arXiv subjects

J. -Q. Yan

Publications and source records attributed to J. -Q. Yan.

At least 19 recordsLinked to original sources

arXiv2026

Magneto-optical Kerr effect in an A-type antiferromagnet

Magneto-optic Kerr effect (MOKE) is a powerful probe of broken time-reversal symmetry ($\mathcal{T}$), typically used to study ferromagnets. While MOKE has been observed in some antiferromagnets (AFMs) with vanishing magnetization, it is often associated with structures whose symmetry is lower than basic collinear, bipartite order. In contrast, theory predicts a mechanism for MOKE intrinsic to all AFMs of A-type, i.e. layered AFMs in which ferromagnetic layers are antiferromagnetically aligned. Here we report the first experimental confirmation of this mechanism in a bulk AFM. We achieve this by measuring the imaginary component of MOKE as a function of photon energy in MnBi$_2$Te$_4$, an A-type AFM where $\mathcal{T}$ is preserved in combination with a translation. By comparing the experimental results with model calculations, we demonstrate that observable MOKE should be expected in all collinear A-type AFMs with out-of-plane spin order, thus enabling optical detection of AFM domains and expanding the scope of MOKE to few-layer AFMs.

cond-mat.mtrl-sci↗

arXiv2025

Role of non-magnetic spacers in the magnetic interactions of antiferromagnetic topological insulators MnBi$_{4}$Te$_{7}$ and MnBi$_{2}$Te$_{4}$

MnBi$_{4}$Te$_{7}$ belongs to a family of antiferromagnetic topological insulators. It forms a natural heterostructure of magnetic (septuple) and non-magnetic (quintuple) topological blocks. Here, we explore the magnetism and magnetic interactions in this compound using inelastic neutron scattering. We find that the interlayer magnetic coupling is much weaker in MnBi$_{4}$Te$_{7}$ as compared to MnBi$_{2}$Te$_{4}$ due to the insertion of non-magnetic quintuple layers in the former. However, other key magnetic energy scales residing within a single septuple block, the single-ion anisotropy and long-range intralayer exchanges, are essentially the same. This identifies a transferable set of magnetic interactions applicable to the extended family of magnetic topological insulators based on MnBi$_2$Te$_4$-Bi$_2$Te$_3$ heterostructures.

cond-mat.str-el↗

arXiv2024

Observation of Phonon Angular Momentum

Angular momentum (AM), a fundamental concept describing the rotation of an object about an axis, profoundly influences all branches of physics. In condensed matter, AM is intimately related to the emergence of topological quantum states, including chiral superconductivity and quantum spin liquids, and various chiral quasiparticles. Recently, it has been predicted that microscopic lattice excitations, known as phonons, can carry finite AM with remarkable macroscopic physical consequences. However, the direct observation of phonon-AM has not been achieved. In this letter, we report the experimental discovery of phonon-AM in the chiral crystal Tellurium. We show that due to AM conservation, applying a time-reversal symmetry breaking thermal gradient along the chiral axis of single crystal Te results in a macroscopic mechanical torque, $τ$, that can be observed using a cantilever-based device. We establish that the mechanical torques change sign by flipping the thermal gradient and disappear in polycrystalline samples that lack a preferred chirality. Based on our experimental settings, we estimate $τ\sim10^{-11}$N$\cdot$m, in agreement with theoretical calculations. Our results uncover phonon-AM and pave the way for phonon-AM enabled quantum states for microelectronic applications.

cond-mat.str-el↗

arXiv2023

Local site behavior of the 5d and 4f ions in the frustrated pyrochlore Ho2Os2O7

The pyrochlore osmate Ho2Os2O7 is a candidate material for a fragile J=0 local singlet ground state, however little is known regarding the single-ion behavior of either the Os or Ho ions. To address this we present polarized neutron powder diffraction (PNPD) and resonant inelastic x-ray scattering (RIXS) measurements that separately probe the local site behavior of the Os and Ho ions. The PNPD results are dominated by Ho3+ scattering and the analysis reveals local site susceptibility behavior consistent with spin ice materials. Complimentary unpolarized neutron powder diffraction show an ordered spin ice ground state in an applied magnetic field. To isolate the Os4+ single-ion behavior we present resonant inelastic x-ray scattering (RIXS) measurements at the osmium L-edge. Analysis of the RIXS spectra parameterize the spin-orbit coupling (0.35 eV), Hund's coupling (0.27 eV) and trigonal distortion (-0.17 eV). The results are considered within the context of a J=0 model and possible departures from this through structural distortions, excitonic interactions and 5d-4f interactions between the Os ion and the surrounding Ho lattice. The experimental methodology employed highlights the complimentary information available in rare earth based 5d pyrochlores from distinct neutron and x-ray scattering techniques that allow for the isolation and determination of the behavior of the different ions.

