arXiv subjects

H. Ding

Publications and source records attributed to H. Ding.

At least 37 records · Page 2Linked to original sources

arXiv2019

Demonstration of a compact plasma accelerator powered by laser-accelerated electron beams

Plasma wakefield accelerators are capable of sustaining gigavolt-per-centimeter accelerating fields, surpassing the electric breakdown threshold in state-of-the-art accelerator modules by 3-4 orders of magnitude. Beam-driven wakefields offer particularly attractive conditions for the generation and acceleration of high-quality beams. However, this scheme relies on kilometer-scale accelerators. Here, we report on the demonstration of a millimeter-scale plasma accelerator powered by laser-accelerated electron beams. We showcase the acceleration of electron beams to 130 MeV, consistent with simulations exhibiting accelerating gradients exceeding 100 GV/m. This miniaturized accelerator is further explored by employing a controlled pair of drive and witness electron bunches, where a fraction of the driver energy is transferred to the accelerated witness through the plasma. Such a hybrid approach allows fundamental studies of beam-driven plasma accelerator concepts at widely accessible high-power laser facilities. It is anticipated to provide compact sources of energetic high-brightness electron beams for quality-demanding applications such as free-electron lasers.

physics.acc-ph↗

arXiv2019

Probing Ultrafast Magnetic-Field Generation by Current Filamentation Instability in Femtosecond Relativistic Laser-Matter Interactions

We present experimental measurements of the femtosecond time-scale generation of strong magnetic-field fluctuations during the interaction of ultrashort, moderately relativistic laser pulses with solid targets. These fields were probed using low-emittance, highly relativistic electron bunches from a laser wakefield accelerator, and a line-integrated $B$-field of $2.70 \pm 0.39\,\rm kT\,μm$ was measured. Three-dimensional, fully relativistic particle-in-cell simulations indicate that such fluctuations originate from a Weibel-type current filamentation instability developing at submicron scales around the irradiated target surface, and that they grow to amplitudes strong enough to broaden the angular distribution of the probe electron bunch a few tens of femtoseconds after the laser pulse maximum. Our results highlight the potential of wakefield-accelerated electron beams for ultrafast probing of relativistic laser-driven phenomena.

physics.plasm-ph↗

arXiv2019

Spin fluctuation induced Weyl semimetal state in the paramagnetic phase of EuCd$_2$As$_2$

Weyl fermions as emergent quasiparticles can arise in Weyl semimetals (WSMs) in which the energy bands are nondegenerate, resulting from inversion or time-reversal symmetry breaking. Nevertheless, experimental evidence for magnetically induced WSMs is scarce. Here, using photoemission spectroscopy, we observe that the degeneracy of Bloch bands is already lifted in the paramagnetic phase of EuCd$_2$As$_2$. We attribute this effect to the itinerant electrons experiencing quasistatic and quasi-long-range ferromagnetic fluctuations. Moreover, the spin nondegenerate band structure harbors a pair of ideal Weyl nodes near the Fermi level. Hence, we show that long-range magnetic order and the spontaneous breaking of time-reversal symmetry are not an essential requirement for WSM states in centrosymmetric systems, and that WSM states can emerge in a wider range of condensed-matter systems than previously thought.

cond-mat.str-el↗

arXiv2019

Doping evolution of the charge excitations and electron correlations in electron-doped superconducting La$_{2-x}$Ce$_{x}$CuO$_{4}$

Electron correlations play a dominant role in the charge dynamics of the cuprates. We use resonant inelastic x-ray scattering (RIXS) to track the doping dependence of the collective charge excitations in electron doped La$_{2-x}$Ce$_{x}$CuO$_{4}$(LCCO). From the resonant energy dependence and the out-of-plane momentum dependence, the charge excitations are identified as three-dimensional (3D) plasmons, which reflect the nature of the electronic structure and Coulomb repulsion on both short and long length scales. With increasing electron doping, the plasmon excitations show monotonic hardening in energy, a consequence of the electron correlation effect on electron structure near the Fermi surface (FS). Importantly, the plasmon excitations evolve from a broad feature into a well defined peak with much increased life time, revealing the evolution of the electrons from incoherent states to coherent quasi-particles near the FS. Such evolution marks the reduction of the short-range electronic correlation, and thus the softening of the Mottness of the system with increasing electron doping.

