-
Non-invertible Symmetries in Weyl Fermions, and Applications to Fermion-Boundary Scattering Problem
Authors:
Pengcheng Wei,
Yunqin Zheng
Abstract:
We construct a family of non-invertible topological defects in two-dimensional theories of $n$ Weyl fermions. The construction relies on the existence of $G$-symmetric conformal boundary conditions for $n$ Dirac fermions. Upon unfolding, these boundary conditions become topological defects $\mathcal D$ of $n$ Weyl fermions that intertwine the two $G$-representations, and they are generically non-i…
▽ More
We construct a family of non-invertible topological defects in two-dimensional theories of $n$ Weyl fermions. The construction relies on the existence of $G$-symmetric conformal boundary conditions for $n$ Dirac fermions. Upon unfolding, these boundary conditions become topological defects $\mathcal D$ of $n$ Weyl fermions that intertwine the two $G$-representations, and they are generically non-invertible. For $G=U(1)^n$, we show that $\mathcal D$ is a duality defect associated with gauging a finite Abelian group $Γ$, and we give an explicit algorithm for determining $Γ$ and its action on the fermions. We also show that the same finite-Abelian gauging description applies in certain restricted examples with non-Abelian $G$. By contrast, for certain non-Abelian symmetry structures, including the $G=SU(2)$ symmetry appearing in the $1$-$5$-$7$-$8$-$9$ problem, we prove that $\mathcal D$ cannot be realized as a duality defect for gauging any finite Abelian group. Finally, we explain how the duality-defect perspective gives a streamlined derivation of fermion scattering from a conformal boundary.
△ Less
Submitted 19 May, 2026;
originally announced May 2026.
-
On the Absence of Symmetric Simple Conformal Boundary Conditions
Authors:
Pengcheng Wei,
Yunqin Zheng
Abstract:
Non-trivial 't Hooft anomaly obstructs the existence of a simple symmetric conformal boundary condition in a CFT. Conversely, there is a common piece of lore that trivial 't Hooft anomaly promises the existence of a simple symmetry conformal boundary condition in a given CFT. Recently, counter examples to this lore was realized in tetracritical Ising CFT [1] and compact boson [2] -- the simple con…
▽ More
Non-trivial 't Hooft anomaly obstructs the existence of a simple symmetric conformal boundary condition in a CFT. Conversely, there is a common piece of lore that trivial 't Hooft anomaly promises the existence of a simple symmetry conformal boundary condition in a given CFT. Recently, counter examples to this lore was realized in tetracritical Ising CFT [1] and compact boson [2] -- the simple conformal boundary conditions preserving certain anomaly-free subsymmetry are absent in these CFTs. In this work, we uncover the underlying reason for the absence of these boundary conditions in counter examples, and propose a criterion diagnosing when the lore fails for any given 2d CFT. The Symmetry TFT description for boundary conditions plays a crucial role.
△ Less
Submitted 29 December, 2025;
originally announced December 2025.
-
Chirality-selective topological magnon phase transition induced by interplay of anisotropic exchange interactions in honeycomb ferromagnet
Authors:
Jin-Yu Ni,
Xia-Ming Zheng,
Peng-Tao Wei,
Da-Yong Liu,
Liang-Jian Zou
Abstract:
A variety of distinct anisotropic exchange interactions commonly exist in one magnetic material due to complex crystal, magnetic and orbital symmetries. Here we investigate the effects of multiple anisotropic exchange interactions on topological magnon in a honeycomb ferromagnet, and find a chirality-selective topological magnon phase transition induced by a complicated interplay of Dzyaloshinsky-…
▽ More
A variety of distinct anisotropic exchange interactions commonly exist in one magnetic material due to complex crystal, magnetic and orbital symmetries. Here we investigate the effects of multiple anisotropic exchange interactions on topological magnon in a honeycomb ferromagnet, and find a chirality-selective topological magnon phase transition induced by a complicated interplay of Dzyaloshinsky-Moriya interaction (DMI) and pseudo-dipolar interaction (PDI), accompanied by the bulk gap close and reopen with chiral inversion. Moreover, this novel topological phase transition involves band inversion at high symmetry points $K$ and $K'$, which can be regarded as a pseudo-orbital reversal, i.e. magnon valley degree of freedom, implying a new manipulation corresponding to a sign change of the magnon thermal Hall conductivity. Indeed, it can be realized in 4$d$ or 5$d$ correlated materials with both spin-orbit coupling and orbital localized states, such as iridates and ruthenates, etc. This novel regulation may have potential applications on magnon devices and topological magnonics.
