arXiv subjects

H. B. Cao

Publications and source records attributed to H. B. Cao.

At least 19 recordsLinked to original sources

arXiv2023

Multi-k magnetic structure and large anomalous Hall effect in candidate magnetic Weyl semimetal NdAlGe

The magnetic structure, magnetoresistance, and Hall effect of non-centrosymmetric magnetic semimetal NdAlGe are investigated revealing an unusual magnetic state and anomalous transport properties that are associated with the electronic structure of this non-centrosymmetric compound. The magnetization and magnetoresistance measurements are both highly anisotropic and indicate an Ising-like magnetic system. The magnetic structure is complex in that it involves three magnetic ordering vectors including an incommensurate spin density wave and commensurate ferrimagnetic state in zero field. We have discovered a large anomalous Hall conductivity that reaches = 430 Ω-1cm-1 implying that it originates from an intrinsic Berry curvature effect stemming from Weyl nodes found in the electronic structure. These electronic structure calculations indicate the presence of nested Fermi surface pockets with nesting wave vectors similar to the measured magnetic ordering wavevector and the presence of Weyl nodes in proximity to the Fermi surface. We associate the incommensurate magnetic structure with the large anomalous Hall response to be the result of the combination of Fermi surface nesting and the Berry curvature associated with Weyl nodes.

cond-mat.mtrl-sci↗

arXiv2021

Quantum spin state transitions in spin-1 equilateral triangular lattice antiferromagnet Na$_2$BaNi(PO$_4$)$_2$

We have grown single crystals of Na$_2$BaNi(PO$_4$)$_2$, a new spin-1 equilateral triangular lattice antiferromagnet (ETLAF), and performed magnetic susceptibility, specific heat and thermal conductivity measurements at ultralow temperatures. The main results are (i) at zero magnetic field, Na$_2$BaNi(PO$_4$)$_2$ exhibits a magnetic ordering at 430 mK with a weak ferromagnetic moment along the $c$ axis. This suggests a canted 120$^\circ$ spin structure, which is in a plane including the crystallographic $c$ axis due to the existence of an easy-axis anisotropy and ferromagnetically stacked along the $c$ axis; (ii) with increasing field along the $c$ axis, a 1/3 magnetization plateau is observed which means the canted 120$^\circ$ spin structure is transformed to a up up down (UUD) spin structure. With even higher fields, the UUD phase further evolves to possible V and V' phases; (iii) with increasing field along the $a$ axis, the canted 120$^\circ$ spin structure is possibly transformed to a umbrella phase and a V phase. Therefore, Na$_2$BaNi(PO$_4$)$_2$ is a rare example of spin-1 ETLAF with single crystalline form to exhibit easy-axis spin anisotropy and series of quantum spin state transitions.

cond-mat.mtrl-sci↗

arXiv2021

Non-magnetic ion site disorder effects on the quantum magnetism of a spin-1/2 equilateral triangular lattice antiferromagnet

With the motivation to study how non-magnetic ion site disorder affects the quantum magnetism of Ba3CoSb2O9, a spin-1/2 equilateral triangular lattice antiferromagnet, we performed DC and AC susceptibility, specific heat, elastic and inelastic neutron scattering measurements on single crystalline samples of Ba2.87Sr0.13CoSb2O9 with Sr doping on non-magnetic Ba2+ ion sites. The results show that Ba2.87Sr0.13CoSb2O9 exhibits (i) a two-step magnetic transition at 2.7 K and 3.3 K, respectively; (ii) a possible canted 120-degree spin structure at zero field with reduced ordered moment as 1.24μB/Co; (iii) a series of spin state transitions for both H // ab-plane and H // c-axis. For H // ab-plane, the magnetization plateau feature related to the up-up-down phase is significantly suppressed; (iv) an inelastic neutron scattering spectrum with only one gapped mode at zero field, which splits to one gapless and one gapped mode at 9 T. All these features are distinctly different from those observed for the parent compound Ba3CoSb2O9, which demonstrates that the non-magnetic ion site disorder (the Sr doping) plays a complex role on the magnetic properties beyond the conventionally expected randomization of the exchange interactions. We propose the additional effects including the enhancement of quantum spin fluctuations and introduction of a possible spatial anisotropy through the local structural distortions.

cond-mat.str-el↗

arXiv2020

Spin-valley locking, bulk quantum Hall effect and chiral surface state in a noncentrosymmetric Dirac semimetal BaMnSb$_2$

