arXiv subjects

Qingming Zhang

Publications and source records attributed to Qingming Zhang.

53 records · Page 3Linked to original sources

arXiv2018

Rare-earth chalcogenides: A large family of triangular lattice spin liquid candidate

Frustrated quantum magnets are expected to host many exotic quantum spin states like quantum spin liquid (QSL), and have attracted numerous interest in modern condensed matter physics. The discovery of the triangular lattice spin liquid candidate YbMgGaO$_4$ stimulated an increasing attention on the rare-earth-based frustrated magnets with strong spin-orbit coupling. Here we report the synthesis and characterization of a large family of rare-earth chalcogenides AReCh$_2$ (A = alkali or monovalent ions, Re = rare earth, Ch = O, S, Se). The family compounds share the same structure (R$\bar{3}$m) as YbMgGaO$_4$, and antiferromagnetically coupled rare-earth ions form perfect triangular layers that are well separated along the $c$-axis. Specific heat and magnetic susceptibility measurements on NaYbO$_2$, NaYbS$_2$ and NaYbSe$_2$ single crystals and polycrystals, reveal no structural or magnetic transition down to 50mK. The family, having the simplest structure and chemical formula among the known QSL candidates, removes the issue on possible exchange disorders in YbMgGaO$_4$. More excitingly, the rich diversity of the family members allows tunable charge gaps, variable exchange coupling, and many other advantages. This makes the family an ideal platform for fundamental research of QSLs and its promising applications.

cond-mat.str-el↗

arXiv2017

Evidence for a spinon Fermi surface in a triangular lattice quantum spin liquid candidate

A quantum spin liquid is an exotic quantum state of matter in which spins are highly entangled and remain disordered down to zero temperature. Such a state of matter is potentially relevant to high-temperature superconductivity and quantum-information applications, and experimental identification of a quantum spin liquid state is of fundamental importance for our understanding of quantum matter. Theoretical studies have proposed various quantum-spin-liquid ground states, most of which are characterized by exotic spin excitations with fractional quantum numbers (termed `spinon'). Here, we report neutron scattering measurements that reveal broad spin excitations covering a wide region of the Brillouin zone in a triangular antiferromagnet YbMgGaO4. The observed diffusive spin excitation persists at the lowest measured energy and shows a clear upper excitation edge, which is consistent with the particle-hole excitation of a spinon Fermi surface. Our results therefore point to a QSL state with a spinon Fermi surface in YbMgGaO4 that has a perfect spin-1/2 triangular lattice as in the original proposal of quantum spin liquids.

cond-mat.str-el↗

arXiv2017

Nearest-neighbor resonating valence bonds in YbMgGaO4

Since its proposal by Anderson, resonating valence bonds (RVB) formed by a superposition of fluctuating singlet pairs have been a paradigmatic concept in understanding quantum spin liquids (QSL). Here, we show that excitations related to singlet breaking on nearest-neighbor bonds describe the high-energy part of the excitation spectrum in YbMgGaO4, the effective spin-1/2 frustrated antiferromagnet on the triangular lattice, as originally considered by Anderson. By a thorough single-crystal inelastic neutron scattering (INS) study, we demonstrate that nearest-neighbor RVB excitations account for the bulk of the spectral weight above 0.5 meV. This renders YbMgGaO4 the first experimental system where putative RVB correlations restricted to nearest neighbors are observed, and poses a fundamental question of how complex interactions on the triangular lattice conspire to form this unique many-body state.

cond-mat.str-el↗

arXiv2017

Interplay of Dirac electrons and magnetism in AMnBi2 (A=Ca, Sr)

Dirac materials exhibit intriguing low-energy carrier dynamics that offer a fertile ground for novel physics discovery. Of particular interest is the interplay of Dirac carriers with other quantum phenomena, such as magnetism. Here we report on a two-magnon Raman scattering study of AMnBi2 (A=Ca, Sr), a prototypical magnetic Dirac system comprising alternating Dirac-carrier and magnetic layers. We present the first accurate determination of the exchange energies in these compounds and, by comparison to the reference compound BaMn2Bi2, we show that the Dirac-carrier layers in AMnBi2 significantly enhance the exchange coupling between the magnetic layers, which in turn drives a charge-gap opening along the Dirac locus. Our findings break new grounds in unveiling the fundamental physics of magnetic Dirac materials, which offer a novel platform for probing a distinct type of spin-Fermion interaction. The outstanding properties of these materials allow a delicate manipulation of the interaction between the Dirac carriers and magnetic moments, thus holding great promise for applications in magnetic Dirac devices.