cond-mat.supr-con↗

arXiv2019

Observation of multiple types of topological fermions in PdBiSe

Topological semimetals with different types of band crossings provide a rich platform to realize novel fermionic excitations, known as topological fermions. In particular, some fermionic excitations can be direct analogues of elementary particles in quantum field theory when both obey the same laws of physics in the low-energy limit. Examples include Dirac and Weyl fermions, whose solid-state realizations have provided new insights into long-sought phenomena in high-energy physics. Recently, theorists predicted new types of fermionic excitations in condensed-matter systems without any high-energy counterpart, and their existence is protected by crystalline symmetries. By studying the topology of the electronic structure in PdBiSe using density functional theory calculations and bulk-sensitive soft X-ray angle-resolved photoemission spectroscopy, we demonstrate a coexistence of four different types of topological fermions: Weyl, Rarita-Schwinger-Weyl, double class-II three-component, and charge-2 fourfold fermions. Our discovery provides a remarkable platform to realize multiple novel fermions in a single solid, charting the way forward to studies of their potentially exotic properties as well as their interplay.

cond-mat.str-el↗

arXiv2019

Moderate hybridization effects revealed by ARPES in heavy-fermion Ce2IrIn8

We utilized high-resolution resonant angle-resolved photoemission spectroscopy (ARPES) to study the band structure and hybridization effect of the heavy-fermion compound Ce2IrIn8. We observe a nearly flat band at the binding energy of 7 meV below the coherent temperature Tcoh ~ 40 K, which characterizes the electrical resistance maximum indicating the onset temperature of hybridization. However, the Fermi vector kF and the Fermi surface (FS) volume have little change around Tcoh, challenging the widely believed evolution from a high-temperature small FS to a low-temperature large FS. Our experimental results of the band structure fit well with the density functional theory plus dynamic mean-field theory (DFT+DMFT) calculations.

cond-mat.str-el↗

arXiv2019

Experimental evidence of anomalously large superconducting gap on topological surface state of $β$-$\rm\mathbf{Bi_2Pd}$ film

Connate topological superconductor (TSC) combines topological surface states with nodeless superconductivity in a single material, achieving effective $p$-wave pairing without interface complication. By combining angle-resolved photoemission spectroscopy and $in$-$situ$ molecular beam epitaxy, we studied the momentum-resolved superconductivity in $β$-$\rm{Bi_2Pd}$ film. We found that the superconducting gap of topological surface state ($Δ_{TSS} \sim$ 3.8 meV) is anomalously enhanced from its bulk value ($Δ_b \sim$ 0.8 meV). The ratio of $2Δ_{TSS}/k_BT_c \sim14.2$, is substantially larger than the BCS value. By measuring $β$-$\rm{Bi_2Pd}$ bulk single crystal as a comparison, we clearly observed the upward-shift of chemical potential in the film. In addition, a concomitant increasing of surface weight on the topological surface state was revealed by our first principle calculation, suggesting that the Dirac-fermion-mediated parity mixing may cause this anomalous superconducting enhancement. Our results establish $β$-$\rm{Bi_2Pd}$ film as a unique case of connate TSCs with a highly enhanced topological superconducting gap, which may stabilize Majorana zero modes at a higher temperature.

cond-mat.supr-con↗

arXiv2019

Observation of topological transition in high-T$_{c}$ superconductor FeTe$_{1-x}$Se$_{x}$/SrTiO$_{3}$(001) monolayers

Superconductors with topological surface or edge states have been intensively explored for the prospect of realizing Majorana bound states, which obey non-Abelian statistics and are crucial for topological quantum computation. The traditional routes for making topological insulator/superconductor and semiconductor/superconductor heterostructures suffer fabrication difficulties and can only work at low temperature. Here, we use angle-resolved photoemission spectroscopy to directly observe the evolution of a topological transition of band structure nearby the Fermi level in two-dimensional high-T$_{c}$ superconductor FeTe$_{1-x}$Se$_{x}$/SrTiO$_{3}$(001) monolayers, fully consistent with our theoretical calculations. Furthermore, evidence of edge states is revealed by scanning tunneling spectroscopy with assistance of theoretical calculations. Our study provides a simple and tunable platform for realizing and manipulating Majorana states at high temperature.

cond-mat.supr-con↗

arXiv2019

New classes of chiral topological nodes with non-contractible surface Fermi arcs in CoSi