Spin-valley locking in the band structure of monolayers of MoS$_2$ and other group-VI dichalcogenides has attracted enormous interest, since it offers potential for valleytronic and optoelectronic applications. Such an exotic electronic state has sparsely been seen in bulk materials. Here, we report spin-valley locking in a bulk Dirac semimetal BaMnSb$_2$. We find valley and spin are inherently coupled for both valence and conduction bands in this material. This is revealed by comprehensive studies using first principle calculations, tight-binding and effective model analyses, angle-resolved photoemission spectroscopy and quantum transport measurements. Moreover, this material also exhibits a stacked quantum Hall effect. The spin-valley degeneracy extracted from the plateau height of quantized Hall resistivity is close to 2. This result, together with the observed Landau level spin splitting, further confirms the spin-valley locking picture. In the extreme quantum limit, we have also observed a two-dimensional chiral metal at the side surface, which represents a novel topological quantum liquid. These findings establish BaMnSb$_2$ as a rare platform for exploring coupled spin and valley physics in bulk single crystals and accessing 3D interacting topological states.

cond-mat.mtrl-sci↗

arXiv2020

Helical magnetic order and Fermi surface nesting in non-centrosymmetric ScFeGe

An investigation of the structural, magnetic, thermodynamic, and charge transport properties of non-centrosymmetric hexagonal ScFeGe reveals it to be an anisotropic metal with a transition to a weak itinerant incommensurate helimagnetic state below $T_N = 36$ K. Neutron diffraction measurements discovered a temperature and field independent helical wavevector \textbf{\textit{k}} = (0 0 0.193) with magnetic moments of 0.53 $μ_{B}$ per formula unit confined to the {\it ab}-plane. Density functional theory calculations are consistent with these measurements and find several bands that cross the Fermi level along the {\it c}-axis with a nearly degenerate set of flat bands just above the Fermi energy. The anisotropy found in the electrical transport is reflected in the calculated Fermi surface, which consists of several warped flat sheets along the $c$-axis with two regions of significant nesting, one of which has a wavevector that closely matches that found in the neutron diffraction. The electronic structure calculations, along with a strong anomaly in the {\it c}-axis conductivity at $T_N$, signal a Fermi surface driven magnetic transition, similar to that found in spin density wave materials. Magnetic fields applied in the {\it ab}-plane result in a metamagnetic transition with a threshold field of $\approx$ 6.7 T along with a sharp, strongly temperature dependent, discontinuity and a change in sign of the magnetoresistance for in-plane currents. Thus, ScFeGe is an ideal system to investigate the effect of in-plane magnetic fields on an easy-plane magnetic system, where the relative strength of the magnetic interactions and anisotropies determine the topology and magnetic structure.

cond-mat.mtrl-sci↗

arXiv2020

Magnetic Field Induced Phase Transition in Spinel GeNi2O4

Cubic spinel GeNi2O4 exhibits intriguing magnetic properties with two successive antiferromagnetic phase transitions (TN1 12.1 and TN2 11.4 K) with the absence of any structural transition. We have performed detailed heat capacity and magnetic measurements in different crystallographic orientations. A new magnetic phase in presence of magnetic field (H > 4 T) along the [111] direction is revealed, which is not observed when the magnetic field is applied along the [100] and [110] directions. High field neutron powder diffraction measurements confirm such a change in magnetic phase, which could be ascribed to a spin reorientation in the presence of magnetic field. A strong magnetic anisotropy and competing magnetic interactions play a crucial role on the complex magnetic behavior in this cubic system.

cond-mat.str-el↗

arXiv2020

Field-induced magnetic phase transitions and the resultant giant anomalous Hall effect in antiferromagnetic half-Heusler DyPtBi

We report field-induced magnetic phase transitions and transport properties of antiferromagnetic DyPtBi. We show that DyPtBi hosts a delicate balance between two different magnetic ground states, which can be controlled by a moderate magnetic field. Furthermore, it exhibits giant anomalous Hall effect (σ_A=1540 (ohm cm)^{-1},θ_{AHE} = 24%) in a field-induced Type-I spin structure, presumably attributed to the enhanced Berry curvature associated with avoided band-crossings near the Fermi energy and / or non-zero spin chirality. The latter mechanism points DyPtBi towards a rare potential realization of anomalous Hall effect in an antiferromagnet with face-center-cubic lattice that was proposed in [Physical Review Letters 87, 116801 (2001)].

