Qiang Zhang’s research while affiliated with University of Science and Technology of China and other places

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Publications (14)


Dirac fermions in antiferromagnetic FeSn kagome lattices with combined space inversion and time-reversal symmetry
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October 2020

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107 Reads

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106 Citations

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Zhenyu Zhang

Symmetry principles play a critical role in formulating the fundamental laws of nature, with a large number of symmetry-protected topological states identified in recent studies of quantum materials. As compelling examples, massless Dirac fermions are jointly protected by the space inversion symmetry P and time-reversal symmetry T supplemented by additional crystalline symmetry, while evolving into Weyl fermions when either P or T is broken. Here, based on first-principles calculations, we reveal that massless Dirac fermions are present in a layered FeSn crystal containing antiferromagnetically coupled ferromagnetic Fe kagome layers, where each of the P and T symmetries is individually broken but the combined PT symmetry is preserved. These stable Dirac fermions, protected by the combined PT symmetry with additional nonsymmorphic S2z symmetry, can be transformed to either massless/massive Weyl or massive Dirac fermions by breaking the PT or S2z symmetry. Our angle-resolved photoemission spectroscopy experiments indeed observed the Dirac states in the bulk and two-dimensional Weyl-like states at the surface. The present paper substantially enriches our fundamental understanding of the intricate connections between symmetries and topologies of matter, especially with the spin degree of freedom playing a vital role.


Fig. 1. Structure characterizations. (a) Crystallographic structure of WP2 with the (021) surface. The blue and purple balls represent W and P atoms, respectively. The black lines indicate a unit cell. (b) TEM and SAED of a WP2 single crystal. The scale bar is 2 nm. (c) Single-crystal XRD pattern of a WP2 single crystal. The insets show the FWHM of the (021) peak in the rocking curve (~ 0.05˚) and the optical image of a shiny as-grown crystal. The scale bar is 3 mm. (d) Powder XRD pattern of grinded WP2 crystals. The measured peaks (blue) perfectly coincide with the calculated results (red). (e) STM image of the cleaved (021) surface with well-resolved terraces. The inset shows the height profile along the green dashed line. The scale bar is 10 nm. (f) STM image of the cleaved (021) surface with atomic resolution. The white solid rectangle denotes a surface unit cell. The scale bar is 1 nm.
Fig. 2. Butterfly-like AMR. (a) Schematic of the measurement configuration. The current I is injected along the [100] direction. The magnetic field B is rotated in the plane consisting of the [100] direction and the normal direction of the (021) surface. The angle between the magnetic field and the [100] direction is denoted as θ. (b) Resistivity ρ as a function of  at 2 K under various magnetic fields. (c) Polar plot of ρ as a function of , illustrating the butterfly-like AMR with four lobes. (d) Calculated Fermi surfaces of WP2. The bow-tie-like closed pockets are electron Fermi surfaces, while the spaghetti-like open pockets are hole Fermi surfaces. (e) Projected electron Fermi surfaces in the rotation plane of B.
Fig. 3. Temperature-dependent evolution of the AMR. (a) ρ as a function of θ with B = 12 T at various temperatures. (b) The angle of the resistivity peak (θ_max) and the AMR ratio ([ρ(θ_max )-ρ(90°)])⁄(ρ(θ_max)) as functions of T.
Fig. 4. Quantum oscillations. (a) Resistivity ρ as a function of B at 2 K for various s. (b) The oscillatory component (∆ρ) as a function of 1/B after subtraction of a smooth MR background from ρ. (c) FFT spectra of ∆ρ at various s. The dashed curves are the calculated angular dependences of the SdH frequencies associated with the individual extremal cross-sectional orbits of the hole Fermi surface pockets (α_1, α_2, β_1, and β_2). (d) Magnitude of the total phase |  | as a function of  for the α_1 branch of the hole Fermi surfaces. The red dashed curve is guide to the eyes for the evolution trend of the total phase. (e) Calculated bulk band structures of WP2 along the Γ-Y-X1-A1 direction. The pink circles mark the band crossing points of the two valence subbands. (f) Theoretically calculated magnitude of the Berry curvature |Ω| of the valence bands along the Γ-Y-X1-A1 direction. |Ω| peaks at the band crossing points. (g) Schematic of the angle-dependent Berry phase in the presence of a monopole when rotating the magnetic field.
Fig. S1. Transport properties of WP2. (a) Resistivity ρ as a function of temperature T at different magnetic fields. (b) ρ as a function of magnetic field B at various temperatures. The magnetic field in (a) and (b) is perpendicular to the (021) plane.