cond-mat.str-el↗

arXiv2023

Helical magnetic state in the vicinity of the pressure-induced superconducting phase in MnP

MnP is a metal that shows successive magnetic transitions from paramagnetic to ferromagnetic and helical magnetic phases at ambient pressure with decreasing temperature. With applied pressure, the magnetic transition temperatures decrease and superconductivity appears around 8 GPa where the magnetic order is fully suppressed and the quantum critical behavior is observed. These results suggest that MnP is an unconventional superconductor in which magnetic fluctuations may be relevant to the superconducting pairing mechanism. In order to elucidate the magnetic ground state adjacent to the superconducting phase first discovered in Mn-based materials, high-pressure neutron diffraction measurements have been performed in hydrostatic pressure up to 7.5 GPa. The helical magnetic structure with the propagation vector along the $b$ axis, reported previously at 3.8 GPa, was found to be robust up to 7.5 GPa. First principles and classical Monte Carlo calculations have also been performed to understand how the pressure-driven magnetic phase transitions are coupled with change of the exchange interactions. The calculations, which qualitatively reproduce the magnetic structures as a function of pressure, suggest that the exchange interactions change drastically with applied pressure and the further-neighbor interactions become more influential at high pressures. Combining the experimental and theoretical results, we describe the detail of exchange interactions in the vicinity of the superconducting phase which is critical to understand the pairing mechanism of the unconventional superconductivity in MnP.

cond-mat.mtrl-sci↗

arXiv2022

Self-selecting vapor growth of transition metal halide single crystals

Transition metal halides can host a large variety of novel phenomena, such as magnetism in the monolayer limit, quantum spin liquid and spiral spin liquid states, and topological magnons and phonons. Sizeable high quality single crystals are necessary for investigations of magnetic and lattice excitations by, for example, inelastic neutron scattering. In this paper, we review a less well-known vapor transport technique, self-selecting vapor growth, and report our growths of transition metal halides using this technique. We report the growth and characterizations of sizable single crystals of RuCl$_3$, CrCl$_3$, Ru$_{1-x}$Cr$_x$Cl$_3$, and CrBr$_3$. In order to expedite the conversion of starting powder to single crystals, we modified the technique by cooling the growth ampoule through an appropriate temperature range. Our work shows that the self-selecting vapor transport technique can provide large single crystals of transition metal halides, demonstrating its potential for providing high quality single crystals of quantum materials.

cond-mat.mtrl-sci↗

arXiv2022

Pseudogap Suppression by Competition with Superconductivity in La-Based Cuprates

We have carried out a comprehensive high-resolution angle-resolved photoemission spectroscopy (ARPES) study of the pseudogap interplay with superconductivity in La-based cuprates. The three systems La$_{2-x}$Sr$_x$CuO$_4$, La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$, and La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$ display slightly different pseudogap critical points in the temperature versus doping phase diagram. We have studied the pseudogap evolution into the superconducting state for doping concentrations just below the critical point. In this setting, near optimal doping for superconductivity and in the presence of the weakest possible pseudogap, we uncover how the pseudogap is partially suppressed inside the superconducting state. This conclusion is based on the direct observation of a reduced pseudogap energy scale and re-emergence of spectral weight suppressed by the pseudogap. Altogether these observations suggest that the pseudogap phenomenon in La-based cuprates is in competition with superconductivity for anti-nodal spectral weight.

