arXiv subjects

C. Y. Guo

Publications and source records attributed to C. Y. Guo.

16 recordsLinked to original sources

arXiv2019

Anomalous thermoelectric effects of ZrTe$_{5}$ in and beyond the quantum limit

Thermoelectric effects are more sensitive and promising probes to topological properties of emergent materials, but much less addressed compared to other physical properties. Zirconium pentatelluride (ZrTe$_{5}$) has inspired active investigations recently because of its multiple topological nature. We study the thermoelectric effects of ZrTe$_{5}$ in a magnetic field and find several anomalous behaviors. The Nernst response has a steplike profile near zero field when the charge carriers are electrons only, suggesting the anomalous Nernst effect arising from a nontrivial profile of Berry curvature. Both the thermopower and Nernst signal exhibit exotic peaks in the strong-field quantum limit. At higher magnetic fields, the Nernst signal has a sign reversal at a critical field where the thermopower approaches to zero. We propose that these anomalous behaviors can be attributed to the Landau index inversion, which is resulted from the competition of the $\sqrt{B}$ dependence of the Dirac-type Landau bands and linear-$B$ dependence of the Zeeman energy ($B$ is the magnetic field). Our understanding to the anomalous thermoelectric properties in ZrTe$_{5}$ opens a new avenue for exploring Dirac physics in topological materials.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic field induced antiferromagnetic tricritical points in Ce$_2 $Sb and Ce$_2 $Bi

We present a detailed investigation of the physical properties of Ce$_2 $Sb and Ce$_2 $Bi single crystals, which undergo antiferromagnetic transitions at around 8.2 and 10 K respectively. When magnetic fields are applied parallel to the $c$ axis, metamagnetic transitions are observed at low temperatures, corresponding to a magnetic field-induced phase transition. It is found that the field-induced transition changes from second-order at higher temperatures, to first-order at low temperatures, suggesting the existence of tricritical points (TCPs) in both compounds. Since replacing Bi with Sb suppresses the TCP to lower temperatures and corresponds to a positive chemical pressure, these results suggest that applying pressure to Ce$_2$Sb may suppress the TCP to lower temperatures, potentially to zero temperature at a quantum tricritical point.

cond-mat.str-el↗

arXiv2018

Evidence for Weyl fermions in a canonical heavy-fermion semimetal YbPtBi

The manifestation of Weyl fermions in strongly correlated electron systems is of particular interest. We report evidence for Weyl fermions in the heavy fermion semimetal YbPtBi from electronic structure calculations, angle-resolved photoemission spectroscopy, magnetotransport and calorimetric measurements. At elevated temperatures where $4f$-electrons are localized, there are triply degenerate points, yielding Weyl nodes in applied magnetic fields. These are revealed by a contribution from the chiral anomaly in the magnetotransport, which at low temperatures becomes negligible due to the influence of electronic correlations. Instead, Weyl fermions are inferred from the topological Hall effect, which provides evidence for a Berry curvature, and a cubic temperature dependence of the specific heat, as expected from the linear dispersion near the Weyl nodes. The results suggest that YbPtBi is a Weyl heavy fermion semimetal, where the Kondo interaction renormalizes the bands hosting Weyl points. These findings open up an opportunity to explore the interplay between topology and strong electronic correlations.

cond-mat.str-el↗

arXiv2018

Magnon Valves Based on YIG/NiO/YIG All-Insulating Magnon Junctions

As an alternative angular momentum carrier, magnons or spin waves can be utilized to encode information and breed magnon-based circuits with ultralow power consumption and non-Boolean data processing capability. In order to construct such a circuit, it is indispensable to design some electronic components with both long magnon decay and coherence length and effective control over magnon transport. Here we show that an all-insulating magnon junctions composed by a magnetic insulator (MI1)/antiferromagnetic insulator (AFI)/magnetic insulator (MI2) sandwich (Y3Fe5O12/NiO/Y3Fe5O12) can completely turn a thermogradient-induced magnon current on or off as the two Y3Fe5O12 layers are aligned parallel or anti-parallel. The magnon decay length in NiO is about 3.5~4.5 nm between 100 K and 200 K for thermally activated magnons. The insulating magnon valve (magnon junction), as a basic building block, possibly shed light on the naissance of efficient magnon-based circuits, including non-Boolean logic, memory, diode, transistors, magnon waveguide and switches with sizable on-off ratios.