cond-mat.supr-con↗

arXiv2017

Raman phonons in the ferroelectric-like metal LiOsO$_3$

The novel ferroelectric-like structural transition observed in metallic LiOsO$_3$ [Y. Shi et al., Nat. Mater. 12, 1024 (2013)], has invoked many theoretical and experimental interests. In this work, we have performed polarized and temperature-dependent Raman scattering measurements on high-quality single crystal LiOsO$_3$ and identified Raman-active modes in both centrosymmetric phase (300 K, R$\bar{3}$c) and non-centrosymmetric phase (10 K, R3c). Only four phonon peaks are observed in the former phase, while there are twelve peaks in the latter phase because of the reduction of crystal symmetry. With the help of careful symmetry analysis and first-principles calculations, we can make a systematic assignment for the observed Raman modes in both phases. The significant changes in line-width and the continuous evolution of Raman frequencies with temperatures were observed for the E$_g$ modes around the transition temperature, which suggests that the ferroelectric-like structural transition is a continuous order-disorder transition. The result sheds light on the coexistence of ferroelectricity and metallicity in the compound.

cond-mat.mtrl-sci↗

arXiv2017

Raman scattering study of large magnetoresistance semimetals TaAs$_2$ and NbAs$_2$

We have performed polarized and temperature-dependent Raman scattering measurements on extremely large magnetoresitance compounds TaAs$_2$ and NbAs$_2$. In both crystals, all the Raman active modes, including six A$_g$ modes and three B$_g$ modes, are clearly observed and well assigned with the combination of symmetry analysis and first-principles calculations. The well-resolved periodic intensity modulations of the observed modes with rotating crystal orientations, verify the symmetry of each assigned mode and are fitted to experimentally determine the elements of Raman tensor matrixes. The broadening of two A$_g$ modes seen in both compounds allows us to estimate electron-phonon coupling constant, which suggests a relatively small electron-phonon coupling in the semimetals TaAs$_2$ and NbAs$_2$. The present study provides the fundamental lattice dynamics information on TaAs$_2$ and NbAs$_2$ and may shed light on the understanding of their extraordinary large magnetoresistance.

cond-mat.mtrl-sci↗

arXiv2017

Evolution of magnetism in cerium doped LaCo2P2 crystals: a magnetic phase diagram

ThCr2Si2-type phosphide ACo2P2 (A=Rare earth elements) has the same structure as iron arsenides, but their magnetic behaviors are quite distinct. In this paper, we for the first time grew a series of La1-xCexCo2P2 single crystals (x=0.0 to1.0), and made structural and magnetic characterizations. This allows us to carry out a careful investigation on the evolution of magnetism with cerium content and build a magnetic phase diagram. We found that the introduction of cerium induces a rapid decrease of c-axis and a change from ferromagnetic (FM) to antiferromagnetic (AFM) states. By employing first-principles band-structure calculations, we identify the formation of P-P bonding with the shortening of c-axis, which effectively drives an increase of AFM interaction and eventually leads to AFM ordering in the high doping region. The present study may shed light on the interplay between the structural collapsing and electronic/magnetic properties in 122 iron pnictides.

cond-mat.mtrl-sci↗

arXiv2017

Crystalline Electric Field Randomness in the Triangular Lattice Spin-Liquid YbMgGaO$_4$

We apply moderate-high-energy inelastic neutron scattering (INS) measurements to investigate Yb$^{3+}$ crystalline electric field (CEF) levels in the triangular spin-liquid candidate YbMgGaO$_4$. Three CEF excitations from the ground-state Kramers doublet are centered at the energies $\hbar ω$ = 39, 61, and 97\,meV in agreement with the effective \mbox{spin-1/2} $g$-factors and experimental heat capacity, but reveal sizable broadening. We argue that this broadening originates from the site mixing between Mg$^{2+}$ and Ga$^{3+}$ giving rise to a distribution of Yb--O distances and orientations and, thus, of CEF parameters that account for the peculiar energy profile of the CEF excitations. The CEF randomness gives rise to a distribution of the effective spin-1/2 $g$-factors and explains the unprecedented broadening of low-energy magnetic excitations in the fully polarized ferromagnetic phase of YbMgGaO$_4$, although a distribution of magnetic couplings due to the Mg/Ga disorder may be important as well.