cond-mat.supr-con↗

arXiv2022

Temperature-induced valence-state transition in double perovskite Ba2-xSrxTbIrO6

In this work, a temperature-induced valence-state transition is studied in a narrow composition range of Ba$_{2-x}$Sr$_x$TbIrO$_6$. The valence-state transition involves an electron transfer between Tb and Ir leading to the valence-state change between Tb$^{3+}$/Ir$^{5+}$ and Tb$^{4+}$/Ir$^{4+}$ phases. This first-order transition has a dramatic effect on the lattice, transport properties, and the long-range magnetic order at low temperatures for both Tb and Ir ions. Ir$^{5+}$ ion has an electronic configuration of 5$d^4$ ($J\rm_{eff}$ = 0) which is expected to be nonmagnetic. In contrast, Ir$^{4+}$ ion with a configuration of 5$d^5$($J\rm_{eff}$ = 1/2) favors a long-range magnetic order. For $x$ = 0.1 with Tb$^{3+}$/Ir$^{5+}$ configuration to the lowest temperature (2 K) investigated in this work, a spin-glass behavior is observed around 5 K indicating Ir$^{5+}$ ($J\rm_{eff}$ = 0) ions act as a spacer reducing the magnetic interactions between Tb$^{3+}$ ions. For $x$ = 0.5 with Tb$^{4+}$/Ir$^{4+}$ configuration below the highest temperature 400 K of this work, a long-range antiferromagnetic order at $T\rm_N$ = 40 K is observed highlighting the importance of Ir$^{4+}$ ($J\rm_{eff}$ = 1/2) ions in promoting the long-range magnetic order of both Tb and Ir ions. For 0.2 $\leqslant x \leqslant$ 0.375, a temperature-induced valence-state transition from high-temperature Tb$^{3+}$/Ir$^{5+}$ phase to low-temperature Tb$^{4+}$/Ir$^{4+}$ phase occurs in the temperature range 180 K $\leqslant T \leqslant$ 325 K and the transition temperature increases with $x$. The compositional dependence demonstrates the ability to tune the the valence state for a critical region of $x$ that leads to a concurrent change in magnetism and structure. This tuning ability could be employed with suitable strain in thin films to act as a switch as the magnetism is manipulated.

cond-mat.str-el↗

arXiv2021

Vapor transport growth of MnBi2Te4 and related compounds

Motivated by fine tuning of the magnetic and topological properties of MnBi$_2$Te$_4$ via defect engineering, in this work, we report the crystal growth of MnBi$_2$Te$_4$ and related compounds using vapor transport method and characterization of vapor transported crystals by measuring elemental analysis, magnetic and transport properties, and scanning tunneling microscopy. For the growth of MnBi$_2$Te$_4$ single crystals, I$_2$, MnI$_2$ , MnCl$_2$, TeCl$_4$, or MoCl$_5$ are all effective transport agents; chemical transportation occurs faster in the presence of iodides than chlorides. MnBi$_2$Te$_4$ crystals can be obtained in the temperature range 500$^\circ$C-590$^\circ$C using I$_2$ as the transport agent. We further successfully grow MnSb$_2$Te$_4$, MnBi$_{2-x}$Sb$_x$Te$_4$, and Sb-doped MnBi$_4$Te$_7$ crystals. A small temperature gradient $<$20$^\circ$C between the hot and cold ends of the growth cmpoule is critical for the successful crystal growth of MnBi$_2$Te$_4$ and related compounds. Compared to flux grown crystals, vapor transported crystals tend to be Mn stoichiometric, and Sb-bearing compositions have more Mn/Sb site mixing. The vapor transport growth provides a new materials synthesis approach to fine tuning the magnetic and topological properties of these intrinsic magnetic topological insulators.

cond-mat.mtrl-sci↗

arXiv2021

Field-induced intermediate phase in $α$-RuCl$_3$: Non-coplanar order, phase diagram, and proximate spin liquid

Frustrated magnets with strong spin-orbit coupling promise to host topological states of matter, with fractionalized excitations and emergent gauge fields. Kitaev's proposal for a honeycomb-lattice Majorana spin liquid has triggered an intense search for experimental realizations, with bond-dependent Ising interaction being the essential building block. A prime candidate is $α$-RuCl$_3$ whose phase diagram in a magnetic field is, however, not understood to date. Here we present conclusive experimental evidence for a novel field-induced ordered phase in $α$-RuCl$_3$, sandwiched between the zigzag and quantum disordered phases at low and high fields, respectively. We provide a detailed theoretical study of the relevant effective spin model which we show to display a field-induced intermediate phase as well. We fully characterize the intermediate phase within this model, including its complex spin structure, and pinpoint the parameters relevant to $α$-RuCl$_3$ based on the experimentally observed critical fields. Most importantly, our study connects the physics of $α$-RuCl$_3$ to that of the Kitaev-$Γ$ model, which displays a quantum spin liquid phase in zero field, and hence reveals the spin liquid whose signatures have been detected in a variety of dynamical probes of $α$-RuCl$_3$.

