arXiv subjects

Jiaqiang Yan

Publications and source records attributed to Jiaqiang Yan.

At least 145 records · Page 8Linked to original sources

arXiv2017

Localized-itinerant dichotomy and unconventional magnetism in SrRu$_2$O$_6$

Electron correlations tend to generate local magnetic moments that usually order if the lattices are not too frustrated. The hexagonal compound SrRu$_2$O$_6$ has a relatively high Neel temperature but small local moments, which seem to be at odds with the nominal valence of Ru$^{5+}$ in the $t_{2g}^3$ configuration. Here, we investigate the electronic property of SrRu$_2$O$_6$ using density functional theory (DFT) combined with dynamical-mean-field theory (DMFT). We find that the strong hybridization between Ru $d$ and O $p$ states results in a Ru valence that is closer to $+4$, leading to the small ordered moment $\sim1.2μ_B$. While this is consistent with a DFT prediction, correlation effects are found to play a significant role. The local moment per Ru site remains finite $\sim2.3μ_B$ in the whole temperature range investigated. Due to the lower symmetry, the $t_{2g}$ manifold is split and the quasiparticle weight is renormalized significantly in the $a_{1g}$ state, while the renormalization in $e_g'$ states is about a factor of 2--3 weaker. Our theoretical Neel temperature $\sim700$~K is in reasonable agreement with experimental observations. SrRu$_2$O$_6$ is a unique system in which localized and itinerant electrons coexist with the proximity to an orbitally-selective Mott transition within the $t_{2g}$ sector.

cond-mat.str-el↗

arXiv2017

Magnetic order and interactions in ferrimagnetic Mn3Si2Te6

The magnetism in Mn$_3$Si$_2$Te$_6$ has been investigated using thermodynamic measurements, first principles calculations, neutron diffraction and diffuse neutron scattering on single crystals. These data confirm that Mn$_3$Si$_2$Te$_6$ is a ferrimagnet below a Curie temperature of $T_C$ approximately 78K. The magnetism is anisotropic, with magnetization and neutron diffraction demonstrating that the moments lie within the basal plane of the trigonal structure. The saturation magnetization of approximately 1.6$μ_B$/Mn at 5K originates from the different multiplicities of the two antiferromagnetically-aligned Mn sites. First principles calculations reveal antiferromagnetic exchange for the three nearest Mn-Mn pairs, which leads to a competition between the ferrimagnetic ground state and three other magnetic configurations. The ferrimagnetic state results from the energy associated with the third-nearest neighbor interaction, and thus long-range interactions are essential for the observed behavior. Diffuse magnetic scattering is observed around the 002 Bragg reflection at 120K, which indicates the presence of strong spin correlations well above $T_C$. These are promoted by the competing ground states that result in a relative suppression of $T_C$, and may be associated with a small ferromagnetic component that produces anisotropic magnetism below $\approx$330K.

cond-mat.str-el↗

arXiv2017

Antiferromagnetism in the van der Waals layered spin-lozenge semiconductor CrTe3

The crystallographic, magnetic, and transport properties of the van der Waals bonded, layered compound CrTe3 have been investigated on single crystal and polycrystalline materials. The crystal structure contains layers made up of lozenge shaped Cr_4 tetramers. Electrical resistivity measurements show the crystals to be semiconducting, with a temperature dependence consistent with a band gap of 0.3eV. The magnetic susceptibility exhibits a broad maximum near 300K characteristic of low dimensional magnetic systems. Weak anomalies are observed in the susceptibility and heat capacity near 55K, and single crystal neutron diffraction reveals the onset of long range antiferromagnetic order at this temperature. Strongly dispersive spin-waves are observed in the ordered state. Significant magneto-elastic coupling is indicated by the anomalous temperature dependence of the lattice parameters and is apparent in structural optimization in van der Waals density functional theory calculations for different magnetic configurations. The cleavability of the compound is apparent from its handling and is confirmed by first principles calculations, which predict a cleavage energy 0.5J/m^2, similar to graphite. Based on these results, CrTe3 is identified as a promising compound for studies of low dimensional magnetism in bulk crystals as well as magnetic order in monolayer materials and van der Waals heterostructures.

