arXiv subjects

X. M. Wang

Publications and source records attributed to X. M. Wang.

18 recordsLinked to original sources

arXiv2024

Inexact Adaptive Cubic Regularization Algorithms on Riemannian Manifolds and Application

The adaptive cubic regularization algorithm employing the inexact gradient and Hessian is proposed on general Riemannian manifolds, together with the iteration complexity to get an approximate second-order optimality under certain assumptions on accuracies about the inexact gradient and Hessian. The algorithm extends the inexact adaptive cubic regularization algorithm under true gradient in [Math. Program., 184(1-2): 35-70, 2020] to more general cases even in Euclidean settings. As an application, the algorithm is applied to solve the joint diagonalization problem on the Stiefel manifold. Numerical experiments illustrate that the algorithm performs better than the inexact trust-region algorithm in [Advances of the neural information processing systems, 31, 2018].

math.OC↗

arXiv2021

Magnetic excitations of the field induced states in BaCo2(AsO4)2 probed by time-domain terahertz spectroscopy

Searching for Kitaev quantum spin liquid (QSL) is a fascinating and challenging problem. Much effort has been devoted to honeycomb lattice candidates with strong spin-orbit coupling in 5d-electron iridates and 4delectron RuCl3. Recently, theoretical studies suggested that the 3d7 Co-based honeycomb materials with high spin state S=3/2 and effective orbital angular momentum L=1 could also be promising candidates of Kitaev QSL. One of the candidates, BaCo2(AsO4)2, was revisited recently. The long range magnetic order in BaCo2(AsO4)2 can be suppressed by very weak in-plane magnetic field, suggesting its proximity to Kitaev QSL. Here we perform time domain terahertz spectroscopy measurement to study the magnetic excitations on BaCo2(AsO4)2. We observe different magnon excitations upon increasing external magnetic field. In particular, the system is easily driven to a field-polarized paramagnetic phase, after the long range magnetic order is suppressed by a weak field Hc 2. The spectra beyond Hc2 are dominated by single magnon and two magnon excitations without showing signature of QSL. We discuss the similarity and difference of the excitation spectra between BaCo2(AsO4)2 and the widely studied Kitaev QSL candidate RuCl3.

cond-mat.str-el↗

arXiv2017

Low-temperature heat transport of CuFe$_{1-x}$Ga$_x$O$_2$ ($x =$ 0--0.12) single crystals

We report a study on the thermal conductivity of CuFe$_{1-x}$Ga$_x$O$_2$ ($x =$ 0--0.12) single crystals at temperatures down to 0.3 K and in magnetic fields up to 14 T. CuFeO$_2$ is a well-known geometrically frustrated triangular lattice antiferromagnet and can be made to display multiferroicity either by applying magnetic field along the $c$ axis or by doping nonmagnetic impurities, accompanied with rich behaviors of magnetic phase transitions. The main experimental findings of this work are: (i) the thermal conductivities ($κ_a$ and $κ_c$) show drastic anomalies at temperature- or field-induced magnetic transitions; (ii) the low-$T$ $κ(H)$ isotherms exhibit irreversibility in a broad region of magnetic fields; (iii) there are phonon scattering effect caused by magnetic fluctuations at very low temperatures. These results demonstrate strong spin-phonon coupling in this material and reveal the non-negligible magnetic fluctuations in the "ground state" of pure and Ga-doped samples.

cond-mat.str-el↗

arXiv2014

PandaX: A Liquid Xenon Dark Matter Experiment at CJPL

PandaX is a large upgradable liquid-xenon detector system that can be used for both direct dark-matter detection and $^{136}$Xe double-beta decay search. It is located in the Jinping Deep-Underground Laboratory in Sichuan, China. The detector operates in dual-phase mode, allowing detection of both prompt scintillation, and ionization charge through proportional scintillation. The central time projection chamber will be staged, with the first stage accommodating a target mass of about 120\,kg. In stage II, the target mass will be increased to about 0.5\,ton. In the final stage, the detector can be upgraded to a multi-ton target mass. In this paper a detailed description of the stage-I detector design and performance results established during the commissioning phase is presented.

physics.ins-det↗

arXiv2013

Growth of (CH$_3$)$_2$NH$_2$CuCl$_3$ single crystals using evaporation method with different temperatures and solvents

The bulk single crystals of of low-dimensional magnet (CH$_3$)$_2$NH$_2$CuCl$_3$ (DMACuCl$_3$ or MCCL) are grown by a slow evaporation method with different kinds of solvents, different degrees of super-saturation of solution and different temperatures of solution, respectively. Among three kinds of solvent, methanol, alcohol and water, alcohol is found to be the best one for growing MCCL crystals because of its structural similarity to the raw materials and suitable evaporation rate. The best growth temperature is in the vicinity of 35 $^{\circ}$C. The problem of the crystals deliquescing in air has been solved through recrystallization process. The crystals are characterized by means of x-ray diffraction, specific heat and magnetic susceptibility.

cond-mat.str-el↗

arXiv2013

Crystal growth and characterization of Haldane chain compound Ni(C$_3$H$_{10}$N$_2$)$_2$NO$_2$ClO$_4$

The bulk single crystals of $S = 1$ chain compound Ni(C$_3$H$_{10}$N$_2$)$_2$NO$_2$ClO$_4$ are grown by using a slow evaporation method at a constant temperature and a slow cooling method. It is found that the optimum condition of growing large crystals is via slow evaporation at 25 $^\circ$C using 0.015 mol Ni(ClO$_4$)$_2$$\cdot$6H$_2$O, 0.015 mol NaNO$_2$, and 0.03 mol 1,3-propanediamine liquid dissolved into 30 ml aqueous solvent. High-quality crystals with size up to $18 \times 7.5 \times 5$ mm$^3$ are obtained. The single crystals are characterized by measurements of x-ray diffraction, magnetic susceptibility, specific heat and thermal conductivity. The susceptibilities along three crystallographic axes are found to exhibit broad peaks at $\sim 55$ K, and then decrease abruptly to zero at lower temperatures, which is characteristic of a Haldane chain system. The specific heat and the thermal conductivity along the $c$ axis can be attributed to the simple phononic contribution and are analyzed using the Debye approximation.

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.

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.

cond-mat.str-el↗

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.

cond-mat.str-el↗

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.

cond-mat.str-el↗

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.

cond-mat.str-el↗

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

Metastable giant moments in Gd-implanted GaN, Si, and sapphire

We report on Gd ion implantation and magnetic characterization of GaN films on sapphire substrates and of bare sapphire and Si substrates to shed light on the mechanism underlying the induced magnetism upon Gd ion implantation. For all three hosts, giant magnetic moments per Gd ion were observed at temperatures of 5 through 300 K. The maximum moment per Gd in GaN was 1800 mu_B, while the moments in Gd-implanted Si and sapphire were only slightly smaller. The apparent induced ferromagnetic response was found to be metastable, disappearing after on the order of 50 days at room temperature, except for the implanted sapphire. We argue that our findings support a defect-based picture of magnetism in Gd-implanted semiconductors and insulators.

cond-mat.mtrl-sci↗

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.

cond-mat.str-el↗

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.

cond-mat.str-el↗

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

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.

cond-mat.str-el↗