In condensed matter systems, chiral topological nodes are robust band crossing points in momentum space that carry nonzero Chern numbers. The chirality is manifested by the presence of surface Fermi arcs connecting the projections of nodes with opposite Chern numbers. In addition to the well-known Weyl nodes, theorists have proposed several other types of chiral topological nodes in condensed matter systems, but the direct experimental evidence of their existence is still lacking. Here, using angle-resolved photoemission spectroscopy, we reveal two types of new chiral nodes, namely the spin-1 nodes and charge-2 Dirac nodes, at the band crossing points near the Fermi level in CoSi, the projections of which on the (001) surface are connected by topologically protected surface Fermi arcs. As these chiral nodes in CoSi are enforced at the Brillouin zone (BZ) center and corner by the crystalline symmetries, the surface Fermi arcs connecting their projections form a non-contractible path traversing the entire (001) surface BZ, in sharp contrast to pairs of Weyl nodes with small separation. Our work marks the first experimental observation of chiral topological nodes beyond the Weyl nodes both in the bulk and on the surface in condensed matter systems.

cond-mat.mtrl-sci↗

arXiv2018

Direct observation of plasma waves and dynamics induced by laser-accelerated electron beams

Plasma wakefield acceleration (PWFA) is a novel acceleration technique with promising prospects for both particle colliders and light sources. However, PWFA research has so far been limited to a few large-scale accelerator facilities world-wide. Here, we present first results on plasma wakefield generation using electron beams accelerated with a 100-TW-class Ti:Sa laser. Due to their ultrashort duration and high charge density, the laser-accelerated electron bunches are suitable to drive plasma waves at electron densities in the order of $10^{19}$ cm$^{-3}$. We capture the beam-induced plasma dynamics with femtosecond resolution using few-cycle optical probing and, in addition to the plasma wave itself, we observe a distinctive transverse ion motion in its trail. This previously unobserved phenomenon can be explained by the ponderomotive force of the plasma wave acting on the ions, resulting in a modulation of the plasma density over many picoseconds. Due to the scaling laws of plasma wakefield generation, results obtained at high plasma density using high-current laser-accelerated electron beams can be readily scaled to low-density systems. Laser-driven PWFA experiments can thus act as miniature models for their larger, conventional counterparts. Furthermore, our results pave the way towards a novel generation of laser-driven PWFA, which can potentially provide ultra-low emittance beams within a compact setup.

physics.acc-ph↗

arXiv2018

Van Hove singularities, chemical pressure and phonons: an angle-resolved photoemission study of KFe$_2$As$_2$ and CsFe$_2$As$_2$

We report an angle-resolved photoemission spectroscopy (ARPES) study of KFe$_2$As$_2$ and CsFe$_2$As$_2$, revealing the existence of a van Hove singularity affecting the electronic properties. As a result of chemical pressure, we find a stronger three-dimensionality in KFe$_2$As$_2$ than in CsFe$_2$As$_2$, notably for the 3$d_{z^2}$ states responsible for the small three-dimensional hole-like Fermi surface pocket reported by quantum oscillations. Supported by first-principles calculations, our ARPES study shows that the van Hove singularity previously reported in KFe$_2$As$_2$ moves closer to the Fermi level under negative chemical pressure. This observation, which suggests that the large density-of-states accompanying the van Hove singularity contributes to the large Sommerfeld coefficient reported for the AFe$_2$As$_2$ (A = K, Rb, Cs) series, is also consistent with the evolution of the inelastic scattering revealed by transport under external pressure, thus offering a possible interpretation for the origin of the apparent change in the superconducting order parameter under pressure. We find that the coherent spectral weight decreases exponentially upon increasing temperature with a characteristic temperature $T^*$. We speculate how the low-energy location of the van Hove singularity and the presence of a low-energy peak in the phonon density-of-states can relate to the high-temperature crossover observed in various electronic and thermodynamic quantities.

cond-mat.supr-con↗

arXiv2018

Evidence of Coulomb interaction induced Lifshitz transition and robust hybrid Weyl semimetal in Td MoTe2

Using soft x-ray angle-resolved photoemission spectroscopy we probed the bulk electronic structure of Td MoTe2. We found that on-site Coulomb interaction leads to a Lifshitz transition, which is essential for a precise description of the electronic structure. A hybrid Weyl semimetal state with a pair of energy bands touching at both type-I and type-II Weyl nodes is indicated by comparing the experimental data with theoretical calculations. Unveiling the importance of Coulomb interaction opens up a new route to comprehend the unique properties of MoTe2, and is significant for understanding the interplay between correlation effects, strong spin-orbit coupling and superconductivity in this van der Waals material.