cond-mat.str-el↗

arXiv2017

Unusual Exciton-Phonon Interactions at van der Waals Engineered Interfaces

Raman scattering is a ubiquitous phenomenon in light-matter interactions which reveals a material's electronic, structural and thermal properties. Controlling this process would enable new ways of studying and manipulating fundamental material properties. Here, we report a novel Raman scattering process at the interface between different van der Waals (vdW) materials as well as between a monolayer semiconductor and 3D crystalline substrates. We find that interfacing a WSe2 monolayer with materials such as SiO2, sapphire, and hexagonal boron nitride (hBN) enables Raman transitions with phonons which are either traditionally inactive or weak. This Raman scattering can be amplified by nearly two orders of magnitude when a foreign phonon mode is resonantly coupled to the A exciton in WSe2 directly, or via an A'1 optical phonon from WSe2. We further showed that the interfacial Raman scattering is distinct between hBN-encapsulated and hBN-sandwiched WSe2 sample geometries. This cross-platform electron-phonon coupling, as well as the sensitivity of 2D excitons to their phononic environments, will prove important in the understanding and engineering of optoelectronic devices based on vdW heterostructures.

cond-mat.mtrl-sci↗

arXiv2017

Interlayer Exciton Optoelectronics in a 2D Heterostructure p-n Junction

Semiconductor heterostructures are backbones for solid state based optoelectronic devices. Recent advances in assembly techniques for van der Waals heterostructures has enabled the band engineering of semiconductor heterojunctions for atomically thin optoelectronic devices. In two-dimensional heterostructures with type II band alignment, interlayer excitons, where Coulomb-bound electrons and holes are confined to opposite layers, have shown promising properties for novel excitonic devices, including a large binding energy, micron-scale in-plane drift-diffusion, and long population and valley polarization lifetime. Here, we demonstrate interlayer exciton optoelectronics based on electrostatically defined lateral p-n junctions in a MoSe2-WSe2 heterobilayer. Applying a forward bias enables the first observation of electroluminescence from interlayer excitons. At zero bias, the p-n junction functions as a highly sensitive photodetector, where the wavelength-dependent photocurrent measurement allows the direct observation of resonant optical excitation of the interlayer exciton. The resulting photocurrent amplitude from the interlayer exciton is about 200 times smaller compared to the resonant excitation of intralayer exciton. This implies that the interlayer exciton oscillator strength is two orders of magnitude smaller than that of the intralayer exciton due to the spatial separation of electron and hole to opposite layers. These results lay the foundation for exploiting the interlayer exciton in future 2D heterostructure optoelectronic devices.

cond-mat.mes-hall↗

arXiv2016

Directional Interlayer Spin-Valley Transfer in Two-Dimensional Heterostructures

Van der Waals heterostructures formed by two different monolayer semiconductors have emerged as a promising platform for new optoelectronic and spin/valleytronic applications. In addition to its atomically thin nature, a two-dimensional semiconductor heterostructure is distinct from its three-dimensional counterparts due to the unique coupled spin-valley physics of its constituent monolayers. Here, we report the direct observation that an optically generated spin-valley polarization in one monolayer can be transferred between layers of a two-dimensional MoSe2-WSe2 heterostructure. Using nondegenerate optical circular dichroism spectroscopy, we show that charge transfer between two monolayers conserves spin-valley polarization and is only weakly dependent on the twist angle between layers. Our work points to a new spin-valley pumping scheme in nanoscale devices, provides a fundamental understanding of spin-valley transfer across the two-dimensional interface, and shows the potential use of two-dimensional semiconductors as a spin-valley generator in 2D spin/valleytronic devices for storing and processing information.