△ Less
Submitted 25 December, 2025;
originally announced December 2025.
-
Multiple topological phases of magnons induced by Dzyaloshinskii-Moriya and pseudodipolar anisotropic exchange interactions in Kagome ferromagnets
Authors:
Jin-Yu Ni,
Xia-Ming Zheng,
Peng-Tao Wei,
Da-Yong Liu,
Liang-Jian Zou
Abstract:
Kagome magnets naturally hosting Dirac points and flat bands exhibit novel topological phases, enabling rich interplays between interactions and topologies. The discovery of two-dimensional (2D) magnets generally coexisting with different types of magnetic interactions poses a challenge for topological magnonic manipulation. Here we investigate the topological magnon phases of 2D Kagome ferromagne…
▽ More
Kagome magnets naturally hosting Dirac points and flat bands exhibit novel topological phases, enabling rich interplays between interactions and topologies. The discovery of two-dimensional (2D) magnets generally coexisting with different types of magnetic interactions poses a challenge for topological magnonic manipulation. Here we investigate the topological magnon phases of 2D Kagome ferromagnet with multiple magnetic anisotropic interactions, i.e. Dzyaloshinskii-Moriya interaction (DMI) and pseudo-dipolar interaction (PDI). It is found that the different sole magnetic anisotropic interactions introduce completely distinct topological phase diagrams and topological states. The multiple topological magnon phases with high Chern number emerge due to the distinct anisotropic interactions. Moreover, the interplay of the multiple anisotropic DMI and PDI interactions involved with Dirac and flat bands controls a variety of topological phase transitions, implying greater manipulation potential. In addition, the sign reversal of thermal Hall and Nernst conductivities induced by temperature is found in particular topological phase regions, namely topological origin, relating to the energy gap and Berry curvature (Chern number) in the vicinity of magnetic phase transition from the thermal fluctuations, providing a possible explanation for the experimental puzzles. All these results demonstrate that the novel topological magnonic properties in Kagome magnet with multiple magnetic anisotropic interactions can realize a potential platform for magnonic devices and quantum computing.
△ Less
Submitted 23 December, 2025;
originally announced December 2025.
-
High Pressure Growth of Transition-Metal Monosilicide RhGe Single Crystals
Authors:
Xiangjiang Dong,
Bowen Zhang,
Xubin Ye,
Peng Wei,
Lie Lian,
Ning Sun,
Youwen Long,
Shangjie Tian,
Shouguo Wang,
Hechang Lei,
Runze Yu
Abstract:
Transition-metal monosilicide RhGe has been reported to exhibit weak itinerant ferromagnetism, superconductivity, and topological properties. In this study, we report the high-pressure growth of high-quality RhGe single crystals up to millimeter size using flux method. Transport measurements reveal the metallic behavior of RhGe between 2-300 K with Fermi liquid behavior at low temperature region.…
▽ More
Transition-metal monosilicide RhGe has been reported to exhibit weak itinerant ferromagnetism, superconductivity, and topological properties. In this study, we report the high-pressure growth of high-quality RhGe single crystals up to millimeter size using flux method. Transport measurements reveal the metallic behavior of RhGe between 2-300 K with Fermi liquid behavior at low temperature region. However, no superconductivity was observed with variations in Ge composition. Magnetic characterizations indicate that RhGe exhibits a paramagnetic behavior between 2-300 K. The high-quality, large-size RhGe single crystals pave the way for further investigation of their topological properties using spectroscopic techniques.
△ Less
Submitted 23 June, 2025;
originally announced June 2025.
-
Laser-driven solid-state synthesis of high-entropy oxides
Authors:
Peng wei,
Yiwen Liu,
Hao Bai,
Lei Zhuang,
Hulei Yu,
Yanhui Chu
Abstract:
The vast compositional and structural landscape of high-entropy oxides (HEOs) grants them a wide range of potentially valuable physicochemical properties. However, the elemental immiscibility and crystal complexity limit their controllable synthesis. Here, we report a laser-driven solid-state synthesis technique that enables high-throughput production of HEOs with different crystal structures, inc…
▽ More
The vast compositional and structural landscape of high-entropy oxides (HEOs) grants them a wide range of potentially valuable physicochemical properties. However, the elemental immiscibility and crystal complexity limit their controllable synthesis. Here, we report a laser-driven solid-state synthesis technique that enables high-throughput production of HEOs with different crystal structures, including rock-salt, perovskite, spinel, fluorite, pyrochlore, tantalate, and silicate, incorporating up to 20 cationic elements. Typically, we successfully synthesize all types of high-entropy rare-earth disilicates (HEREDs), including A-, α-, \b{eta}-, γ-, δ-, F-, and G-type phase structures, with up to 15 rare-earth elements in the A site and 5 transition-metal elements in the B site. Benefiting from their unique G-type phase structure and 20-cation composition, HEREDs are endowed with the new functionality of microwave absorption (effective absorption bandwidth of 4.3 GHz). Our work not only realizes the controllable synthesis of HEOs with vast compositional and structural space but also offers them new physicochemical properties, making them highly promising for a diverse array of structural and functional applications.