cond-mat.str-el↗

arXiv2020

Magnetic structure and exchange interactions in the layered semiconductor CrPS4

Compounds with two-dimensional (2D) layers of magnetic ions weakly connected by van der Waals bonding offer routes to enhance quantum behavior, stimulating both fundamental and applied interest. CrPS4 is one such magnetic van der Waals material, however, it has undergone only limited investigation. Here we present a comprehensive series of neutron scattering measurements to determine the magnetic structure and exchange interactions. The observed magnetic excitations allow a high degree of constraint on the model parameters not normally associated with measurements on a powder sample. The results demonstrate the 2D nature of the magnetic interactions, while also revealing the importance of interactions along 1D chains within the layers. The subtle role of competing interactions is observed, which manifest in a non-trivial magnetic transition and a tunable magnetic structure in a small applied magnetic field through a spin-flop transition. Our results on the bulk compound provide insights that can be applied to an understanding of the behavior of reduced layer CrPS4.

cond-mat.str-el↗

arXiv2020

Superconductivity with Tc=7 K under pressure for Cu-, and Au-doped BaFe2As2

It is noteworthy that chemical substitution of BaFe2As2 (122) with the noble elements Cu and Au gives superconductivity with a maximum Tc=3 K, while Ag substitution (Ag-122) stays antiferromagnetic. For Ba(Fe1-xTMx)2As2, TM= Cu, Au, or Ag, and by doping an amount of x=0.04, a-lattice parameter slightly increases (0.4%) for all TM dopants, while c-lattice decreases (-0.2%) for TM=Cu, barely moves (0.05%) for Au, and increases (0.2%) for Ag. Despite the naive expectation that the noble elements of group 11 should affect the quantum properties of 122 similarly, they produce significant differences extending to the character of the ground state. For the Ag-122 crystal, evidence of only a filamentary superconductivity is noted with pressure. However, for Au and Cu doping (x=0.03) we find a substantial improvement in the superconductivity, with Tc increasing to 7 K and 7.5 K, respectively, under 20 kbar of pressure. As with the ambient pressure results, the identity of the dopant therefore has a substantial impact on the ground state properties. Density functional theory calculations corroborate these results and find evidence of strong electronic scattering for Au and Ag dopants, while Cu is comparatively less disruptive to the 122 electronic structure.

cond-mat.supr-con↗

arXiv2019

Partial Antiferromagnetic Helical Order in Fe$_3$PO$_4$O$_3$

Magnetic frustration in Fe$_3$PO$_4$O$_3$ has been shown to produce to an unusual magnetic state below T$_N = 163$ K, where incommensurate antiferromagnetic order is restricted to nanosized needle-like domains, as inferred from neutron powder diffraction. Here we show using single-crystal neutron diffraction that Fe$_3$PO$_4$O$_3$ does not exhibit a preferred ordering wavevector direction in the $ab$ plane despite having a well-defined ordering wavevector length. This results in the observation of continuous rings of scattering rather than satellite Bragg peaks. The lack of a preferred incommensurate ordering wavevector direction can be understood in terms of an antiferromagnetic Heisenberg model with nearest-neighbor ($J_1$) and second-neighbor ($J_2$) interactions, which produces a quasi-degenerate manifold of ordering wavevectors. This state appears to be similar to the partially ordered phase of MnSi, but in Fe$_3$PO$_4$O$_3$ arises in a frustrated antiferromagnet rather than a chiral ferromagnet.

cond-mat.str-el↗

arXiv2019

Magnetic order in single crystals of Na3Co2SbO6 with a honeycomb arrangement of 3d$^7$ Co$^{2+}$ ions

We have synthesized single crystals of Na$_3$Co$_2$SbO$_6$ and characterized the structure and magnetic order by measuring anisotropic magnetic properties, heat capacity, x-ray and neutron single crystal diffraction. Magnetic properties and specific heat of polycrystalline Na$_3$Co$_2$SbO$_6$ were also measured for comparison. Na$_3$Co$_2$SbO$_6$ crystallizes in a monoclinic structure (space group $C2/m$) with [Co$_2$SbO$_6$]$^{3-}$ layers separated by Na$^+$ ions. The temperature dependence of magnetic susceptibility shows significant anisotropic behavior in the whole temperature range 2\,K-350\,K investigated in this work. An effective moment of about 5.5\,$μ_B$/Co$^{2+}$ from a Curie-Weiss fitting of the magnetic susceptibility is larger than the spin only value and signals significant orbital contribution. Na$_3$Co$_2$SbO$_6$ single crystal undergoes a transition into a long-range antiferromagnetically ordered state below $T_N$=5\,K. Neutron single crystal diffraction confirmed the zigzag magnetic structure with a propagation vector k\,=\,(0.5, 0.5, 0). The ordered moment is found to be 0.9\,$μ_B$ at 4\,K and align along the crystallographic \textit{b}-axis. Density functional theory calculations suggest that the experimentally observed zigzag order is energetically competing with the Neel order. It is also found that the covalency between Co $d$ and O $p$ is quite strong and competes with the local spin-orbit coupling, suggesting a $J_{eff}$=1/2 ground state may not be realized in this compound.