cond-mat.str-el↗

arXiv2018

Continuous doping of a cuprate surface: new insights from in-situ ARPES

The cuprate superconductors distinguish themselves from the conventional superconductors in that a small variation in the carrier doping can significantly change the superconducting transition temperature (T_c), giving rise to a superconducting dome where a pseudogap (ref. 1,2) emerges in the underdoped region and a Fermi liquid appears in the overdoped region. Thus a systematic study of the properties over a wide doping range is critical for understanding the superconducting mechanism. Here, we report a new technique to continuously dope the surface of Bi2Sr2CaCu2O8+x through an ozone/vacuum annealing method. Using in-situ ARPES, we obtain precise quantities of energy gaps and the coherent spectral weight over a wide range of doping. We discover that the d-wave component of the quasiparticle gap is linearly proportional to the Nernst temperature that is the onset of superconducting vortices (ref. 3), strongly suggesting that the emergence of superconducting pairing is concomitant with the onset of free vortices, with direct implications for the onset of superconducting phase coherence at T_c and the nature of the pseudogap phenomena.

cond-mat.supr-con↗

arXiv2018

Observation of a nodal chain with Dirac surface states in TiB2

Topological nodal-line semimetals (TNLSMs) are characterized by symmetry-protected band crossings extending along one-dimensional lines in momentum space. The nodal lines exhibit a variety of possible configurations, such as nodal ring, nodal link, nodal chain, and nodal knot. Here, using angle-resolved photoemission spectroscopy, we observe nodal rings in the orthogonal kz = 0 and kx = 0 planes of the Brillouin zone in TiB2. The nodal rings connect with each other on the intersecting line Γ-K of the orthogonal planes, forming a remarkable nodal-chain structure. Furthermore, we observe surface states (SSs) on the (001) cleaved surface, which are consistent with the calculated SSs considering the contribution from both Ti and B terminations. The calculated SSs have novel Dirac-cone-like band structures, which are distinct from the usual drumhead SSs with a single flat band proposed in other TNLSMs.

cond-mat.mtrl-sci↗

arXiv2018

Trivial topological phase of CaAgP and the topological nodal-line transition in CaAg(P1-xAsx)

By performing angle-resolved photoemission spectroscopy and first-principles calculations, we address the topological phase of CaAgP and investigate the topological phase transition in CaAg(P1-xAsx). We reveal that in CaAgP, the bulk band gap and surface states with a large bandwidth are topologically trivial, in agreement with hybrid density functional theory calculations. The calculations also indicate that application of "negative" hydrostatic pressure can transform trivial semiconducting CaAgP into an ideal topological nodal-line semimetal phase. The topological transition can be realized by partial isovalent P/As substitution at x = 0.38.

cond-mat.mtrl-sci↗

arXiv2018

Decoupled Pairing Amplitude and Electronic Coherence in Iron-Based Superconductors

Here we use angle-resolved photoemission spectroscopy to study superconductivity that emerges in two extreme cases, from a Fermi liquid phase (LiFeAs) and an incoherent bad-metal phase (FeTe0.55Se0.45). We find that although the electronic coherence can strongly reshape the single-particle spectral function in the superconducting state, it is decoupled from the maximum superconducting pairing amplitude, which shows a universal scaling that is valid for all FeSCs. Our observation excludes pairing scenarios in the BCS and the BEC limit for FeSCs and calls for a universal strong coupling pairing mechanism for the FeSCs.

cond-mat.supr-con↗

arXiv2018

Quantitative characterization of the nanoscale local lattice strain induced by Sr dopants in La$_{1.92}$Sr$_{0.08}$CuO$_4$

The nanometer scale lattice deformation brought about by the dopants in high temperature superconducting cuprate La$_{2-x}$Sr$_x$CuO$_4$(x=0.08) was investigated by measuring the associated X-ray diffuse scattering around multiple Bragg peaks. A characteristic diffuse scattering pattern was observed, which can be well described by continuum elastic theory. With the fitted dipole force parameters, the acoustic type lattice deformation pattern was re-constructed and found to be of similar size to lattice thermal vibration at 7 K. Our results address the long-term concern of dopant introduced local lattice inhomogeneity, and show that the associated nanometer scale lattice deformation is marginal and cannot, alone, be responsible for the patched variation in the spectral gaps observed with scanning tunneling microscopy in the cuprates.

cond-mat.str-el↗

arXiv2018

Research towards high-repetition rate laser-driven X-ray sources for imaging applications

Laser wakefield acceleration of electrons represents a basis for several types of novel X-ray sources based on Thomson scattering or betatron radiation. The latter provides a high photon flux and a small source size, both being prerequisites for high-quality X-ray imaging. Furthermore, proof-of-principle experiments have demonstrated its application for tomographic imaging. So far this required several hours of acquisition time for a complete tomographic data set. Based on improvements to the laser system, detectors and reconstruction algorithms, we were able to reduce this time for a full tomographic scan to 3 minutes. In this paper, we discuss these results and give a prospect to future imaging systems.

physics.med-ph↗