△ Less
Submitted 10 April, 2025;
originally announced April 2025.
-
Low driving-force stable bending cooling via fatigue-resistant hierarchical NiTi shape memory alloy
Authors:
Kai Yan,
Kangjie Chu,
Peng Hua,
Pengbo Wei,
Hanlin Gu,
Qiming Zhuang,
Weifeng He,
Fuzeng Ren,
Qingping Sun,
Robert O. Ritchie
Abstract:
Elastocaloric cooling with shape memory alloys (SMAs) is emerging as a promising candidate for next-generation, environmentally friendly refrigeration. However, its development is hindered by the large driving force and low efficiency associated with uniaxial loading modes. In response, we present an innovative elastocaloric air cooling approach that utilizes the bending of NiTi beams, offering a…
▽ More
Elastocaloric cooling with shape memory alloys (SMAs) is emerging as a promising candidate for next-generation, environmentally friendly refrigeration. However, its development is hindered by the large driving force and low efficiency associated with uniaxial loading modes. In response, we present an innovative elastocaloric air cooling approach that utilizes the bending of NiTi beams, offering a low-force and energy-efficient solution. We achieve a continuous maximum temperature drop of 0.91 K and a dissipated energy of 15.1 N mm at a low specific driving force of 220 N. Notably, the specimen achieves over 5 million cycles under a maximum surface tensile strain of 1.94% for macroscopic cyclic bending, via a pre-strained, warm laser shock peening (pw-LSP) method. This unprecedented fatigue resistance originates from the formation of a hierarchical microstructure and a large compressive residual stress of over 1 GPa. This work demonstrates the great potential of bending induced elastocaloric cooling in the near future.
△ Less
Submitted 26 December, 2024;
originally announced December 2024.
-
Effects of residual stress on the isothermal tensile behavior of nanocrystalline superelastic NiTi shape memory alloy
Authors:
Kai Yan,
Pengbo Wei,
Weifeng He,
Qingping Sun
Abstract:
The residual stress greatly affects the mechanical behavior of a material. In this work, the effect of residual stress on the isothermal tensile behavior of a NiTi shape memory alloy is studied. The focused ion beam and digital image correlation are combined to measure the two-dimensional residual stress in nanocrystalline NiTi plates processed with prestrain laser shock peening. A four-point bend…
▽ More
The residual stress greatly affects the mechanical behavior of a material. In this work, the effect of residual stress on the isothermal tensile behavior of a NiTi shape memory alloy is studied. The focused ion beam and digital image correlation are combined to measure the two-dimensional residual stress in nanocrystalline NiTi plates processed with prestrain laser shock peening. A four-point bending experiment verified the accuracy of this measurement method. The FIB-DIC method is an attractive tool for measuring the two-dimensional residual stress in phase transition nanocrystalline materials. The internal residual stress significantly decreases the phase transition stress, and the mechanism is studied via finite element and theoretical analyses. This work implies that the mechanical behavior of NiTi shape memory alloys can be tailored via residual stress engineering.
△ Less
Submitted 20 September, 2024;
originally announced September 2024.