cond-mat.str-el↗

arXiv2018

Lattice distortion in the spin-orbital entangled state in RVO3 perovskites

We report a thorough study of Y$_{0.7}$La$_{0.3}$VO$_3$ single crystals by measuring magnetic properties, specific heat, thermal conductivity, x-ray and neutron diffraction with the motivation of revealing the lattice response to the spin-orbital entanglement in \textit{R}VO$_3$. Upon cooling from room temperature, the orbitally disordered paramagnetic state changes around T*$\sim$220\,K to spin-orbital entangled state which is then followed by a transition at T$_N$=116\,K to C-type orbital ordered (OO) and G-type antiferromagnetic ordered (AF) ground state. In the temperature interval T$_N<T<T^*$, the VO$_{6/2}$ octahedra have two comparable in-plane V-O bonds which are longer than the out-of-plane V-O1 bond. This local structural distortion supports the spin-orbital entanglement of partially filled and degenerate yz/zx orbitals. However, this distortion is incompatible with the steric octahedral site distortion intrinsic to orthorhombic perovskites. Their competition induces a second order transition from the spin-orbital entangled state to C-OO/G-AF ground state where the long range OO suppresses the spin-orbital entanglement. Our analysis suggests that the spin-orbital entangled state and G-OO are comparable in energy and compete with each other. Rare earth site disorder favors the spin-orbital entanglement rather than a cooperative Jahn-Teller distortion. The results also indicate for LaVO$_3$ a C-OO/G-AF state in T$_t$\,$\leq$\,T\,$\leq$T$_N$ and an orbital flipping transition at T$_t$.

cond-mat.str-el↗

arXiv2018

Magnetic Properties of the low dimensional BaM$_2$Si$_2$O$_7$(M= Cu, Co, Mn) system

We performed susceptibility, magnetization, specific heat, and single crystal neutron diffraction measurements on single crystalline BaMn$_2$Si$_2$O$_7$. Based on the results, we revisited its spin structure with a more accurate solution and constructed a magnetic phase diagram with applied field along the $b$-axis, which contains a spin flop transition around 6 T. We also used susceptibility, magnetization, and specific heat results confirmed the ferrimagnetic-like magnetism in polycrystalline BaCo$_2$Si$_2$O$_7$. Furthermore, we performed LSDA + U calculations for the BaM$_2$Si$_2$O$_7$ (M = Cu, Co, and Mn) system. Our discussions based on the comparison among the obtained magnetic exchange interactions suggest the different structures and electronic configurations are the reasons for the different magnetic properties among the system members.

cond-mat.str-el↗

arXiv2018

Anomalous metamagnetism in the low carrier density Kondo lattice YbRh3Si7

We report complex metamagnetic transitions in single crystals of the new low carrier Kondo antiferromagnet YbRh3Si7. Electrical transport, magnetization, and specific heat measurements reveal antiferromagnetic order at T_N = 7.5 K. Neutron diffraction measurements show that the magnetic ground state of YbRh3Si7 is a collinear antiferromagnet where the moments are aligned in the ab plane. With such an ordered state, no metamagnetic transitions are expected when a magnetic field is applied along the c axis. It is therefore surprising that high field magnetization, torque, and resistivity measurements with H||c reveal two metamagnetic transitions at mu_0H_1 = 6.7 T and mu_0H_2 = 21 T. When the field is tilted away from the c axis, towards the ab plane, both metamagnetic transitions are shifted to higher fields. The first metamagnetic transition leads to an abrupt increase in the electrical resistivity, while the second transition is accompanied by a dramatic reduction in the electrical resistivity. Thus, the magnetic and electronic degrees of freedom in YbRh3Si7 are strongly coupled. We discuss the origin of the anomalous metamagnetism and conclude that it is related to competition between crystal electric field anisotropy and anisotropic exchange interactions.

cond-mat.str-el↗

arXiv2018

The nature of spin excitations in the one-third magnetization plateau phase of Ba$_3$CoSb$_2$O$_9$