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Butterfly-Like Anisotropic Magnetoresistance and Angle-Dependent Berry Phase in a Type-II Weyl Semimetal WP 2
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  • Full-text available

September 2020

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181 Reads

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11 Citations

The Weyl semimetal has emerged as a new topologically nontrivial phase of matter, hosting low-energy excitations of massless Weyl fermions. Here, we present a comprehensive study of a type-II Weyl semimetal WP 2 . Transport studies show a butterfly-like magnetoresistance at low temperature, reflecting the anisotropy of the electron Fermi surfaces. This four-lobed feature gradually evolves into a two-lobed variant with an increase in temperature, mainly due to the reduced relative contribution of electron Fermi surfaces compared to hole Fermi surfaces for magnetoresistance. Moreover, an angle-dependent Berry phase is also discovered, based on quantum oscillations, which is ascribed to the effective manipulation of extremal Fermi orbits by the magnetic field to feel nearby topological singularities in the momentum space. The revealed topological character and anisotropic Fermi surfaces of the WP 2 substantially enrich the physical properties of Weyl semimetals, and show great promises in terms of potential topological electronic and Fermitronic device applications.

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Fig. 3. Temperature-dependent evolution of the AMR. (a) ρ as a function of θ with B = 12 T at various temperatures. (b) The angle of the resistivity peak (í µí¼ƒ max ) and the AMR ratio [í µí¼Œ(í µí¼ƒ max ) − í µí¼Œ(90°)] í µí¼Œ(í µí¼ƒ max ) ⁄ as functions of T.
Fig. 4. Quantum oscillations. (a) Resistivity ρ as a function of B at 2 K for various s. (b) The oscillatory component (∆ρ) as a function of 1/B after subtraction of a smooth MR background from ρ. (c) FFT spectra of ∆ρ at various s. The dashed curves are the calculated angular dependences of the SdH frequencies associated with the individual extremal cross-sectional orbits of the hole Fermi surface pockets ( í µí»¼ 1 , í µí»¼ 2 , í µí»½ 1 , and í µí»½ 2 ). (d)
Fig. S3. SdH oscillation component of the í µí»¼ 1 branch of the hole Fermi surfaces at various θs. The curves are shifted vertically for clarity. The black curves are the fitting curves based on the LifshitzKosevich model.
Butterfly-like anisotropic magnetoresistance and angle-dependent Berry phase in Type-II Weyl semimetal WP2

August 2020

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178 Reads

Weyl semimetal emerges as a new topologically nontrivial phase of matter, hosting low-energy excitations of massless Weyl fermions. Here, we present a comprehensive study of the type-II Weyl semimetal WP2. Transport studies show a butterfly-like magnetoresistance at low temperature, reflecting the anisotropy of the electron Fermi surfaces. The four-lobed feature gradually evolves into a two-lobed one upon increasing temperature, mainly due to the reduced relative contribution of electron Fermi surfaces compared to hole Fermi surfaces for the magnetoresistance. Moreover, angle-dependent Berry phase is further discovered from the quantum oscillations, which is ascribed to the effective manipulation of the extremal Fermi orbits by the magnetic field to feel the nearby topological singularities in the momentum space. The revealed topological characters and anisotropic Fermi surfaces of WP2 substantially enrich the physical properties of Weyl semimetals and hold great promises in topological electronic and Fermitronic device applications.


FIG. 4: Electronic band structure of the FeSn (001) surface. (a),(b) ARPES data (a) with a 35 eV-photon energy and their second derivatives (b) along the K-Γ-K' line. In (b), the projected DFT band structures of the Sn-terminated (blue color circles) and Fe 3 Sn-terminated (purple color circles) surfaces are overlapped. Here, the radii of circles are proportional to the local density of states projected onto the topmost Sn and Fe 3 Sn surface layers. (c) Constant energy surface with respect to chemical potential, obtained by ARPES with a 35 eVphoton energy. The ARPES data are symmetrized with respect to k x = 0. (d) Projected DFT constant energy surfaces of the Sn-terminated (blue color circles) and Fe 3 Sn-terminated (green color circles) surfaces.
Dirac Fermions in Antiferromagnetic FeSn Kagome Lattices with Combined Space Inversion and Time Reversal Symmetry