-
Signatures of a Spin-Active Interface and Locally Enhanced Zeeman field in a Superconductor-Chiral Material Heterostructure
Authors:
Cliff Chen,
Jason Tran,
Anthony McFadden,
Raymond Simmonds,
Keisuke Saito,
En-De Chu,
Daniel Morales,
Varrick Suezaki,
Yasen Hou,
Joe Aumentado,
Patrick A. Lee,
Jagadeesh S. Moodera,
Peng Wei
Abstract:
A localized Zeeman field, intensified at heterostructure interfaces, could play a crucial role in a broad area including spintronics and unconventional superconductors. Conventionally, the generation of a local Zeeman field is achieved through magnetic exchange coupling with a magnetic material. However, magnetic elements often introduce defects, which could weaken or destroy superconductivity. Al…
▽ More
A localized Zeeman field, intensified at heterostructure interfaces, could play a crucial role in a broad area including spintronics and unconventional superconductors. Conventionally, the generation of a local Zeeman field is achieved through magnetic exchange coupling with a magnetic material. However, magnetic elements often introduce defects, which could weaken or destroy superconductivity. Alternatively, the coupling between a superconductor with strong spin-orbit coupling and a non-magnetic chiral material could serve as a promising approach to generate a spin active interface. In this study, we leverage an interface superconductor, namely induced superconductivity in noble metal surface states, to probe the spin active interface. Our results unveil an enhanced interface Zeeman field, which selectively closes the surface superconducting gap while preserving the bulk superconducting pairing. The chiral material, i.e. trigonal tellurium, also induces Andreev bound states (ABS) exhibiting spin polarization. The field dependence of ABS manifests a substantially enhanced interface Landé g-factor (g_eff ~ 12), thereby corroborating the enhanced interface Zeeman energy.
△ Less
Submitted 28 August, 2024;
originally announced August 2024.
-
Highly Efficient Superconducting Diodes and Rectifiers for Quantum Circuitry
Authors:
Josep Ingla-Aynés,
Yasen Hou,
Sarah Wang,
En-De Chu,
Oleg A. Mukhanov,
Peng Wei,
Jagadeesh S. Moodera
Abstract:
Superconducting electronics is essential for energy-efficient quantum and classical high-end computing applications. Towards this goal, non-reciprocal superconducting circuit elements, such as superconducting diodes (SDs) can fulfill many critical needs. SDs have been the subject of multiple studies, but integrating several SDs in a superconducting circuit remains a challenge. Here we implement th…
▽ More
Superconducting electronics is essential for energy-efficient quantum and classical high-end computing applications. Towards this goal, non-reciprocal superconducting circuit elements, such as superconducting diodes (SDs) can fulfill many critical needs. SDs have been the subject of multiple studies, but integrating several SDs in a superconducting circuit remains a challenge. Here we implement the first SD bridge with multiple SDs exhibiting reproducible characteristics operating at temperatures of a few Kelvin. We demonstrate its functionality as a full wave rectifier using elemental superconductors and insulating ferromagnets, with efficiency up to 43%, and ac to dc signal conversion capabilities at frequencies up to 40 kHz. Our results show a pathway with a highly scalable thin film platform for nonreciprocal superconducting circuits. They could significantly reduce energy consumption as well as decohering thermal and electromagnetic noise in quantum computing.
△ Less
Submitted 21 June, 2024; v1 submitted 17 June, 2024;
originally announced June 2024.
-
Topological magnons in one-dimensional ferromagnetic Su-Schrieffer-Heeger model with anisotropic interaction
Authors:
Peng-Tao Wei,
Jin-Yu Ni,
Xia-Ming Zheng,
Da-Yong Liu,
Liang-Jian Zou
Abstract:
Topological magnons in a one-dimensional (1D) ferromagnetic (FM) Su-Schrieffer-Heeger (SSH) model with anisotropic exchange interactions are investigated. Apart from the inter-cellular isotropic Heisenberg interaction, the intercellular anisotropic exchange interactions, i.e. Dzyaloshinskii-Moriya interaction (DMI) and pseudo-dipolar interaction (PDI), also can induce the emergence of the non-triv…
▽ More
Topological magnons in a one-dimensional (1D) ferromagnetic (FM) Su-Schrieffer-Heeger (SSH) model with anisotropic exchange interactions are investigated. Apart from the inter-cellular isotropic Heisenberg interaction, the intercellular anisotropic exchange interactions, i.e. Dzyaloshinskii-Moriya interaction (DMI) and pseudo-dipolar interaction (PDI), also can induce the emergence of the non-trivial phase with two degenerate in-gap edge states separately localized at the two ends of the 1D chain, while the intracellular interactions instead unfavors the topological phase. The interplay among them has synergistic effects on the topological phase transition, very different from that in the two-dimensional (2D) ferromagnet. These results demonstrate that the 1D magnons possess rich topological phase diagrams distinctly different from the electronic version of the SSH model and even the 2D magnons. Due to the lower dimensional structural characteristics of this 1D topological magnonic system, the magnonic crystals can be constructed from bottom to top, which has important potential applications in the design of novel magnonic devices.
△ Less
Submitted 27 October, 2022;
originally announced October 2022.