cond-mat.str-el↗

arXiv2021

Evolution of Magnetic Interactions in Sb-substituted MnBi2Te4

The Mn(Bi$_{1-x}$Sb$_x$)$_2$Te$_4$ series is purported to span from antiferromagnetic (AF) topological insulator at x = 0 to a trivial AF insulator at x = 1. Here we report on neutron diffraction and inelastic neutron scattering studies of the magnetic interactions across this series. All compounds measured possess ferromagnetic (FM) triangular layers and we find a crossover from AF to FM interlayer coupling near x = 1 for our samples. The large spin gap at x = 0 closes rapidly and the average FM exchange interactions within the triangular layer increase with Sb substitution. Similar to a previous study of MnBi$_2$Te$_4$, we find severe spectral broadening which increases dramatically across the compositional series. In addition to broadening, we observe an additional sharp magnetic excitation in MnSb$_2$Te$_4$ that may indicate the development of local magnetic modes based on recent reports of antisite disorder between Mn and Sb sublattices. The results suggest that both substitutional and antisite disorder contribute substantially to the magnetism in Mn(Bi$_{1-x}$Sb$_x$)$_2$Te$_4$.

cond-mat.str-el↗

arXiv2021

Thermopower across the phase diagram of the cuprate La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ : signatures of the pseudogap and charge-density-wave phases

The Seebeck coefficient (thermopower) $S$ of the cuprate superconductor La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ was measured across its doping phase diagram (from $p = 0.12$ to $p = 0.25$), at various temperatures down to $T \simeq 2$ K, in the normal state accessed by suppressing superconductivity with a magnetic field up to $H = 37.5$ T. The magnitude of $S/T$ in the $T=0$ limit is found to suddenly increase, by a factor $\simeq 5$, when the doping is reduced below $p^{\star} = 0.23$, the critical doping for the onset of the pseudogap phase. This confirms that the pseudogap phase causes a large reduction of the carrier density $n$, consistent with a drop from $n = 1 + p$ above $p^{\star}$ to $n = p$ below $p^{\star}$, as previously inferred from measurements of the Hall coefficient, resistivity and thermal conductivity. When the doping is reduced below $p = 0.19$, a qualitative change is observed whereby $S/T$ decreases as $T \to 0$, eventually to reach negative values at $T=0$. In prior work on other cuprates, negative values of $S/T$ at $T \to 0$ were shown to result from a reconstruction of the Fermi surface caused by charge-density-wave (CDW) order. We therefore identify $p_{\rm CDW} \simeq 0.19$ as the critical doping beyond which there is no CDW-induced Fermi surface reconstruction. The fact that $p_{\rm CDW}$ is well separated from $p^{\star}$ reveals that there is a doping range below $p^{\star}$ where the transport signatures of the pseudogap phase are unaffected by CDW correlations, as previously found in YBa$_2$Cu$_3$O$_y$ and La$_{2-x}$Sr$_x$CuO$_4$.

cond-mat.supr-con↗

arXiv2020

Nature of Magnetic Excitations in the High-Field Phase of $α$-RuCl$_3$

We present comprehensive electron spin resonance (ESR) studies of in-plane oriented single crystals of $α$-RuCl$_3$, a quasi-two-dimensional material with honeycomb structure, focusing on its high-field spin dynamics. The measurements were performed in magnetic fields up to 16 T, applied along the [110] and [100] directions. Several ESR modes were detected. Combining our findings with recent inelastic neutron- and Raman-scattering data, we identify most of the observed excitations. Most importantly, we show that the low-temperature ESR response beyond the boundary of the magnetically ordered region is dominated by single- and two-particle processes with magnons as elementary excitations. The peculiarities of the excitation spectrum in the vicinity of the critical field are discussed.

cond-mat.str-el↗

arXiv2020

Coexistence of Surface Ferromagnetism and Gapless Topological State in MnBi$_2$Te$_4$

Surface magnetism and its correlation with the electronic structure are critical to understand the gapless topological surface state in the intrinsic magnetic topological insulator MnBi$_2$Te$_4$. Here, using static and time resolved angle-resolved photoemission spectroscopy (ARPES), we find a significant ARPES intensity change together with a gap opening on a Rashba-like conduction band. Comparison with a model simulation strongly indicates that the surface magnetism on cleaved MnBi$_2$Te$_4$ is the same as its bulk state. The coexistence of surface ferromagnetism and a gapless TSS uncovers the novel complexity of MnBi$_2$Te$_4$ that may be responsible for the low quantum anomalous Hall temperature of exfoliated MnBi$_2$Te$_4$.

