493 research outputs found

    Pressure-Induced Magnetic Quantum Phase Transitions from Gapped Ground State in TlCuCl3

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    Magnetization maesurements under hydrostatic pressure were performed on an S=1/2 coupled spin system TlCuCl3 with a gapped ground state under magnetic field H parallel to the [2,0,1] direction. With increasing applied pressure P, the gap decreases and closes completely at Pc=0.42 kbar. For P>Pc, TlCuCl3 undergoes antiferromagnetic ordering. A spin-flop transition was observed at Hsf=0.7T. The spin-flop field is approximately independent of pressure, although the sublattice magnetization increases with pressure. The gap and Neel temperature are presented as function is attributed to to the relative enhancement of the interdimer exchange interactions compared with the intradimer exchange interaction.Comment: 4pages,3figures To be published in J. Phys. Soc. Jpn. Vol.73 No.1

    Drastic Change of Magnetic Phase Diagram in Doped Quantum Antiferromagnet TlCu1−x_{1-x}Mgx_xCl3_3

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    TlCuCl3_3 is a coupled spin dimer system, which has a singlet ground state with an excitation gap of Δ/gμB\Delta/g\mu_{\mathrm B} = 5.5 T. TlCu1−x_{1-x}Mgx_xCl3_3 doped with nonmagnetic Mg2+^{2+} ions undergoes impurity-induced magnetic ordering. Because triplet excitation with a finite gap still remains, this doped system can also undergo magnetic-field-induced magnetic ordering. By specific heat measurements and neutron scattering experiments under a magnetic field, we investigated the phase diagram in TlCu1−x_{1-x}Mgx_xCl3_3 with x∼0.01x\sim 0.01, and found that impurity- and field-induced ordered phases are the same. The gapped spin liquid state observed in pure TlCuCl3_3 is completely wiped out by the small amount of doping.Comment: 9 pages, 5 figures, jpsj2 class file, to be published in J. Phy. Soc. Jpn. Vol.75 No.3 (2006); layout changed, unrelated figure remove

    Neutron Scattering Study of Magnetic Ordering and Excitations in the Doped Spin Gap System Tl(Cu1−x_{1-x}Mgx_x)Cl3_3

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    Neutron elastic and inelastic scattering measurements have been performed in order to investigate the spin structure and the magnetic excitations in the impurity-induced antiferromagnetic ordered phase of the doped spin gap system Tl(Cu1−x_{1-x}Mgx_x)Cl3_3 with x=0.03x=0.03. The magnetic Bragg reflections indicative of the ordering were observed at Q=(h,0,l){\pmb Q}=(h, 0, l) with integer hh and odd ll below TN=3.45T_{\rm N}=3.45 K. It was found that the spin structure of the impurity-induced antiferromagnetic ordered phase on average in Tl(Cu1−x_{1-x}Mgx_x)Cl3_3 with x=0.03x=0.03 is the same as that of the field-induced magnetic ordered phase for H∥b{\pmb H} \parallel b in the parent compound TlCuCl3_3. The triplet magnetic excitation was clearly observed in the a∗a^*-c∗c^* plane and the dispersion relations of the triplet excitation were determined along four different directions. The lowest triplet excitation corresponding to the spin gap was observed at Q=(h,0,l){\pmb Q}=(h, 0, l) with integer hh and odd ll, as observed in TlCuCl3_3. It was also found that the spin gap increases steeply below TNT_{\rm N} upon decreasing temperature. This strongly indicates that the impurity-induced antiferromagnetic ordering coexists with the spin gap state in Tl(Cu1−x_{1-x}Mgx_x)Cl3_3 with x=0.03x=0.03.Comment: 24 pages, 7 figures, 11 eps files, revtex style, will appear in Phys. Rev.

    Random Bond Effect in the Quantum Spin System (Tl1−x_{1-x}Kx_{x})CuCl3_3

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    The effect of exchange bond randomness on the ground state and the field-induced magnetic ordering was investigated through magnetization measurements in the spin-1/2 mixed quantum spin system (Tl1−x_{1-x}Kx_{x})CuCl3_3 for x<0.36x<0.36. Both parent compounds TlCuCl3_3 and KCuCl3_3 are coupled spin dimer systems, which have the singlet ground state with excitation gaps Δ/kB=7.7{\Delta}/k_{\rm B}=7.7 K and 31 K, respectively. Due to bond randomness, the singlet ground state turns into the magnetic state with finite susceptibility, nevertheless, the excitation gap remains. Field-induced magnetic ordering, which can be described by the Bose condensation of excited triplets, magnons, was observed as in the parent systems. The phase transition temperature is suppressed by the bond randomness. This behavior may be attributed to the localization effect.Comment: 19 pages, 7 figures, 12 eps files, revtex, will appear in PR

    Impurity-Induced Antiferromagnetic Ordering in the Spin Gap System TlCuCl_3

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    The magnetization measurements have been performed on the doped spin gap system TlCu_{1-x}Mg_xCl_3 with x <= 0.025. The parent compound TlCuCl_3 is a three-dimensional coupled spin dimer system with the excitation gap Delta/k_B = 7.7 K. The impurity-induced antiferromagnetic ordering was clearly observed. The easy axis lies in the (0,1,0) plane. It was found that the transition temperature increases with increasing Mg^{2+} concentration x, while the spin-flop transition field is almost independent of x. The magnetization curve suggests that the impurity-induced antiferromagnetic ordering coexists with the spin gap for x <= 0.017.Comment: 5 pages, 6 figures, revtex styl

    Cu-NMR study on the disordered quantum spin magnet with the Bose-glass ground state

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    Cu-NMR study has been performed on the disordered spin-gap system Tl1-xKxCuCl3 In the high-field H > HC=\Delta/\mu_B, where \Delta is the spin-gap, the hyperfine field becomes extremely inhomogeneous at low temperatures due to the field-induced magnetic order, indicating that the ordered spin state must be different from the pure TlCuCl3. In the low field H < HC, a saturating behavior in the longitudinal nuclear spin relaxation rate 1/T1 was observed at low temperatures, indicating existence of the magnetic ground state proposed to be Bose-glass phase by Fisher.Comment: RHMF200

    Spin-resonance modes of the spin-gap magnet TlCuCl_3

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    Three kinds of magnetic resonance signals were detected in crystals of the spin-gap magnet TlCuCl_3. First, we have observed the microwave absorption due to the excitation of the transitions between the singlet ground state and the excited triplet states. This mode has the linear frequency-field dependence corresponding to the previously known value of the zero-field spin-gap of 156 GHz and to the closing of spin-gap at the magnetic field H_c of about 50 kOe. Second, the thermally activated resonance absorption due to the transitions between the spin sublevels of the triplet excitations was found. These sublevels are split by the crystal field and external magnetic field. Finally, we have observed antiferromagnetic resonance absorption in the field-induced antiferromagnetic phase above the critical field H_c. This resonance frequency is strongly anisotropic with respect to the direction of the magnetic field.Comment: v.2: typo correction (one of the field directions was misprinted in the v.1
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