physics.app-ph↗

arXiv2018

Evidence for triplet superconductivity near an antiferromagnetic instability in CrAs

Superconductivity was recently observed in CrAs as the helimagnetic order is suppressed by applying pressure, suggesting possible unconventional superconductivity. To reveal the nature of the superconducting order parameter of CrAs, here we report the angular dependence of the upper critical field under pressure. Upon rotating the field by 360 degrees in the $bc$-plane, six maxima are observed in the upper critical field, where the oscillations have both six-fold and two-fold symmetric components. Our analysis suggests the presence of an unconventional odd-parity spin triplet state.

cond-mat.supr-con↗

arXiv2017

Evidence for nodal superconductivity in a layered compound Ta$_4$Pd$_3$Te$_{16}$

We report an investigation of the London penetration depth $Δλ(T)$ on single crystals of the layered superconductor Ta$_4$Pd$_3$Te$_{16}$, where the crystal structure has quasi-one-dimensional characteristics. A linear temperature dependence of $Δλ(T)$ is observed for $T\ll T_c$, in contrast to the exponential decay of fully gapped superconductors. This indicates the existence of line nodes in the superconducting energy gap. A detailed analysis shows that the normalized superfluid density $ρ_s(T)$, which is converted from $Δλ(T)$, can be well described by a multigap scenario, with nodes in one of the superconducting gaps, providing clear evidence for nodal superconductivity in Ta$_4$Pd$_3$Te$_{16}$.

cond-mat.supr-con↗

arXiv2017

Charge-Stripe Order and Superconductivity in $\mathrm{Ir_{1-x}Pt_xTe_2}$

A combined resistivity and hard x-ray diffraction study of superconductivity and charge ordering in $\mathrm{Ir_{1-x}Pt_xTe_2}$, as a function of Pt substitution and externally applied hydrostatic pressure, is presented. Experiments are focused on samples near the critical composition $x_c\sim 0.045$ where competition and switching between charge order and superconductivity is established. We show that charge order as a function of pressure in $\mathrm{Ir_{0.95}Pt_{0.05}Te_{2}}$ is preempted - and hence triggered - by a structural transition. Charge ordering appears uniaxially along the short crystallographic (1,0,1) domain axis with a $\mathrm{(\frac{1}{5},0,\frac{1}{5})}$ modulation. Based on these results we draw a charge-order phase diagram and discuss the relation between stripe ordering and superconductivity.

cond-mat.supr-con↗

arXiv2017

Possible Weyl fermions in the magnetic Kondo system CeSb

Materials where the electronic bands have unusual topologies allow for the realization of novel physics and have a wide range of potential applications. When two electronic bands with linear dispersions intersect at a point, the excitations could be described as Weyl fermions which are massless particles with a particular chirality. Here we report evidence for the presence of Weyl fermions in the ferromagnetic state of the low-carrier density, strongly correlated Kondo lattice system CeSb, from electronic structure calculations and angle-dependent magnetoresistance measurements. When the applied magnetic field is parallel to the electric current, a pronounced negative magnetoresistance is observed within the ferromagnetic state, which is destroyed upon slightly rotating the field away. These results give evidence for CeSb belonging to a new class of Kondo lattice materials with Weyl fermions in the ferromagnetic state.