cond-mat.mes-hall↗

arXiv2016

Many-body effects in nonlinear optical responses of 2D layered semiconductors

We performed ultrafast degenerate pump-probe spectroscopy on monolayer WSe2 near its exciton resonance. The observed differential reflectance signals exhibit signatures of strong many-body interactions including the exciton-exciton interaction and free carrier induced band gap renormalization. The exciton-exciton interaction results in a resonance blue shift which lasts for the exciton lifetime (several ps), while the band gap renormalization manifests as a resonance red shift with several tens ps lifetime. Our model based on the many-body interactions for the nonlinear optical susceptibility fits well the experimental observations. The power dependence of the spectra shows that with the increase of pump power, the exciton population increases linearly and then saturates, while the free carrier density increases superlinearly, implying that exciton Auger recombination could be the origin of these free carriers. Our model demonstrates a simple but efficient method for quantitatively analyzing the spectra, and indicates the important role of Coulomb interactions in nonlinear optical responses of such 2D materials.

cond-mat.mes-hall↗

arXiv2016

Neutron tomography of magnetic Majorana fermions in a proximate quantum spin liquid

Quantum matter provides an effective vacuum out of which arise emergent particles not corresponding to any experimentally detected elementary particle. Topological quantum materials in particular have become a focus of intense research in part because of the remarkable possibility to realize Majorana fermions, with their potential for new, decoherence-free quantum computing architectures. In this paper we undertake a study on high-quality single crystal of $α-RuCl_3$ which has been identified as a material realizing a proximate Kitaev state, a topological quantum state with magnetic Majorana fermions. Four-dimensional tomographic reconstruction of dynamical correlations measured using neutrons is uniquely powerful for probing such magnetic states. We discover unusual signals, including an unprecedented column of scattering over a large energy interval around the Brillouin zone center which is remarkably stable with temperature. This is straightforwardly accounted for in terms of the Majorana excitations present in Kitaev's topological quantum spin liquid. Other, more delicate, features in the scattering can be transparently associated with perturbations to an ideal model. This opens a window on emergent magnetic Majorana fermions in correlated materials.

cond-mat.mtrl-sci↗

arXiv2016

Spectroscopic evidence for type II Weyl semimetal state in MoTe2

In a type I Dirac or Weyl semimetal, the low energy states are squeezed to a single point in momentum space when the chemical potential Ef is tuned precisely to the Dirac/Weyl point. Recently, a type II Weyl semimetal was predicted to exist, where the Weyl states connect hole and electron bands, separated by an indirect gap. This leads to unusual energy states, where hole and electron pockets touch at the Weyl point. Here we present the discovery of a type II topological Weyl semimetal (TWS) state in pure MoTe2, where two sets of WPs (W2+-, W3+-) exist at the touching points of electron and hole pockets and are located at different binding energies above Ef. Using ARPES, modeling, DFT and calculations of Berry curvature, we identify the Weyl points and demonstrate that they are connected by different sets of Fermi arcs for each of the two surface terminations. We also find new surface "track states" that form closed loops and are unique to type II Weyl semimetals. This material provides an exciting, new platform to study the properties of Weyl fermions.

cond-mat.mes-hall↗

arXiv2016

Interference Evidence for Rashba-Type Spin-Split on Semimetallic WTe2 Surface

Semimetallic tungsten ditelluride (WTe2) displays an extremely large non-saturating magnetoresistance (XMR), which is the subject of intense interest. This phenomenon is thought to arise from the combination of perfect n-p charge compensation with low carrier densities in WTe2 and presumably details of its band structure. Recently, "spin texture" induced by strong spin-orbital coupling (SOC) has been observed in WTe2 by angle-resolved photoemission spectroscopy (ARPES). This provides a mechanism for protecting backscattering for the states involved and thus was proposed to play an important role in the XMR of WTe2. Here, based on our density functional calculations for bulk WTe2, we found a strong Rashba spin-orbit effect in the calculated band structure due to its non-centrosymmetric structure. This splits bands and two-fold spin degeneracy of bands is lifted. A prominent Umklapp interference pattern (a spectroscopic feature with involving reciprocal lattice vectors) can be observed by scanning tunneling microscopic (STM) measurements on WTe2 surface at 4.2 K. This differs distinctly from the surface atomic structure demonstrated at 77 K. The energy dependence of Umklapp interference shows a strong correspondence with densities of states integrated from ARPES measurement, manifesting a fact that the bands are spin-split on the opposites side of Gamma point. Spectroscopic survey reveals the ratio of electron/hole asymmetry changes alternately with lateral locations along b axis, providing a microscopic picture for double-carrier transport of semimetallic WTe2. The calculated band structure and Fermi surface is further supported by our ARPES results and Shubnikov-de Haas (SdH) oscillations measurements.