June 2019

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542 Reads

Symmetry principles play a critical role in formulating the fundamental laws of nature1^{1}, with a large number of symmetry-protected topological states identified in recent studies of quantum materials2,3^{2,3}. As compelling examples, massless Dirac fermions are jointly protected by the space inversion symmetry P and time reversal symmetry T supplemented by additional crystalline symmetry48^{4-8}, while evolving into Weyl fermions when either P or T is broken4,911^{4,9-11}. Strikingly, such massless Dirac fermions are expected to survive when each of the P and T symmetries is individually broken but the combined PT symmetry is preserved12^{12}. To date, this conceptually intriguing prediction remains to be experimentally validated. Here, based on angle-resolved photoemission spectroscopy (ARPES) aided by first-principles calculations, we present the first experimental observation of massless Dirac fermions in a layered FeSn crystal containing antiferromagnetically coupled ferromagnetic Fe kagome layers. In this system, each of the P and T symmetries is individually broken, but the stable Dirac points are protected by the combined PT symmetry with additional non-symmorphic S2zS_{2z} symmetry. We further demonstrate that by breaking the PT or S2zS_{2z} symmetry, we can transform the massless Dirac fermions into massless Weyl or massive Dirac fermions. The present study substantially enriches our fundamental understanding of the intricate connections between symmetries and topologies of matter, especially with the spin degree of freedom playing a vital role.


Epitaxial Growth of Optically Thick, Single Crystalline Silver Films for Plasmonics

January 2019

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62 Reads

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24 Citations

ACS Applied Materials & Interfaces

Single crystalline Ag films on dielectric substrates has received tremendous attention recently due to their technological potentials as low loss plasmonic materials. Two different growth approaches have been used to produce single crystalline Ag films previously. One approach is based on repetitive cycles of a two-step process (low temperature deposition followed by RT annealing) using molecular beam epitaxy (MBE) which is extremely time-consuming due to the need to repeat growth cycles. Another approach is based on rapid e-beam deposition which is capable of growing thick single crystalline Ag films (>300 nm), but lacks the precision in thickness control of thin epitaxial films. Here we report a universal approach to grow atomically smooth epitaxial Ag films by eliminating the repetitive cycles used in the previous two-step MBE method while maintaining the precise thickness control from a few monolayers to the optically thick regime, thus overcoming the limitations of the two aforementioned methods. In addition, we develop an in-situ growth of aluminum oxide as the capping layer to protect the epitaxial Ag films. The quality of the epitaxial Ag films has been evaluated using a variety of techniques, and the superior optical performance of the films is demonstrated by measuring the propagation length of surface plasmon polaritons (~80 μm at 632 nm), as well as their capability to support a plasmonic nanolaser in infrared incorporating an InGaAsP quantum well as the gain media.


Flatbands and Emergent Ferromagnetic Ordering in Fe 3 Sn 2 Kagome Lattices

August 2018

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391 Reads

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403 Citations

Physical Review Letters

A flatband representing a highly degenerate and dispersionless manifold state of electrons may offer unique opportunities for the emergence of exotic quantum phases. To date, definitive experimental demonstrations of flatbands remain to be accomplished in realistic materials. Here, we present the first experimental observation of a striking flatband near the Fermi level in the layered Fe3Sn2 crystal consisting of two Fe kagome lattices separated by a Sn spacing layer. The band flatness is attributed to the local destructive interferences of Bloch wave functions within the kagome lattices, as confirmed through theoretical calculations and modelings. We also establish high-temperature ferromagnetic ordering in the system and interpret the observed collective phenomenon as a consequence of the synergetic effect of electron correlation and the peculiar lattice geometry. Specifically, local spin moments formed by intramolecular exchange interaction are ferromagnetically coupled through a unique network of the hexagonal units in the kagome lattice. Our findings have important implications to exploit emergent flat-band physics in special lattice geometries.