cond-mat.str-el↗

arXiv2017

Extremely large magnetoresistance and Fermi surface topology of PrSb

We report magnetotransport measurements of PrSb in high magnetic fields. Our results show that PrSb exhibits extremely large magnetoresistance(XMR) at low temperatures. Meanwhile angle-dependent magnetoresistance measurements were used to probe the Fermi surface via Shubnikov-de Haas (SdH) oscillations. The angular dependence of the frequencies of the $α$-branch indicate a two-dimensional character for this Fermi surface sheet, while the effective mass of this branch as a function of angle shows a four-fold signature. The evolution of the Fermi surface with field was also studied up to 32~T. An enlargement of the Fermi surface up to 14~T is observed, before the oscillation frequencies become constant at higher fields. Meanwhile our analysis of the residual Landau index from the high field data reveals a zero Berry phase and therefore trivial topology of the Fermi surface.

cond-mat.str-el↗

arXiv2017

Antiferromagnetism with divalent Eu in EuNi$_5$As$_3$

We have successfully synthesized single crystals of EuNi$_5$As$_3$ using a flux method and we present a comprehensive study of the physical properties using magnetic susceptibility, specific heat, electrical resistivity, thermoelectric power and x-ray absorption spectroscopy (XAS) measurements. EuNi$_5$As$_3$ undergoes two close antiferromagnetic transitions at respective temperatures of $T_{N1}$ = 7.2 K and $T_{N2}$ = 6.4 K, which are associated with the Eu$^{2+}$ moments. Both transitions are suppressed upon applying a field and we map the temperature-field phase diagrams for fields applied parallel and perpendicular to the easy $a$ axis. XAS measurements reveal that the Eu is strongly divalent, with very little temperature dependence, indicating the localized Eu$^{2+}$ nature of EuNi$_5$As$_3$, with a lack of evidence for heavy fermion behavior.

cond-mat.str-el↗

arXiv2017

Disruption of the accidental Dirac semimetal state in ZrTe$_{5}$ under hydrostatic pressure

We study the effect of hydrostatic pressure on the magnetotransport properties of the zirconium pentatelluride. The magnitude of resistivity anomaly gets enhanced with increasing pressure, but the transition temperature $T^{\ast}$ is almost independent of it. In the case of H $\parallel$ $b$, the quasi-linear magnetoresistance decreases drastically from 3300$\%$ (9 T) at ambient pressure to 400$\%$ (14 T) at 2.5 GPa. Besides, the change of the quantum oscillation phase from topological nontrivial to trivial is revealed around 2 GPa. Both demonstrate that the pressure breaks the accidental Dirac node in ZrTe$_{5}$. For H $\parallel$ $c$, in contrast, subtle changes can be seen in the magnetoresistance and quantum oscillations. In the presence of pressure, ZrTe$_{5}$ evolves from a highly anisotropic to a nearly isotropic electronic system, which accompanies with the disruption of the accidental Dirac semimetal state. It supports the assumption that ZrTe$_{5}$ is a semi-3D Dirac system with linear dispersion along two directions and a quadratic one along the third.

cond-mat.mtrl-sci↗

arXiv2016

Superconductivity and multiple pressure-induced phases in BaPt$_2$As$_2$

The newly discovered BaPt$_2$As$_2$ shows a structural distortion at around 275~K, followed by the emergence of superconductivity at lower temperatures. Here we identify the presence of charge density wave (CDW) order at room temperature and ambient pressure using single crystal x-ray diffraction, with both a superlattice and an incommensurate modulation, where there is a change of the superlattice structure below $\simeq$ 275~K. Upon applying pressure, BaPt$_2$As$_2$ shows a rich temperature-pressure phase diagram with multiple pressure-induced transitions at high temperatures, the emergence or disappearance of which are correlated with sudden changes in the superconducting transition temperature $T_c$. These findings demonstrate that BaPt$_2$As$_2$ is a promising new system for studying competing interactions and the relationship between high-temperature electronic instabilities and superconductivity.

cond-mat.supr-con↗

arXiv2015

Crossover from a heavy fermion to intermediate valence state in noncentrosymmetric Yb2Ni12(P,As)7

