arXiv subjects

X. F. Sun

Publications and source records attributed to X. F. Sun.

At least 55 records · Page 3Linked to original sources

arXiv2013

Magnetically originated phonon-glass-like behavior in Tb_2Ti_2O_7 single crystal

We report a study on the thermal conductivity (κ) of Tb_2Ti_2O_7 single crystals at low temperatures. It is found that in zero field this material has an extremely low phonon thermal conductivity in a broad temperature range. The mean free path of phonons is even smaller than that of amorphous materials and is 3--4 orders of magnitude smaller than the sample size at 0.3 K. The strong spin fluctuation of the spin-liquid state is discussed to be the reason of the strong phonon scattering. The magnetic-field dependence of κand comparison with Y_2Ti_2O_7 and TbYTi_2O_7 confirm the magnetic origin of this phonon-glass-like behavior.

cond-mat.str-el↗

arXiv2013

An irreversible magnetic-field dependence of low-temperature heat transport of spin-ice compound Dy_2Ti_2O_7 in a [111] field

We study the low-temperature thermal conductivity (κ) of Dy_2Ti_2O_7 along and perpendicular to the (111) plane and under the magnetic field along the [111] direction. Besides the step-like decreases of κat the field-induced transitions from the spin-ice state to the kagomé-ice state and then to the polarized state, an abnormal phenomenon is that the κ(H) isotherms show a clear irreversibility at very low temperatures upon sweeping magnetic field up and down. This phenomenon surprisingly has no correspondence with the well-known magnetization hysteresis. Possible origins for this irreversibility are discussed; in particular, a pinning effect of magnetic monopoles in spin ice compound by the weak disorders is proposed.

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arXiv2013

Low-Temperature Low-Field Phases of the Pyrochlore Quantum Magnet Tb$_2$Ti$_2$O$_7$

By means of ac magnetic-susceptibility measurements, we find evidence for a new magnetic phase of Tb$_2$Ti$_2$O$_7$ below about 140 mK in zero magnetic field. In magnetic fields parallel to [111], this phase---exhibiting frequency- and amplitude-dependent susceptibility and an extremely slow spin dynamics---extends to about 70 mT, at which it gives way to another phase. The field dependence of the susceptibility of this second phase, which extends to about 0.6 T, indicates the presence of a weak magnetization plateau below 50 mK, as has been predicted by a single-tetrahedron four-spin model, giving support to the underlying proposal that the disordered low-field ground state of Tb$_2$Ti$_2$O$_7$ is a quantum spin ice.

cond-mat.str-el↗

arXiv2012

Large magnetic heat transport in a Haldane chain material Ni(C$_3H_{10}N_2)_2NO_2ClO_4

We report a study on the heat transport of an S = 1 Haldane chain compound Ni(C_3H_{10}N_2)_2NO_2ClO_4 at low temperatures and in magnetic fields. The zero-field thermal conductivities show a remarkable anisotropy for the heat current along the spin-chain direction (κ_b) and the vertical direction (κ_c), implying a magnetic contribution to the heat transport along the spin-chain direction. The magnetic-field-induced change of the spin spectrum has obviously opposite impacts on κ_b and κ_c. In particular, κ_b(H) and κ_c(H) curves show peak-like increases and dip-like decreases, respectively, at \sim 9 T, which is the critical field that minimizes the spin gap. These results indicate a large magnetic thermal transport in this material.

cond-mat.str-el↗

arXiv2012

Low-temperature heat transport and magnetic-structure transition of the hexagonal TmMnO_3 single crystals

We study the low-temperature heat transport, as well as the magnetization and the specific heat, of TmMnO_3 single crystals to probe the transitions of magnetic structure induced by magnetic field. It is found that the low-T thermal conductivity (κ) shows strong magnetic-field dependence and the overall behaviors can be understood in the scenario of magnetic scattering on phonons. In addition, a strong "dip"-like feature shows up in κ(H) isotherms at 3.5--4 T for H \parallel c, which is related to a known spin re-orientation of Mn^{3+} moments. The absence of this phenomenon for H \parallel a indicates that the magnetic-structure transition of TmMnO_3 cannot be driven by the in-plane field. In comparison, the magnetothermal conductivity of TmMnO_3 is much larger than that of YMnO_3 but smaller than that of HoMnO_3, indicating that the magnetisms of rare-earth ions are playing the key role in the spin-phonon coupling of the hexagonal manganites.