cond-mat.str-el↗

arXiv2016

Raman scattering in transition metal dichalcogenides MTe2 (M = Mo, W)

We performed comparable polarized Raman scattering studies of MoTe2 and WTe2. By rotating crystals to tune the angle between the principal axis of the crystals and the polarization of the incident/scattered light, we obtained the angle dependence of the intensities for all the observed modes, which is perfectly consistent with careful symmetry analysis. Combining these results with first-principles calculations, we clearly identified the observed phonon modes in the different phases of both crystals. Fifteen Raman-active phonon modes (10Ag+5Bg) in the high-symmetry phase 1T'-MoTe2 (300 K) were well assigned, and all the symmetry-allowed Raman modes (11A1+6A2) in the low-symmetry phase Td-MoTe2 (10 K) and 12 Raman phonons (8A1+4A2) in Td-WTe2 were observed and identified. The present work provides basic information about the lattice dynamics in transition-metal dichalcogenides and may shed some light on the understanding of the extremely large magnetoresistance (MR) in this class of materials.

cond-mat.mtrl-sci↗

arXiv2016

$μ$SR evidence for the U(1) quantum spin liquid ground state in the triangular antiferromagnet YbMgGaO$_4$

Muon spin relaxation and rotation ($μ$SR) experiments on single crystals of the structurally perfect triangular antiferromagnet YbMgGaO$_4$ indicate the absence of both static long-range magnetic order and spin freezing down to 0.048 K in zero field. Below 0.4 K, the $μ^{+}$ spin relaxation rates, which are proportional to the dynamic correlation function of the Yb$^{3+}$ spins,exhibit temperature-independent plateaus. Same behavior is revealed by temperature-independent local susceptibilities extracted from the Knight shifts of the $μ^{+}$ spin rotation frequencies under a transverse field of 20 Oe. All these $μ$SR results unequivocally support the formation of a gapless U(1) quantum spin liquid ground state in the triangular antiferromagnet YbMgGaO$_4$.

cond-mat.str-el↗

arXiv2015

Rare-earth triangular lattice spin liquid: a single-crystal study of YbMgGaO$_{4}$

YbMgGaO$_{4}$, a structurally perfect two-dimensional triangular lattice with odd number of electrons per unit cell and spin-orbit entangled effective spin-1/2 local moments of Yb$^{3+}$ ions, is likely to experimentally realize the quantum spin liquid ground state. We report the first experimental characterization of single crystal YbMgGaO$_{4}$ samples. Due to the spin-orbit entanglement, the interaction between the neighboring Yb$^{3+}$ moments depends on the bond orientations and is highly anisotropic in the spin space. We carry out the thermodynamic and the electron spin resonance measurements to confirm the anisotropic nature of the spin interaction as well as to quantitatively determine the couplings. Our result is a first step towards the theoretical understanding of the possible quantum spin liquid ground state in this system and sheds new lights on the search of quantum spin liquids in strong spin-orbit coupled insulators.

cond-mat.str-el↗

arXiv2015

Gapless quantum spin liquid ground state in the two-dimensional spin-1/2 triangular antiferromagnet YbMgGaO$_4$

Quantum spin liquid (QSL) is a novel state of matter which refuses the conventional spin freezing even at 0 K. Experimentally searching for the structurally perfect candidates is a big challenge in condensed matter physics. Here we report the successful synthesis of a new spin-1/2 triangular antiferromagnet YbMgGaO$_4$ with R$\bar{3}$m symmetry. The compound with an ideal two-dimensional and spatial isotropic magnetic triangular-lattice has no site-mixing magnetic defects and no antisymmetric Dzyaloshinsky-Moriya (DM) interactions. No spin freezing down to 60 mK (despite $Θ$$_w$ $\sim$ -4 K), the low-T power-law temperature dependence of heat capacity and nonzero susceptibility suggest that YbMgGaO$_4$ is a promising gapless ($\leq$ $|$$Θ$$_w$$|$/100) QSL candidate. The residual spin entropy, which is accurately determined with a non-magnetic reference LuMgGaO$_4$, approaches zero ($<$ 0.6 \%). This indicates that the possible QSL ground state (GS) of the frustrated spin system has been experimentally achieved at the lowest measurement temperatures.