-
Signature of a pair of Majorana zero modes in superconducting gold surface states
Authors:
Sujit Manna,
Peng Wei,
Yingming Xie,
Kam Tuen Law,
Patrick Lee,
Jagadeesh Moodera
Abstract:
Under certain conditions, a fermion in a superconductor can separate in space into two parts known as Majorana zero modes, which are immune to decoherence from local noise sources and are attractive building blocks for quantum computers. Promising experimental progress has been made to demonstrate Majorana zero modes in materials with strong spin-orbit coupling proximity coupled to superconductors…
▽ More
Under certain conditions, a fermion in a superconductor can separate in space into two parts known as Majorana zero modes, which are immune to decoherence from local noise sources and are attractive building blocks for quantum computers. Promising experimental progress has been made to demonstrate Majorana zero modes in materials with strong spin-orbit coupling proximity coupled to superconductors. Here we report signatures of Majorana zero modes in a new material platform utilizing the surface states of gold. Using scanning tunneling microscope to probe EuS islands grown on top of gold nanowires, we observe two well separated zero bias tunneling conductance peaks aligned along the direction of the applied magnetic field, as expected for a pair of Majorana zero modes. This platform has the advantage of having a robust energy scale and the possibility of realizing complex designs using lithographic methods.
△ Less
Submitted 11 April, 2020; v1 submitted 9 November, 2019;
originally announced November 2019.
-
Strain controlled superconductivity in few-layer NbSe2
Authors:
Cliff Chen,
Protik Das,
Ece Aytan,
Weimin Zhou,
Justin Horowitz,
Biswarup Satpati,
Alexander A. Balandin,
Roger K. Lake,
Peng Wei
Abstract:
The controlled tunability of superconductivity in low-dimensional materials may enable new quantum devices. Particularly in triplet or topological superconductors, tunneling devices such as Josephson junctions etc. can demonstrate exotic functionalities. The tunnel barrier, an insulating or normal material layer separating two superconductors, is a key component for the junctions. Thin layers of N…
▽ More
The controlled tunability of superconductivity in low-dimensional materials may enable new quantum devices. Particularly in triplet or topological superconductors, tunneling devices such as Josephson junctions etc. can demonstrate exotic functionalities. The tunnel barrier, an insulating or normal material layer separating two superconductors, is a key component for the junctions. Thin layers of NbSe2 have been shown as a superconductor with strong spin orbit coupling, which can give rise to topological superconductivity if driven by a large magnetic exchange field. Here we demonstrate the superconductor-insulator transitions in epitaxially grown few-layer NbSe2 with wafer-scale uniformity on insulating substrates. We provide the electrical transport, Raman spectroscopy, cross-sectional transmission electron microscopy, and X-ray diffraction characterizations of the insulating phase. We show that the superconductor-insulator transition is driven by strain, which also causes characteristic energy shifts of the Raman modes. Our observation paves the way for high quality hetero-junction tunnel barriers to be seamlessly built into epitaxial NbSe2 itself, thereby enabling highly scalable tunneling devices for superconductor-based quantum electronics.
△ Less
Submitted 31 July, 2020; v1 submitted 11 July, 2019;
originally announced July 2019.
-
Spin Seebeck effect from antiferromagnetic magnons and critical spin fluctuations in epitaxial FeF2 films
Authors:
Junxue Li,
Zhong Shi,
Victor H. Ortiz,
Mohammed Aldosary,
Cliff Chen,
Vivek Aji,
Peng Wei,
Jing Shi
Abstract:
We report a longitudinal spin Seebeck effect (SSE) study in epitaxially grown FeF2(110) antiferromagnetic (AFM) thin films with strong uniaxial anisotropy over the temperature range of 3.8 - 250 K. Both the magnetic field- and temperature-dependent SSE signals below the Néel temperature (TN=70 K) of the FeF2 films are consistent with a theoretical model based on the excitations of AFM magnons with…
▽ More
We report a longitudinal spin Seebeck effect (SSE) study in epitaxially grown FeF2(110) antiferromagnetic (AFM) thin films with strong uniaxial anisotropy over the temperature range of 3.8 - 250 K. Both the magnetic field- and temperature-dependent SSE signals below the Néel temperature (TN=70 K) of the FeF2 films are consistent with a theoretical model based on the excitations of AFM magnons without any net induced static magnetic moment. In addition to the characteristic low-temperature SSE peak associated with the AFM magnons, there is another SSE peak at TN which extends well into the paramagnetic phase. All the SSE data taken at different magnetic fields up to 12 T near and above the critical point TN follow the critical scaling law very well with the critical exponents for magnetic susceptibility of 3D Ising systems, which suggests that the AFM spin correlation is responsible for the observed SSE near TN.