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arXiv2012

Paramagnetic ground state with field-induced partial order in Nd_3Ga_5SiO_{14} probed by low-temperature heat transport

We study the low-temperature heat transport of Nd_3Ga_5SiO_{14}, which is a spin-liquid candidate, to probe the nature of ground state and the effect of magnetic field on the magnetic properties. The thermal conductivity (κ) shows a purely phononic transport in zero field. The external magnetic field along the c axis induces a dip-like behavior of κ(H), which can be attributed to a simple paramagnetic scattering on phonons. However, the magnetic field along the ab plane induces another step-like decrease of κ. This kind of κ(H) behavior is discussed to be related to a field-induced partial order, which yields low-energy magnetic excitations that significantly scatter phonons. These results point to a paramagnetic ground state that partial magnetic order can be induced by magnetic field along the ab plane, which is also signified by the low-T specific heat data.

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arXiv2012

Heat transport of quasi-one-dimensional Ising-like antiferromagnet BaCo_2V_2O_8 in the longitudinal and transverse fields

The very-low-temperature thermal conductivity (κ) is studied for BaCo_2V_2O_8, a quasi-one-dimensional Ising-like antiferromagnet exhibiting an unusual magnetic-field-induced order-to-disorder transition. The nearly isotropic transport in the longitudinal field indicates that the magnetic excitations scatter phonons rather than conduct heat. The field dependence of κshows a sudden drop at \sim 4 T, where the system unndergoes the transition from the Néel order to the incommensurate state. Another dip at lower field of \sim 3 T indicates an unknown magnetic transition, which is likely due to the spin-flop transition. Moreover, the κ(H) in the transverse field shows a very deep valley-like feature, which moves slightly to higher field and becomes sharper upon lowering the temperature. This indicates a magnetic transition induced by the transverse field, which however is not predicted by the present theories for this low-dimensional spin system.

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arXiv2011

Heat transport of the quasi-one-dimensional alternating spin chain material (CH_{3})_{2}NH_{2}CuCl_{3}

We report a study of the low-temperature heat transport in the quasi-one-dimensional S = 1/2 alternating antiferromagnetic-ferromagnetic chain compound (CH_{3})_{2}NH_{2}CuCl_{3}. Both the temperature and magnetic-field dependencies of thermal conductivity are very complicated, pointing to the important role of spin excitations. It is found that magnetic excitations act mainly as the phonon scatterers in a broad temperature region from 0.3 to 30 K. In magnetic fields, the thermal conductivity show drastic changes, particularly at the field-induced transitions from the low-field Néel state to the spin-gapped state, the field-induced magnetic ordered state, and the spin polarized state. In high fields, the phonon conductivity is significantly enhanced because of the weakening of spin fluctuations.

cond-mat.str-el↗

arXiv2011

Low-temperature heat transport of the layered spin-dimer compound Ba_3Mn_2O_8

We report the study on the low-temperature heat transport of Ba_3Mn_2O_8 single crystal, a layered spin-dimer compound exhibiting the magnetic-field-induced magnetic order or the magnon Bose-Einstein condensation. The thermal conductivities (κ) along both the ab plane and the c axis show nearly isotropic dependence on magnetic field, that is, κis strongly suppressed with increasing field, particularly at the critical fields of magnetic phase transitions. These results indicate that the magnetic excitations play a role of scattering phonons and the scattering effect is enhanced when the magnetic field closes the gap in the spin spectrum. In addition, the magnons in the BEC state of this materials do not show notable ability of carrying heat.