Single Crystalline Silver Films for Plasmonics: From Monolayer to Optically Thick Film

August 2018

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67 Reads

Epitaxial growth of single crystalline noble metals on dielectric substrates has received tremendous attention recently due to their technological potentials as low loss plasmonic materials. Currently there are two different growth approaches, each with its strengths and weaknesses. One adopts a sophisticated molecular beam epitaxial procedure to grow atomically smooth epitaxial Ag films. However, the procedure is rather slow and becomes impractical to grow films with thickness > 50 nm. Another approach adopts a growth process using rapid e-beam deposition which is capable of growing single crystalline Ag films in the thick regime (> 300 nm). However, the rapid growth procedure makes it difficult to control film thickness precisely, i.e., the method is not applicable to growing thin epitaxial films. Here we report a universal approach to grow atomically smooth epitaxial Ag films with precise thickness control from a few monolayers to the optically thick regime, overcoming the limitations of the two aforementioned methods. In addition, we develop an in-situ growth of aluminum oxide as the capping layer which exhibits excellent properties protecting the epitaxial Ag films. The performance of the epitaxial Ag films as a function of the film thickness is investigated by directly measuring the propagation length of the surface plasmon polaritons (SPPs) as well as their device performance to support a waveguide plasmonic nanolaser in infrared incorporating an InGaAsP quantum well as the gain media.


Tuning Band Gap and Work Function Modulations in Monolayer hBN/Cu(111) Heterostructures with Moiré Patterns

August 2018

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134 Reads

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50 Citations

ACS Nano

The moiré pattern formed between a two-dimensional (2D) material and the substrate has played a crucial role in tuning the electronic structure of the 2D material. Here, by using scanning tunneling microscopy and spectroscopy (STM/S), we found a moiré-pattern-dependent band gap and work function modulation in hBN/Cu(111) heterostructures, whose amplitudes increase with the moiré pattern wavelength. Moreover, the work function modulation shifts agree well with the conduction band (CB) edge shifts, indicating a spatially constant electron affinity for the hBN layer. Density functional theory (DFT) calculations showed that these observations in hBN/Cu(111) heterostructures are mainly originated from the hybridization of N 3pz orbital and Cu 4s orbital in different atomic configurations. Our results show that the twist-angle dependence of moiré patterns in hBN/Cu(111) heterostructures can be used to tailor the electronic properties including band gap and work function.


Atomically flat and thermally stable graphene on Si(111) with preserved intrinsic electronic properties

April 2018

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64 Reads

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5 Citations

Nanoscale

Silicon and graphene are two wonder materials, and their hybrid heterostructures are expected to be very interesting fundamentally and practically. In the present study, by adopting fast dry transfer and ultra-high vacuum annealing, atomically flat monolayer graphene is successfully prepared on the chemically active Si(111) substrate. More importantly, the graphene overlayer largely maintains its intrinsic electronic properties, validated from the energy-dependent electronic transparency, Dirac point observation, quantum coherence characteristics, and further confirmed by the first-principles calculations. The survival of graphene's intrinsic properties can hold up to 1030 K. The atomically flat and thermally stable graphene on the chemically active silicon surface with preserved inherent characteristics renders the graphene/silicon hybrid system promising to design high-performance devices and exploit interfacial topological quantum effects.



Citations (9)


... A notable feature of FeSn's band structure is the coexistence of Dirac fermions and flat bands close to the Fermi level 16 . Angle-resolved photoemission spectroscopy (ARPES) measurement on FeSn single crystals confirmed the existence of massless Dirac fermions in bulk and 2D Weyl-like states at the surface 17 . The presence of spin-polarized 2D flat bands at the surface of FeSn thin films has also been confirmed by planar tunnelling spectroscopy and first-principles calculation 18 . ...

Reference:

Challenges and insights in growing epitaxial FeSn thin films on GaAs(111) substrate using molecular beam epitaxy
Dirac fermions in antiferromagnetic FeSn kagome lattices with combined space inversion and time-reversal symmetry
  • Citing Article
  • October 2020

... Meanwhile, such SOT occurs in a ferromagnet Fe 3 GeTe 2 without any heavy-metal layer and is deeply rooted in its band topology and Berry curvature 17 . Unlike normal metals or Weyl semimetals 75,76 , Fe 3 GeTe 2 has a topological Nodal-line band structure and consequent large Berry curvature 72 , and thus eventually large ME effect and SOT magnitude 17 . One interesting question from this work 17 is how to explore a new larger SOT system with band topology. ...

Butterfly-Like Anisotropic Magnetoresistance and Angle-Dependent Berry Phase in a Type-II Weyl Semimetal WP 2

... The optical properties of Ag thin films play an extremely important role in these applications. Thus, most previous investigations mainly concentrated on the surface plasmon polaritons (SPPs) of Ag films in the visible and near-infrared (VIS-NIR) regions [12][13][14][15][16][17][18][19][20][21][22]. By contrast, little attention has been paid to the optical characteristics of Ag thin films in the mid-infrared (MIR) and far-infrared (FIR) regions. ...