We report measurements of the physical properties and electronic structure of the hexagonal compounds Yb2Ni12Pn7 (Pn = P, As) by measuring the electrical resistivity, magnetization, specific heat and partial fluorescence yield x-ray absorption spectroscopy (PFY-XAS). These demonstrate a crossover upon reducing the unit cell volume, from an intermediate valence state in Yb2Ni12As7 to a heavy-fermion paramagnetic state in Yb2Ni12P7, where the Yb is nearly trivalent. Application of pressure to Yb2Ni12P7 suppresses T_FL, the temperature below which Fermi liquid behavior is recovered, suggesting the presence of a quantum critical point (QCP) under pressure. However, while there is little change in the Yb valence of Yb2Ni12P7 up to 30 GPa, there is a strong increase for Yb2Ni12As7 under pressure, before a near constant value is reached. These results indicate that any magnetic QCP in this system is well separated from strong valence fluctuations. The pressure dependence of the valence and lattice parameters of Yb2Ni12As7 are compared and at 1 GPa, there is an anomaly in the unit cell volume as well as a change in the slope of the Yb valence, indicating a correlation between structural and electronic changes.

cond-mat.str-el↗

arXiv2015

Evidence for two distinct superconducting phases in EuBiS$_2$F under pressure

We present a pressure study of the electrical resistivity, AC magnetic susceptibility and powder x-ray diffraction (XRD) of the newly discovered BiS$_2$-based superconductor EuBiS$_2$F. At ambient pressure, EuBiS$_2$F shows an anomaly in the resistivity at around $T_0\approx 280$ K and a superconducting transition at $T_c\approx 0.3$ K. Upon applying hydrostatic pressure, there is little change in $T_0$ but the amplitude of the resistive anomaly is suppressed, whereas there is a dramatic enhancement of $T_c$ from 0.3 K to about 8.6 K at a critical pressure of $p_c$ $\approx{1.4}$ GPa. XRD measurements confirm that this enhancement of $T_c$ coincides with a structural phase transition from a tetragonal phase ($P4/nmm$) to a monoclinic phase ($P2_1$/m), which is similar to that observed in isostructural LaO$_{0.5}$F$_{0.5}$BiS$_2$. Our results suggest the presence of two different superconducting phases with distinct crystal structures in EuBiS$_2$F, which may be a general property of this family of BiS$_2$-based superconductors.

cond-mat.supr-con↗

arXiv2014

Superconductivity and structural distortion in BaPt$_2$As$_2$

We successfully synthesized the BaPt$_2$As$_2$ single crystals and studied their structural and physical properties at low temperatures. BaPt$_2$As$_2$ crystallizes in the CaBe$_2$Ge$_2$-type tetragonal structure (P4/nmm) at room temperature and undergoes a first-order structural transition at $T_S\simeq 275$ K, which is likely associated with a charge-density-wave (CDW) instability. BCS-type superconductivity with two subsequent transitions at $T_{c1}=1.67$K and $T_{c2}$=1.33K are observed in this compound. Thus, BaPt$_2$As$_2$ may serve as a new system for studying the interplay of superconductivity and the CDW order.

cond-mat.supr-con↗

arXiv2014

BCS-like superconductivity in NdO$_{1-x}$F$_{x}$BiS$_{2}$ ($x$ = 0.3 and 0.5) single crystals

We measure the magnetic penetration depth $Δλ(T)$ for NdO$_{1-x}$F$_{x}$BiS$_{2}$ ($x$ = 0.3 and 0.5) using the tunnel diode oscillator technique. The $Δλ(T)$ shows an upturn in the low-temperature limit which is attributed to the paramagnetism of Nd ions. After subtracting the paramagnetic contributions, the penetration depth $Δλ(T)$ follows exponential-type temperature dependence at $T\ll T_c$. Both $Δλ(T)$ and the corresponding superfluid density $ρ_s(T)$ can be described by the BCS model with an energy gap of $Δ(0)$ $\approx$ 2.0 $k_BT_c$ for both $x$ = 0.3 and 0.5, suggesting strong-coupling BCS superconductivity in the presence of localized moments for NdO$_{1-x}$F$_{x}$BiS$_{2}$.

cond-mat.supr-con↗