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arXiv2011

Low-temperature heat transport of Nd_2CuO_4: Roles of Nd magnons and spin-structure transitions

We report the magnetic-field dependence of thermal conductivity (κ) of an insulating cuprate Nd_2CuO_4 at very low temperatures down to 0.3 K. It is found that apart from the paramagnetic moments scattering on phonons, the Nd^{3+} magnons can act as either heat carriers or phonon scatterers, which strongly depends on the long-range antiferromagnetic transition and the field-induced transitions of spin structure. In particular, the Nd^{3+} magnons can effectively transport heat in the spin-flopped state of the Nd^{3+} sublattice. However, both the magnon transport and the magnetic scattering are quenched at very high fields. The spin re-orientations under the in-plane field can be conjectured from the detailed field dependence of κ.

cond-mat.str-el↗

arXiv2011

Thermal conductivity of pure and Zn-doped LiCu_2O_2 single crystals

We report a study of the low-temperature thermal conductivity (κ) of pure and Zn-doped LiCu_2O_2 single crystals. The κ(T) of pure LiCu_2O_2 single crystal shows a double-peak behavior, with two peaks locating at 48 K and 14 K, respectively. The different dependences of the peaks on the Zn concentration indicate that the high-T peak is likely due to the phonon transport while the low-T one is attributed to the magnon transport in the spin spiral ordering state. In addition, the magnetic field can gradually suppress the low-T peak but does not affect the high-T one; this further confirms that the low-T peak is originated from the magnon heat transport.

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arXiv2010

Magnetic phase transitions and magnetoelectric coupling of GdFeO_3 single crystals probed by low-temperature heat transport

The low-temperature thermal conductivity (κ) of GdFeO_3 single crystals is found to be strongly dependent on magnetic field. The low-field κ(H) curves show two "dips" for H \parallel a and only one "dip" for H \parallel c, with the characteristic fields having good correspondence with the spin-flop and the spin-polarization transitions. A remarkable phenomenon is that the subKelvin thermal conductivity shows hysteretic behaviors on the history of applying magnetic field, that is, the κ(H) isotherms measured with field increasing are larger than those with field decreasing. Intriguingly, the broad region of magnetic field (\sim 0--3 T) showing the irreversibility of heat transport coincides with that presenting the ferroelectricity. It is discussed that the irreversible κ(H) behaviors are due to the phonon scattering by ferroelectric domain walls. This result shows an experimental feature that points to the capability of controlling the ferroelectric domain structures by magnetic field in multiferroic materials.

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arXiv2010

Large magnetothermal conductivity of HoMnO_3 single crystals and its relation to the magnetic-field induced transitions of magnetic structure

We study the low-temperature heat transport of HoMnO_3 single crystals to probe the magnetic structures and their transitions induced by magnetic field. It is found that the low-T thermal conductivity (κ) shows very strong magnetic-field dependence, with the strongest suppression of nearly 90% and the biggest increase of 20 times of κcompared to its zero-field value. In particular, some ``dip"-like features show up in κ(H) isotherms for field along both the ab plane and the c axis. These behaviors are found to shed new light on the complex H-T phase diagram and the field-induced re-orientations of Mn^{3+} and Ho^{3+} spin structures. The results also demonstrate a significant spin-phonon coupling in this multiferroic compound.

cond-mat.str-el↗

arXiv2009

Deviation from the Wiedemann-Franz law induced by nonmagnetic impurities in overdoped La_{2-x}Sr_{x}CuO_{4}

To investigate the validity of the Wiedemann-Franz (WF) law in disordered but metallic cuprates, the low-temperature charge and heat transport properties are carefully studied for a series of impurity-substituted and carrier-overdoped La_{1.8}Sr_{0.2}Cu_{1-z}M_zO_4 (M = Zn or Mg) single crystals. With moderate impurity substitution concentrations of z = 0.049 and 0.082 (M = Zn), the resistivity shows a clear metallic behavior at low temperature and the WF law is confirmed to be valid. With increasing impurity concentration to z = 0.13 (M = Zn) or 0.15 (M = Mg), the resistivity shows a low-T upturn but its temperature dependence indicates a finite conductivity in the T \to 0 limit. In this weakly-localized metallic state that is intentionally achieved in the overdoped regime, a {\it negative} departure from the WF law is found, which is opposite to the theoretical expectation.