Epitaxial Growth of Optically Thick, Single Crystalline Silver Films for Plasmonics
  • Citing Article
  • January 2019

ACS Applied Materials & Interfaces

... However, it also uniquely hosts an ideal flat band, which has garnered significant attention for real systems incorporating kagome lattices [1,2]. Several classes of compounds with intriguing physical properties that include kagome lattices are worth highlighting: (i) AV 3 Sb 5 (A = alkali metals), a family of vanadium-based compounds, presents a rare coexistence of charge density wave and superconductivity [3][4][5][6]; (ii) AV 6 Sb 6 (A = alkali metals), a vanadium kagome bilayer system, features Dirac nodal lines near the Fermi level and superconductivity under pressure [7][8][9][10]; (iii) RV 6 Sn 6 (R = rare earth), another vanadium kagome bilayer system, is notable for its charge density wave properties [11][12][13]; (iv) Co 3 Sn 2 S 2 , a Weyl semimetal with a ferromagnetic cobalt kagome lattice, exhibits a giant anomalous Hall effect [14][15][16][17][18][19]; (v) topological ferrimagnetic or ferromagnetic kagome metals, such as FeSn [20][21][22], Fe 3 Sn 2 [23][24][25][26], and RMn 6 Sn 6 (R = rare earth) [27][28][29][30][31][32][33][34], further exemplify the noteworthy physics of kagome lattices. ...

Flatbands and Emergent Ferromagnetic Ordering in Fe 3 Sn 2 Kagome Lattices

Physical Review Letters

... We observe some defects within the MOF domain, as well as some DCA-only regions (discussed in Supplementary Note 11). The long-range modulation of the MOF STM apparent height follows the hBN/Cu(111) moiré pattern, which arises due to a mismatch between the hBN and Cu(111) lattices (giving rise to pore, P, and wire, W, regions-see upper inset) [29][30][31] . This moiré pattern has been shown to affect the electronic properties of adsorbates 31 , including one previous example of a MOF 32 . ...

Tuning Band Gap and Work Function Modulations in Monolayer hBN/Cu(111) Heterostructures with Moiré Patterns
  • Citing Article
  • August 2018

ACS Nano

... [21] In addition, annealing can promote the rearrangement of atoms and the formation of more stable structures. [21,22] As a result, annealing can routinely enhance the transport properties of graphene and improve the reliability and stability of sensor device. ...

Atomically flat and thermally stable graphene on Si(111) with preserved intrinsic electronic properties
  • Citing Article
  • April 2018

Nanoscale

... In this section of the review paper, we will discuss some of the works looking into growing MBE monolayer MoSe 2 and WSe 2 . Some of the first works relating to growing monolayer/few-layer TMDs using MBE were selenides in 2014 and 2015 by Zhang et al, Liu et al and Jiao et al [27][28][29][30][31]; all of the studies used MBE to grow MoSe 2 using graphene or highly ordered pyrolytic graphite (HOPG). In particular, Jiao et al investigated the growth and boundary formation of MoSe 2 on graphene and HOPG; they demonstrated that when using these substrates, MoSe 2 growth occurs over a wide range of growth conditions by the nucleation of 2D islands on the surface [31]. ...

Band gap renormalization and work function tuning in MoSe2/hBN/Ru(0001) heterostructures

... Ferromagnetic (FM) state can be obtained at slightly higher energies via either a magnetic field or a transverse electric field [15], and parallel magnetic structures are electrically conductive. In addition to intrinsic graphene, some interesting properties have been obtained by joining two or more adjacent nanoribbons in different ways to form nanojunctions [17,18]. Due to the presence of its spin alternation rule, the application of non-intrinsic graphene nanoribbons in spintronics is greatly extended [17]. ...

Carbon Tetragons as Definitive Spin Switches in Narrow Zigzag Graphene Nanoribbons
  • Citing Article
  • January 2016

Physical Review Letters

... Recent experimental evidence and first-principles simulations for In/Si(111) suggest that the metallic uniform state remains metastable below the critical temperature and that the transition is first order in this system [9][10][11][12][13][14][15][16]. In Ref. [14] it was shown that the first-principles simulation results, the metastability of the metallic uniform state, and the first-order transition could be understood within a grand-canonical Peierls theory based on an effective one-dimensional low-energy model. ...

Stabilization and Manipulation of Electronically Phase-Separated Ground States in Defective Indium Atom Wires on Silicon
  • Citing Article
  • November 2014

Physical Review Letters