Takayuki Kawamata

Enhancement of thermal conductivity due to spinons in the one-dimensional spin system SrCuO2

We have measured the thermal conductivity along the c-axis parallel to the spin-chains, κc, of the one-dimensional antiferromagnetic spin system SrCuO2, using as-grown and O2-annealed single-crystals grown from raw materials with 99.9 % (3N) and 99.99 % (4N) purity. The value of κc around 50 K, where large contribution of the thermal conductivity due to spinons, κspinon, is observed, is markedly enhanced by both the increase of the purity of raw materials and the O2-annealing. Therefore, the increase of κc implies that κspinon is enhanced due to the decrease of spin defects caused by impurities in raw materials and by oxygen defects. The mean free path of spinons is as large as about 24000 A at low temperatures in the O2-annealed single-crystal grown from raw materials with 4N purity.

Hole trapping by Ni, Kondo effect, and electronic phase diagram in nonsuperconducting Ni-substituted La2-xSrxCu1-yNiyO4

In order to investigate the electronic state in the normal state of high-Tc cuprates in a wide range of temperature and hole-concentration, specific-heat, electrical-resistivity, magnetization and muon-spin-relaxation (muSR) measurements have been performed in non-superconducting Ni-substituted La2-xSrxCu1-yNiyO4 where the superconductivity is suppressed through the partial substitution of Ni for Cu without disturbing the Cu-spin correlation in the CuO2 plane so much. In the underdoped regime, it has been found that there exist both weakly localized holes around Ni and itinerant holes at high temperatures. With decreasing temperature, all holes tend to be localized, followed by the occurrence of variable-range hopping conduction at low temperatures. Finally, in the ground state, it has been found that each Ni2+ ion traps a hole strongly and that a magnetically ordered state appears. In the overdoped regime, on the other hand, it has been found that a Kondo-like state is formed around each Ni2+ spin at low temperatures. In conclusion, the ground state of non-superconducting La2-xSrxCu1-yNiyO4 changes upon hole doping from a magnetically ordered state with the strong hole-trapping by Ni2+ to a metallic state with Kondo-like behavior due to Ni2+ spins, and the quantum phase transition is crossover-like due to the phase separation into short-range magnetically ordered and metallic regions.

Thermal conductivity of the quasi one-dimensional spin system Sr2V3O9

We have measured the thermal conductivity along the [101] direction, kappa_[101]_, along the [10-1] direction, kappa_[10-1]_, and along the b-axis, kappa_b_, of the quasi one-dimensional S=1/2 spin system Sr_2_V_3_O_9_ in magnetic fields up to 14 T, in order to find the thermal conductivity due to spinons and to clarify whether the spin-chains run along the [101] or [10-1] direction. It has been found that both kappa_[101]_, kappa_[10-1]_ and kappa_b_ show one peak around 10 K in zero field and that the magnitude of kappa_[10-1]_ is larger than those of kappa_[101]_ and kappa_b_. By the application of magnetic field along the heat current, the peak of kappa_[10-1]_ is markedly suppressed, while the peaks of kappa_[101]_ and kappa_b_ little change. These results indicate that there is a large contribution of spinons to kappa_[10-1]_ and suggest that the spin-chains run along the [10-1] direction.

Thermal conductivity in the Bose-Einstein Condensed state of triplons in the bond-alternating spin-chain system Pb2V3O9

In order to clarify the origin of the enhancement of the thermal conductivity in the Bose-Einstein Condensed (BEC) state of field-induced triplons, we have measured the thermal conductivity along the [101] direction parallel to spin-chains, κ||[101], and perpendicular to spin-chains, κ⊥[101], of the S=1/2 bond-alternating spin-chain system Pb2V3O9 in magnetic fields up to 14 T. With increasing field at 3 K, it has been found that both κ||[101] and κ⊥[101] are suppressed in the gapped normal state in low fields. In the BEC state of field-induced triplons in high fields, on the other hand, κ||[101] is enhanced with increasing field, while κ⊥[101] is suppressed. That is, the thermal conductivity along the direction, where the magnetic interaction is strong, is markedly enhanced in the BEC state. Accordingly, our results suggest that the enhancement of κ||[101] in the BEC state is caused by the enhancement of the thermal conductivity due to triplons on the basis of the two-fluid model, as in the case of the superfluid state of liquid 4He.

Single-crystal growth of Pb2V3O9 and the Bose-Einstein condensed state of triplons studied by thermal conductivity, specific heat and magnetization measurements

We have succeeded in growing large-size single crystals of the quasi-one-dimensional S = 1/2 spin-dimer system Pb2V3O9 by the floating-zone method, and also investigated the thermal conductivity, specific heat and magnetization in magnetic fields. In high magnetic fields, it has been found that the suppression of the thermal conductivity by the application of magnetic field is relaxed. This behavior is caused by the enhancement of the thermal conductivity due to triplons and/or phonons owing to the extension of the mean free path of triplons and/or phonons in the Bose-Einstein condensed (BEC) state of field-induced triplons. We have estimated the critical field Hc(T) between the BEC state of triplons and the gapped normal state from the thermal conductivity, specific heat and magnetization measurements and obtained the value of the critical exponent oslash; ~ 2.0 in [Hc(T) – Hc(0)] ∝ To using Hc(T)s at low temperatures.