cond-mat.mtrl-sci↗

arXiv2016

Low-Resistance 2D/2D Ohmic Contacts: A Universal Approach to High-Performance WSe2, MoS2, and MoSe2 Transistors

We report a new strategy for fabricating 2D/2D low-resistance ohmic contacts for a variety of transition metal dichalcogenides (TMDs) using van der Waals assembly of substitutionally doped TMDs as drain/source contacts and TMDs with no intentional doping as channel materials. We demonstrate that few-layer WSe2 field-effect transistors (FETs) with 2D/2D contacts exhibit low contact resistances of ~ 0.3 k ohm.um, high on/off ratios up to > 109, and high drive currents exceeding 320 uA um-1. These favorable characteristics are combined with a two-terminal field-effect hole mobility ~ 2x102 cm2 V-1 s-1 at room temperature, which increases to >2x103 cm2 V-1 s-1 at cryogenic temperatures. We observe a similar performance also in MoS2 and MoSe2 FETs with 2D/2D drain and source contacts. The 2D/2D low-resistance ohmic contacts presented here represent a new device paradigm that overcomes a significant bottleneck in the performance of TMDs and a wide variety of other 2D materials as the channel materials in post-silicon electronics.

cond-mat.mes-hall↗

arXiv2016

Expansion of a Valley-Polarized Exciton Cloud in a 2D Heterostructure

Heterostructures comprising different monolayer semiconductors provide a new system for fundamental science and device technologies, such as in the emerging field of valleytronics. Here, we realize valley-specific interlayer excitons in monolayer WSe2-MoSe2 vertical heterostructures. We create interlayer exciton spin-valley polarization by circularly polarized optical pumping and determine a valley lifetime of 40 nanoseconds. This long-lived polarization enables the visualization of the expansion of a valley-polarized exciton cloud over several microns. The spatial pattern of the polarization evolves into a ring with increasing exciton density, a manifestation of valley exciton exchange interactions. Our work lays a foundation for quantum optoelectronics and valleytronics based on interlayer excitons in van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2015

The Chiral Anomaly and Ultrahigh Mobility in Crystalline HfTe5

HfTe5 is predicted to be a promising platform for studying topological phases. Here through an electrical transport study, we present the first observation of chiral anomaly and ultrahigh mobility in HfTe5 crystals. Negative magneto-resistivity in HfTe5 is observed when the external magnetic and electrical fields are parallel (B//E) and quickly disappears once B deviates from the direction of E. Quantitative fitting further confirms the chiral anomaly as the underlying physics. Moreover, by analyzing the conductivity tensors of longitudinal and Hall traces, ultrahigh mobility and ultralow carrier density are revealed in HfTe5, which paves the way for potential electronic applications.

cond-mat.mtrl-sci↗

arXiv2015

Trion Formation Dynamics in Monolayer Transition Metal Dichalcogenides

We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides (TMDs), specifically molybdenum diselenide (MoSe2), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond timescale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by ~ 50%. This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in TMDs. The quasiparticle formation and conversion processes are important for interpreting photoluminescence and photoconductivity in TMDs.

cond-mat.mes-hall↗

arXiv2015

Electron-phonon coupling and thermal transport in the thermoelectric compound $\mathrm{Mo_3Sb_{7-x}Te_x}$