△ Less
Submitted 10 May, 2019;
originally announced May 2019.
-
Superconductivity in the surface state of noble metal gold and its Fermi level tuning by EuS dielectric
Authors:
Peng Wei,
Sujit Manna,
Marius Eich,
Patrick Lee,
Jagadeesh Moodera
Abstract:
The induced superconductivity (SC) in a robust and scalable quantum material with strong Rashba spin-orbit coupling is particularly attractive for generating topological superconductivity and Majorana bound states (MBS). Gold (111) thin film has been proposed as a promising candidate because of the large Rashba energy, the predicted topological nature and the possibility for large-scale MBS device…
▽ More
The induced superconductivity (SC) in a robust and scalable quantum material with strong Rashba spin-orbit coupling is particularly attractive for generating topological superconductivity and Majorana bound states (MBS). Gold (111) thin film has been proposed as a promising candidate because of the large Rashba energy, the predicted topological nature and the possibility for large-scale MBS device fabrications. We experimentally demonstrate two important steps towards achieving such a goal. We successfully show induced SC in the Shockley surface state (SS) of ultrathin Au(111) layers grown over epitaxial vanadium films, which is easily achievable on a wafer scale. The emergence of SC in the SS, which is physically separated from a bulk superconductor, is attained by indirect quasiparticle scattering processes instead of by conventional interfacial Andreev reflections. We further show the ability to tune the SS Fermi level (E_F) by interfacing SS with a high-k dielectric ferromagnetic insulator EuS. The shift of E_F from ~ 550 mV to ~34mV in superconducting SS is an important step towards realizing MBS in this robust system.
△ Less
Submitted 25 February, 2019;
originally announced February 2019.
-
Systematic design and realization of double-negative acoustic metamaterials by topology optimization
Authors:
Hao-Wen Dong,
Sheng-Dong Zhao,
Peijun Wei,
Li Cheng,
Chuanzeng Zhang,
Yue-Sheng Wang
Abstract:
Double-negative acoustic metamaterials (AMMs) offer the promising ability of superlensing for applications in ultrasonography, biomedical sensing and nondestructive evaluation. Here, under the simultaneous increasing or non-increasing mechanisms, we develop a unified topology optimization framework considering the different microstructure symmetries, minimal structural feature sizes and dispersion…
▽ More
Double-negative acoustic metamaterials (AMMs) offer the promising ability of superlensing for applications in ultrasonography, biomedical sensing and nondestructive evaluation. Here, under the simultaneous increasing or non-increasing mechanisms, we develop a unified topology optimization framework considering the different microstructure symmetries, minimal structural feature sizes and dispersion extents of effective parameters. Then we apply the optimization framework to furnish the heuristic resonance-cavity-based and space-coiling metamaterials with broadband double negativity. Meanwhile, we demonstrate the essences of double negativity derived from the novel artificial multipolar LC and Mie resonances which can be induced by controlling mechanisms in optimization. Furthermore, abundant numerical simulations validate the double negativity, negative refraction, enhancements of evanescent waves and subwavelengh imaging for the optimized AMMs. Finally, we experimentally show the desired broadband subwavelengh imaging using the 3D-printed optimized space-coiling metamaterial. The present methodology and broadband metamaterials provide the ideal strategy of constructing AMMs for subwavelengh imaging technology.
△ Less
Submitted 7 November, 2018;
originally announced November 2018.
-
Experimental Observation of the Quantum Anomalous Hall Effect in a Magnetic Topological Insulator
Authors:
Cui-Zu Chang,
Jinsong Zhang,
Xiao Feng,
Jie Shen,
Zuocheng Zhang,
Minghua Guo,
Kang Li,
Yunbo Ou,
Pang Wei,
Li-Li Wang,
Zhong-Qing Ji,
Yang Feng,
Shuaihua Ji,
Xi Chen,
Jinfeng Jia,
Xi Dai,
Zhong Fang,
Shou-Cheng Zhang,
Ke He,
Yayu Wang,
Li Lu,
Xu-Cun Ma,
Qi-Kun Xu
Abstract:
The quantized version of the anomalous Hall effect has been predicted to occur in magnetic topological insulators, but the experimental realization has been challenging. Here, we report the observation of the quantum anomalous Hall (QAH) effect in thin films of Cr-doped (Bi,Sb)2Te3, a magnetic topological insulator. At zero magnetic field, the gate-tuned anomalous Hall resistance reaches the predi…
▽ More
The quantized version of the anomalous Hall effect has been predicted to occur in magnetic topological insulators, but the experimental realization has been challenging. Here, we report the observation of the quantum anomalous Hall (QAH) effect in thin films of Cr-doped (Bi,Sb)2Te3, a magnetic topological insulator. At zero magnetic field, the gate-tuned anomalous Hall resistance reaches the predicted quantized value of h/e^2,accompanied by a considerable drop of the longitudinal resistance. Under a strong magnetic field, the longitudinal resistance vanishes whereas the Hall resistance remains at the quantized value. The realization of the QAH effect may lead to the development of low-power-consumption electronics.