cond-mat.supr-con↗

arXiv2009

Low-Temperature Heat Transport in the Low-Dimensional Quantum Magnet NiCl_{2}-4SC(NH_{2})_{2}

We report a study of the low-temperature thermal conductivity of NiCl_{2}-4SC(NH_{2})_{2}, which is a spin-1 chain system exhibiting the magnon Bose-Einstein condensation (BEC) in magnetic field. It is found that the low-T thermal conductivity along the spin-chain direction shows strong anomalies at the lower and upper critical fields of the magnon BEC state. In this state, magnons act mainly as phonon scatterers at relatively high temperature, but change their role to heat carriers upon temperature approaching zero. The result demonstrates a large thermal conductivity in the magnon BEC state and points to a direct analog between the magnon BEC and the conventional one.

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arXiv2009

Comment on "Low-temperature phonon thermal conductivity of single-crystalline Nd_{2}CuO_{4}: Effects of sample size and surface roughness" by Li et al.

In this Comment, we show that the experimental data reported by Li {\it et al.} (Phys. Rev. B {\bf 77}, 134501 (2008)) do not support the phonon specular reflection at low temperatures for high-T_c cuprates, because the phonon mean free path \ell calculated from their thermal conductivity data is much smaller than the averaged sample width above \sim 100 mK for both Nd_{2}CuO_{4} and YBa_{2}Cu_{3}O_{6.0} single crystals.

cond-mat.supr-con↗

arXiv2008

Doping dependence of phonon and quasiparticle heat transport of pure and Dy-doped Bi_2Sr_2CaCu_2O_{8+δ} single crystals

The temperature and magnetic-field (H) dependences of thermal conductivity (κ) of Bi_2Sr_2CaCu_2O_{8+δ} (Bi2212) are systematically measured for a broad doping range by using both pure Bi2212 single crystals with tuned oxygen contents and Bi_2Sr_2Ca_{1-x}Dy_xCu_2O_{8+δ} (Dy-Bi2212) single crystals with different Dy contents x. In the underdoped samples, the quasiparticle (QP) peak below T_c is strongly suppressed, indicating strong QP scattering by impurities or oxygen defects, whereas the phonon conductivity is enhanced in moderately Dy-doped samples and a phonon peak at 10 K is observed for the first time in Bi2212 system, which means Dy^{3+} ions not only introduce the impurities or point defects but also stabilize the crystal lattice. The subkelvin data show that the QP heat conductivity gradually decreases upon lowering the hole doping level. The magnetic-field dependence of κat temperature above 5 K is mainly due to the QP scattering off vortices. While the underdoped pure Bi2212 show very weak field dependence of κ, the Dy-doped samples present an additional "dip"-like term of κ(H) at low field, which is discussed to be related to the phonon scattering by free spins of Dy^{3+} ions. For non-superconducting Dy-Bi2212 samples with x \simeq 0.50, an interesting "plateau" feature shows up in the low-T κ(H) isotherms with characteristic field at 1 -- 2 T, for which we discuss the possible revlevance of magnon excitations.

cond-mat.supr-con↗

arXiv2008

Large magnetothermal conductivity in GdBaCo_{2}O_{5+x} single crystals

To study the effects of paramagnetic spins on phonons, both the in-plane and the c-axis heat transport of GdBaCo_{2}O_{5+x} (GBCO) single crystals are measured at low temperature down to 0.36 K and in magnetic field up to 16 T. It is found that the phonon heat transport is very strongly affected by the magnetic field and nearly 5 times increase of the thermal conductivity in several Tesla field is observed at 0.36 K. It appears that phonons are resonantly scattered by paramagnetic spins in zero field and the application of magnetic field removes such strong scattering, but the detailed mechanism is to be elucidated.

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