Magnetization plateaus in quantum magnets---where bosonic quasiparticles crystallize into emergent spin superlattices---are spectacular yet simple examples of collective quantum phenomena escaping classical description. While magnetization plateaus have been observed in a number of spin-1/2 antiferromagnets, the description of their magnetic excitations remains an open theoretical and experimental challenge. Here, we investigate the dynamical properties of the triangular-lattice spin-1/2 antiferromagnet Ba$_3$CoSb$_2$O$_9$ in its one-third magnetization plateau phase using a combination of nonlinear spin-wave theory and neutron scattering measurements. The agreement between our theoretical treatment and the experimental data demonstrates that magnons behave semiclassically in the plateau in spite of the purely quantum origin of the underlying magnetic structure. This allows for a quantitative determination of Ba$_3$CoSb$_2$O$_9$ exchange parameters. We discuss the implication of our results to the deviations from semiclassical behavior observed in zero-field spin dynamics of the same material and conclude they must have an intrinsic origin.

cond-mat.str-el↗

arXiv2018

Anisotropic susceptibilities in the honeycomb Kitaev system $α$-RuCl$_{3}$

The magnetic insulator $α$-RuCl$_{3}$ is a promising candidate to realize Kitaev interactions on a quasi-2D honeycomb lattice. We perform extensive susceptibility measurements on single crystals of $α$-RuCl$_{3}$, including angle-dependence of the in-plane longitudinal and transverse susceptibilities, which reveal a unidirectional anisotropy within the honeycomb plane. By comparing the experimental results to a high-temperature expansion of a Kitaev-Heisenberg-$Γ$ spin Hamiltonian with bond anisotropy, we find excellent agreement with the observed phase shift and periodicity of the angle-resolved susceptibilities. Within this model, we show that the pronounced difference between in-plane and out-of-plane susceptibilities as well as the finite transverse susceptibility are rooted in strong symmetric off-diagonal $Γ$ spin exchange. The $Γ$ couplings and relationships between other terms in the model Hamiltonian are quantified by extracting relevant Curie-Weiss intercepts from the experimental data.

cond-mat.mtrl-sci↗

arXiv2018

Temperature- and field-driven spin reorientations in triple-layer ruthenate Sr$_4$Ru$_3$O$_{10}$

Sr$_4$Ru$_3$O$_{10}$, the $n$ = 3 member of the Ruddlesden-Popper type ruthenate Sr$_{n+1}$Ru$_n$O$_{3n+1}$, is known to exhibit a peculiar metamagnetic transition in an in-plane magnetic field. However, the nature of both the temperature- and field-dependent phase transitions remains as a topic of debate. Here, we have investigated the magnetic transitions of Sr$_4$Ru$_3$O$_{10}$ via single-crystal neutron diffraction measurements. At zero field, we find that the system undergoes a ferromagnetic transition with both in-plane and out-of-plane magnetic components at $T_{c}$ ~ 100 K. Below $T^{*}$ ~ 50 K, the magnetic moments incline continuously toward the out-of-plane direction. At $T$ ~ 1.5 K, where the spins are nearly aligned along the $c$ axis, a spin reorientation occurs above a critical field $B_c$, giving rise to a spin component perpendicular to the plane defined by the field direction and the $c$ axis. We suggest that both the temperature- and field-driven spin reorientations are associated with a change in the magnetocrystalline anisotropy, which is strongly coupled to the lattice degrees of freedom. This study elucidates the long-standing puzzles on the zero-field magnetic orders of Sr$_4$Ru$_3$O$_{10}$ and provides new insights into the nature of the field-induced metamagnetic transition.

cond-mat.str-el↗

arXiv2017

On the origin of critical nematic fluctuations in pnictide superconductors

We employ polarization-resolved Raman spectroscopy to study critical nematic fluctuations in Ba(Fe$_{1-x}$Au$_x$)$_2$As$_2$ superconductors above and across well separated tetragonal to orthorhombic phase transition at temperature $T_S(x)$ and the Néel transition at $T_N(x)$. The static Raman susceptibility in $XY$ symmetry channel increases upon cooling from room temperature following the Curie-Weiss law, with Weiss temperature $T_θ(x)$ several tens of degrees lower than $T_S(x)$. Data reveals a hidden nematic quantum critical point at $x_{c} = 0.031$ when the system becomes superconducting, indicating a direct connection between quantum critical nematic fluctuations and unconventional superconductivity. We attribute the origin of the nematicity to charge quadrupole fluctuations due to electron transfer between the nearly degenerate $d_{xz}/d_{yz}$ orbitals.

cond-mat.supr-con↗