resistivity with decreasing temperature. This observation clearly demonstrates a close relation between the ferromagnetism with extremely high-Tc and the carrier transport in the Mn-doped GaN film.Comment: 9 pages, 3 figure

Nuclear Spin-Lattice Relaxation in One-Dimensional Heisenberg Ferrimagnets: Three-Magnon versus Raman Processes

Nuclear spin-lattice relaxation in one-dimensional Heisenberg ferrimagnets is studied by means of a modified spin-wave theory. We consider the second-order process, where a nuclear spin flip induces virtual spin waves which are then scattered thermally via the four-magnon exchange interaction, as well as the first-order process, where a nuclear spin directly interacts with spin waves via the hyperfine interaction. We point out a possibility of the three-magnon relaxation process predominating over the Raman one and suggest model experiments.Comment: to be published in J. Phys. Soc. Jpn. 73, No. 6 (2004

ΛΛ\Lambda\Lambda-ΞN\Xi N-ΣΣ\Sigma\Sigma coupling in ΛΛ 6^{~6}_{\Lambda\Lambda}He with the Nijmegen soft-core potentials

The $\Lambda\Lambda$-$\Xi N$-$\Sigma\Sigma$ coupling in $^{~6}_{\Lambda\Lambda}$He is studied with the [$\alpha$ + $\Lambda$ + $\Lambda$] + [$\alpha$ + $\Xi$ + $N$] + [$\alpha$ + $\Sigma$ + $\Sigma$] model, where the $\alpha$ particle is assumed as a frozen core. We use the Nijmegen soft-core potentials, NSC97e and NSC97f, for the valence baryon-baryon part, and the phenomenological potentials for the $\alpha-B$ parts ($B$=$N$, $\Lambda$, $\Xi$ and $\Sigma$). We find that the calculated $\Delta B_{\Lambda\Lambda}$ of $^{~6}_{\Lambda\Lambda}$He for NSC97e and NSC97f are, respectively, 0.6 and 0.4 MeV in the full coupled-channel calculation, the results of which are about half in comparison with the experimental data, $\Delta B^{exp}_{\Lambda\Lambda}=1.01\pm0.20^{+0.18}_{-0.11}$ MeV. Characteristics of the $S=-2$ sector in the NSC97 potentials are discussed in detail.Comment: 18 pages, 4 figure

Charge echo in a Cooper-pair box

A spin-echo-type technique is applied to an artificial two-level system that utilizes charge degree of freedom in a small superconducting electrode. Gate-voltage pulses are used to produce the necessary pulse sequence in order to eliminate the inhomogeneity effect in the time-ensemble measurement and to obtain refocused echo signals. Comparison of the decay time of the observed echo signal with estimated decoherence time suggests that low-frequency energy-level fluctuations due to the 1/f charge noise dominate the dephasing in the system.Comment: 4 pages, 3 figure

Ground State Property of an Alternating Spin Ladder Involving Two Kinds of Inter-Chain Interactions

The ground state property of the alternating spin ladder is studied in the case that the system involves an antiferromagnetic intra-chain interaction as well as two kinds of inter-chain interactions; one is between spins of the same magnitude and the other is between spins with different magnitudes. The calculation has been carried out by the exact diagonalization method. As a consequence of the competition among interactions, the system is revealed to show an interesting variety of phases in the ground state property. Its phase diagram is exhibited in the parameter space of the system. We find that, however small the total amount of the inter-chain interactions is, the ferrimagnetic ground state becomes unstable in a certain region. In this case, which of the ferrimagnetic and the singlet ground state to appear is determined only by the ratio between the inter-chain interactions regardless of their total amount. The nature of two phases appearing in the singlet region of the phase diagram and the type of the phase transition between them are also discussed. The results are ensured by comparing with those of obtained in other models which are contained in our model as special limiting cases.Comment: 12 pages, 9 PostScript figure

Nuclear spin-lattice relaxation in ferrimagnetic clusters and chains: A contrast between zero and one dimensions

Motivated by ferrimagnetic oligonuclear and chain compounds synthesized by Caneschi et al., both of which consist of alternating manganese(II) ions and nitronyl-nitroxide radicals, we calculate the nuclear spin-lattice relaxation rate 1/T_1 employing a recently developed modified spin-wave theory. 1/T_1 as a function of temperature drastically varies with the location of probe nuclei in both clusters and chains, though the relaxation time scale is much larger in zero dimension than in one dimension. 1/T_1 as a function of an applied field in long chains forms a striking contrast to that in finite clusters, diverging with decreasing field like inverse square root at low temperatures and logarithmically at high temperatures.Comment: to be published in Phys. Rev. B 68 August 01 (2003

Temperature square dependence of the low frequency 1/f charge noise in the Josephson junction qubits

To verify the hypothesis about the common origin of the low frequency 1/f noise and the quantum f noise recently measured in the Josephson charge qubits, we study temperature dependence of the 1/f noise and decay of coherent oscillations. T^2 dependence of the 1/f noise is experimentally demonstrated, which supports the hypothesis. We also show that dephasing in the Josephson charge qubits off the electrostatic energy degeneracy point is consistently explained by the same low frequency 1/f noise that is observed in the transport measurements.Comment: 4 pages, 2 figure