△ Less
Submitted 27 May, 2016;
originally announced May 2016.
-
Induced superconductivity and engineered Josephson tunneling devices in epitaxial (111)-oriented gold/vanadium heterostructures
Authors:
Peng Wei,
Ferhat Katmis,
Cui-Zu Chang,
Jagadeesh S. Moodera
Abstract:
We report a unique experimental approach to create topological superconductors by inducing superconductivity into epitaxial metallic thin film with strong spin-orbit coupling. Utilizing molecular beam epitaxy technique under ultra-high vacuum condition, we are able to achieve (111) oriented single phase of gold (Au) thin film grown on a well-oriented vanadium (V) s-wave superconductor film with cl…
▽ More
We report a unique experimental approach to create topological superconductors by inducing superconductivity into epitaxial metallic thin film with strong spin-orbit coupling. Utilizing molecular beam epitaxy technique under ultra-high vacuum condition, we are able to achieve (111) oriented single phase of gold (Au) thin film grown on a well-oriented vanadium (V) s-wave superconductor film with clean interface. We obtained atomically smooth Au thin films with thicknesses even down to below a nanometer showing near-ideal surface quality. The as-grown V/Au bilayer heterostructure exhibits superconducting transition at around 4 K. Clear Josephson tunneling and Andreev reflection are observed in S-I-S tunnel junctions fabricated from the epitaxial bi-layers. The barrier thickness dependent tunneling and the associated subharmonic gap structures (SGS) confirmed the induced superconductivity in Au (111), paving the way for engineering thin film heterostructure based p-wave superconductors and nano devices for Majorana fermion.
△ Less
Submitted 27 January, 2016;
originally announced January 2016.
-
Strong interfacial exchange field in the graphene/EuS heterostructure
Authors:
Peng Wei,
Sunwoo Lee,
Florian Lemaitre,
Lucas Pinel,
Davide Cutaia,
Wujoon Cha,
Ferhat Katmis,
Yu Zhu,
Donald Heiman,
James Hone,
Jagadeesh S. Moodera,
Ching-Tzu Chen
Abstract:
Exploiting 2D materials for spintronic applications can potentially realize next-generation devices featuring low-power consumption and quantum operation capability. The magnetic exchange field (MEF) induced by an adjacent magnetic insulator enables efficient control of local spin generation and spin modulation in 2D devices without compromising the delicate material structures. Using graphene as…
▽ More
Exploiting 2D materials for spintronic applications can potentially realize next-generation devices featuring low-power consumption and quantum operation capability. The magnetic exchange field (MEF) induced by an adjacent magnetic insulator enables efficient control of local spin generation and spin modulation in 2D devices without compromising the delicate material structures. Using graphene as a prototypical 2D system, we demonstrate that its coupling to the model magnetic insulator (EuS) produces a substantial MEF (> 14 T) with potential to reach hundreds of Tesla, which leads to orders-of-magnitude enhancement in the spin signal originated from Zeeman spin-Hall effect. Furthermore, the new ferromagnetic ground state of Dirac electrons resulting from the strong MEF may give rise to quantized spin-polarized edge transport. The MEF effect shown in our graphene/EuS devices therefore provides a key functionality for future spin logic and memory devices based on emerging 2D materials in classical and quantum information processing.
△ Less
Submitted 28 April, 2016; v1 submitted 20 October, 2015;
originally announced October 2015.