Phonon properties of $\mathrm{Mo_3Sb_{7-x}Te_x}$ ($x=0,1.5, 1.7$), a potential high-temperature thermoelectric material, have been studied with inelastic neutron and x-ray scattering, and with first-principles simulations. The substitution of Te for Sb leads to pronounced changes in the electronic structure, local bonding, phonon density of states (DOS), dispersions, and phonon lifetimes. Alloying with tellurium shifts the Fermi level upward, near the top of the valence band, resulting in a strong suppression of electron-phonon screening, and a large overall stiffening of interatomic force-constants. The suppression in electron-phonon coupling concomitantly increases group velocities and suppresses phonon scattering rates, surpassing the effects of alloy-disorder scattering, and resulting in a surprising increased lattice thermal conductivity in the alloy. We also identify that the local bonding environment changes non-uniformly around different atoms, leading to variable perturbation strengths for different optical phonon branches. The respective roles of changes in phonon group velocities and phonon lifetimes on the lattice thermal conductivity are quantified. Our results highlight the importance of the electron-phonon coupling on phonon mean-free-paths in this compound, and also estimates the contributions from boundary scattering, umklapp scattering, and point-defect scattering.

cond-mat.mtrl-sci↗

arXiv2015

Effects of chemical pressure on the magnetic ground states of the osmate double perovskites SrCaCoOsO6 and Ca2CoOsO6

The magnetic ground state in the double perovskite system Sr2-xCaxCoOsO6 changes from an antiferromagnet (x = 0), to a spin glass (x = 1), to a ferrimagnet (x = 2) as the Ca content increases. This crossover is driven by chemical pressure effects that control the relative strength of magnetic exchange interactions. The synthesis, crystal structure, and magnetism of SrCaCoOsO6 and Ca2CoOsO6 are investigated and compared with Sr2CoOsO6. Both compounds adopt a monoclinic crystal structure with rock salt ordering of Co2+ and Os6+ and a-a-b+ octahedral tilting, but the average Co-O-Os bond angle evolves from 158.0(3) degrees in SrCaCoOsO6 to 150.54(9) degrees in Ca2CoOsO6 as the smaller Ca2+ ion replaces Sr2+. While this change may seem minor it has a profound effect on the magnetism, changing the magnetic ground state from antiferromagnetic in Sr2CoOsO6 (TN1 = 108 K, TN2 = 70 K), to a spin glass in SrCaCoOsO6 (Tf1 = 32 K, Tf2 = 13 K), to ferrimagnetic in Ca2CoOsO6 (TC = 145 K). In the first two compounds the observation of two transitions is consistent with weak coupling between the Co and Os sublattices.

cond-mat.str-el↗

arXiv2015

Effect of Chemical Pressure on High Temperature Ferrimagnetic Double Perovskites Sr2CrOsO6 and Ca2CrOsO6

The ordered double perovskites Sr2CrOsO6 and Ca2CrOsO6 have been synthesized and characterized with neutron powder diffraction, electrical transport measurements, and high field magnetization experiments. Sr2CrOsO6 and Ca2CrOsO6 crystallize with R-3 and P21/n space group symmetry, respectively. Both materials are found to be ferrimagnetic insulators with saturation magnetizations near 0.2 μB. Sr2CrOsO6 orders at 660 K, showing non-monotonic magnetization temperature dependence, while Ca2CrOsO6 orders at 490 K and does not show non-monotonic behavior. Evidence for a theoretically predicted canted magnetic structure in Sr2CrOsO6 is sought and not found.

cond-mat.str-el↗

arXiv2015

Structural and magnetic phase transitions in EuTi1-xNbxO3

We investigate the structural and magnetic phase transitions in EuTi1-xNbxO3 with synchrotron powder X-ray diffraction (XRD), resonant ultrasound spectroscopy (RUS), and magnetization measurements. Upon Nb-doping, the Pm-3m to I4/mcm structural transition shifts to higher temperatures and the room temperature lattice parameter increases while the magnitude of the octahedral tilting decreases. In addition, Nb substitution for Ti destabilizes the antiferromagnetic ground state of the parent compound and long range ferromagnetic order is observed in the samples containing more than 10% Nb. The structural transition in pure and doped compounds is marked by a step-like softening of the elastic moduli in a narrow temperature interval near TS, which resembles that of SrTiO3 and can be adequately modeled using the Landau free energy model employing the same coupling between strain and octahedral tilting order parameter as previously used to model SrTiO3.

cond-mat.mtrl-sci↗