cond-mat.mtrl-sci↗

arXiv2020

A-type Antiferromagnetic order in MnBi4Te7 and MnBi6Te10 single crystals

MnBi$_4$Te$_{7}$ and MnBi$_6$Te$_{10}$ are two members with n=2 and 3 in the family of MnBi$_{2n}$Te$_{3n+1}$ where the n=1 member, MnBi$_2$Te$_{4}$, has been intensively investigated as the first intrinsic antiferromagnetic topological insulator. Here we report the A-type antiferromagnetic order in these two compounds by measuring magnetic properties, electrical and thermal transport, specific heat, and single crystal neutron diffraction. Both compounds order into an A-type antiferromagnetic structure as does MnBi$_2$Te$_{4}$ with ferromagnetic planes coupled antiferromagnetically along the crystallographic \textit{c} axis. While no evidence for any in-plane ordered moment is found for MnBi$_2$Te$_{4}$ or MnBi$_6$Te$_{10}$, weak reflections at half-L positions along the [0 0 L] direction are observed for MnBi$_4$Te$_{7}$ suggesting an in-plane ordered moment around 0.15$μ_{B}$/Mn. The ordering temperature, T$_N$, is 13\,K for MnBi$_4$Te$_{7}$ and 11\,K for MnBi$_6$Te$_{10}$. The magnetic order is also manifested in the anisotropic magnetic properties. For both compounds, the interlayer coupling is weak and a spin flip transition occurs when a magnetic field of around 1.6\,kOe is applied along the \textit{c}-axis at 2\,K. As observed in MnBi$_2$Te$_4$, when cooling across T$_N$, no anomaly was observed in the temperature dependence of thermopower. On the other hand, critical scattering effects are observed in thermal conductivity although the effect is less pronounced than that in MnBi$_2$Te$_{4}$.

cond-mat.mtrl-sci↗

arXiv2019

Evidence for charge transfer and proximate magnetism in graphene/a-RuCl_3 heterostructures

We report a study of electronic transport in van der Waals heterostructures composed of flakes of the antiferromagnetic Mott insulator a-RuCl_3 placed on top of monolayer graphene Hall bars. While the zero-field transport shows a strong resemblance to that of isolated graphene, we find a consistently $p$-type Hall effect suggestive of multiband conduction, along with a non-monotonic and gate-voltage-dependent excursion of the resistivity at low temperatures that is reminiscent of transport in the presence of a magnetic phase transition. We interpret these data as evidence for charge transfer from graphene to a-RuCl_3 in an inhomogeneous device yielding both highly- and lightly-doped regions of graphene, while the latter shows a particular sensitivity to magnetism in the a-RuCl_3. Thus proximity to graphene is a means to access magnetic properties of thin layers of magnetic insulators.

cond-mat.str-el↗

arXiv2019

Competing magnetic interactions in the antiferromagnetic topological insulator MnBi$_{2}$Te$_{4}$

The antiferromagnetic (AF) compound MnBi$_{2}$Te$_{4}$ is suggested to be the first realization of an antiferromagnetic (AF) topological insulator. Here we report on inelastic neutron scattering studies of the magnetic interactions in MnBi$_{2}$Te$_{4}$ that possess ferromagnetic (FM) triangular layers with AF interlayer coupling. The spin waves display a large spin gap and pairwise exchange interactions within the triangular layer are frustrated due to large next-nearest neighbor AF exchange. The degree of frustration suggests proximity to a variety of magnetic phases, potentially including skyrmion phases, that could be accessed in chemically tuned compounds or upon the application of symmetry-breaking fields.

cond-mat.str-el↗

arXiv2019

Polarization-resolved Raman spectroscopy of α-RuCl3 and evidence of room temperature two-dimensional magnetic scattering

Polarization-resolved Raman spectroscopy was performed and analyzed from large, high quality, mono-domain single crystal of α-RuCl3, a proximate Kitaev quantum spin liquid. Spectra were collected with laser polarizations parallel and perpendicular to the honeycomb plane. Pairs of nearly degenerate phonons were discovered and show either a 4-fold or 2-fold polarization angle dependence in their Raman intensity, thereby providing evidence to definitively assign the bulk crystal point group as C2h. The low frequency continuum that is often attributed to scattering from pairs of Majorana fermions was also examined and found to disappear when the laser excitation and scattered photon polarizations were perpendicular to the honeycomb plane. This disappearance, along with the behavior of the phonon spectrum in the same polarization configuration, strongly suggests that the scattering continuum is 2-dimensional. We argue that this scattering continuum originates from the Kitaev magnetic interactions that survives up to room temperature, a scale larger than the bare Kitaev exchange energy of approximately 50 K.

cond-mat.str-el↗