cond-mat.str-el↗

arXiv2015

Ultralow-frequency collective compression mode and strong interlayer coupling in multilayer black phosphorus

The recent renaissance of black phosphorus (BP) as a two-dimensional 2D layered material has generated tremendous interest in its tunable electronic band gap and highly anisotropic transport properties that offer new opportunities for device applications. Many of these outstanding properties are attributed to its unique structural characters that still need elucidation. Here we show Raman measurements that reveal an ultralow-frequency collective compression mode (CCM), which is unprecedented among similar 2D layered materials. This novel CCM indicates an unusually strong interlayer coupling in BP, which is quantitatively supported by a phonon frequency analysis and first-principles calculations. Moreover, the CCM and another branch of low-frequency Raman modes shift sensitively with changing number of layers, allowing an accurate determination of the thickness up to tens of atomic layers, which is considerably higher than those previously achieved by using high-frequency Raman modes. These results offer fundamental insights and practical tools for exploring multilayer BP in new device applications.

cond-mat.mes-hall↗

arXiv2015

Anomalous valley polarization in monolayer MoSe2

Modern electronic devices heavily rely on the accurate control of charge and spin of electrons. The emergence of controllable valley degree of freedom brings new possibilities and presents a promising prospect towards valleytronics. Recently, valley excitation selected by chiral optical pumping has been observed in monolayer MoS2. In this work, we report polarized photoluminescence (PL) measurements for monolayer MoSe2, another member of the family of transition-metal-dichalcogenides (MX2), and observe drastic difference from the outcomes of MoS2. In particular, we identify a valley polarization (VP) up to 70% for B exciton, while that for A exciton is less than 3%. Besides, we also find a small but finite negative VP for A- trion. These results reveal several new intra- and inter-valley scattering processes which significantly affect valley polarization, hence provide new insights into exciton physics in monolayer MX2 and possible valleytronic applications.

cond-mat.mtrl-sci↗

arXiv2014

Electronic Raman Scattering On Individual Single Walled Carbon Nanotubes

We report experimental measurements of electronic Raman scattering under resonant conditions by electrons in individual single-walled carbon nanotubes (SWNTs). The inelastic Raman scattering at low frequency range reveals a single particle excitation feature and the dispersion of electronic structure around the center of Brillouin zone of a semiconducting SWNT (14, 13) is extracted.

cond-mat.mes-hall↗

arXiv2013

Re-entrance of Gapless Quantum Spin Liquids Observed in a Newly Synthesized Spin-1/2 Kagome Antiferromagnet $ZnCu_{3}(OH)_{6}SO_{4}$

Quantum spin liquid (QSL) is a novel state of matter with exotic excitations and was theoretically predicted to be realized most possibly in an S=1/2 kagome antiferromagnet. Experimentally searching for the candidate materials is a big challenge in condensed matter physics and only two such candidates were reported so far. Here we report the successful synthesis of a new spin-1/2 kagome antiferromagnet ZnCu3(OH)6SO4. No magnetic ordering is observed down to 50 mK, despite a moderately high Weiss temperature of θW ~ -79 K. It strongly suggests that the material is a new QSL candidate. Most interestingly, the magnetic specific heat clearly exhibits linear behaviors in two low-temperature regions. Both behaviors exactly correspond to two temperature-independent susceptibilities. These consistently reveal a novel re-entrance phenomenon of gapless QSL state at the lowest temperatures. The findings provide new insights into QSL ground and excited states and will inspire new theoretical and experimental studies.

cond-mat.str-el↗

arXiv2013

Transition-metal distribution in kagome antiferromagnet CoCu3(OH)6Cl2 revealed by resonant x-ray diffraction

The distribution of chemically similar transition-metal ions is a fundamental issue in the study of herbertsmithite-type kagome antiferromagnets. Using synchrotron radiation, we have performed resonant powder x-ray diffractions on newly synthesized CoCu3(OH)6Cl2, which provide an exact distribution of transition-metal ions in the frustrated antiferromagnet. Both magnetic susceptibility and specific heat measurements are quantitatively consistent with the occupation fractions determined by resonant x-ray diffraction. The distribution of transition-metal ions and residual magnetic entropy suggest a novel low temperature (T < 4 K) magnetism, where the interlayer triangular spins undergo a spin-glass freezing while the kagome spins still keep highly frustrated.

cond-mat.str-el↗