-
Independent tuning of electronic properties and induced ferromagnetism in topological insulators with heterostructure approach
Authors:
Zilong Jiang,
Cui-Zu Chang,
Chi Tang,
Peng Wei,
Jagadeesh S. Moodera,
Jing Shi
Abstract:
The quantum anomalous Hall effect (QAHE) has been recently demonstrated in Cr- and V-doped three-dimensional topological insulators (TIs) at temperatures below 100 mK. In those materials, the spins of unfilled d-electrons in the transition metal dopants are exchange coupled to develop a long-range ferromagnetic order, which is essential for realizing QAHE. However, the addition of random dopants d…
▽ More
The quantum anomalous Hall effect (QAHE) has been recently demonstrated in Cr- and V-doped three-dimensional topological insulators (TIs) at temperatures below 100 mK. In those materials, the spins of unfilled d-electrons in the transition metal dopants are exchange coupled to develop a long-range ferromagnetic order, which is essential for realizing QAHE. However, the addition of random dopants does not only introduce excess charge carriers that require readjusting the Bi/Sb ratio, but also unavoidably introduces paramagnetic spins that can adversely affect the chiral edge transport in QAHE. In this work, we show a heterostructure approach to independently tune the electronic and magnetic properties of the topological surface states in (BixSb1-x)2Te3 without resorting to random doping of transition metal elements. In heterostructures consisting of a thin (BixSb1-x)2Te3 TI film and yttrium iron garnet (YIG), a high Curie temperature (~ 550 K) magnetic insulator, we find that the TI surface in contact with YIG becomes ferromagnetic via proximity coupling which is revealed by the anomalous Hall effect (AHE). The Curie temperature of the magnetized TI surface ranges from 20 to 150 K but is uncorrelated with the Bi fraction x in (BixSb1-x)2Te3. In contrast, as x is varied, the AHE resistivity scales with the longitudinal resistivity. In this approach, we decouple the electronic properties from the induced ferromagnetism in TI. The independent optimization provides a pathway for realizing QAHE at higher temperatures, which is important for novel spintronic device applications.
△ Less
Submitted 19 August, 2015;
originally announced August 2015.
-
Zero-field dissipationless chiral edge transport and the nature of dissipation in the quantum anomalous Hall state
Authors:
Cui-Zu Chang,
Weiwei Zhao,
Duk Y. Kim,
Peng Wei,
J. K. Jain,
Chaoxing Liu,
Moses H. W. Chan,
Jagadeesh S. Moodera
Abstract:
The quantum anomalous Hall (QAH) effect is predicted to possess, at zero magnetic field, chiral edge channels that conduct spin polarized current without dissipation. While edge channels have been observed in previous experimental studies of the QAH effect, their dissipationless nature at a zero magnetic field has not been convincingly demonstrated. By a comprehensive experimental study of the gat…
▽ More
The quantum anomalous Hall (QAH) effect is predicted to possess, at zero magnetic field, chiral edge channels that conduct spin polarized current without dissipation. While edge channels have been observed in previous experimental studies of the QAH effect, their dissipationless nature at a zero magnetic field has not been convincingly demonstrated. By a comprehensive experimental study of the gate and temperature dependences of local and nonlocal magnetoresistance, we unambiguously establish the dissipationless edge transport. By studying the onset of dissipation, we also identify the origin of dissipative channels and clarify the surprising observation that the critical temperature of the QAH effect is two orders of magnitude smaller than the Curie temperature of ferromagnetism.
△ Less
Submitted 4 July, 2015; v1 submitted 7 May, 2015;
originally announced May 2015.
-
Inducing magnetism onto the surface of a topological crystalline insulator
Authors:
Badih A. Assaf,
Ferhat Katmis,
Peng Wei,
Cui-Zu Chang,
Biswarup Satpati,
Jagadeesh S. Moodera,
Don Heiman
Abstract:
Inducing magnetism onto a topological crystalline insulator (TCI) has been predicted to result in several novel quantum electromagnetic effects. This is a consequence of the highly strain-sensitive band topology of such symmetry-protected systems. We thus show that placing the TCI surface of SnTe in proximity to EuS, a ferromagnetic insulator, induces magnetism at the interface between SnTe and Eu…
▽ More
Inducing magnetism onto a topological crystalline insulator (TCI) has been predicted to result in several novel quantum electromagnetic effects. This is a consequence of the highly strain-sensitive band topology of such symmetry-protected systems. We thus show that placing the TCI surface of SnTe in proximity to EuS, a ferromagnetic insulator, induces magnetism at the interface between SnTe and EuS and thus breaks time-reversal-symmetry in the TCI. Magnetotransport experiments on SnTe-EuS-SnTe trilayer devices reveal a hysteretic lowering of the resistance at the TCI surface that coincides with an increase in the density of magnetic domain walls. This additional conduction could be a signature of topologically-protected surface states within domain walls. Additionally, a hysteretic anomalous Hall effect reveals that the usual in-plane magnetic moment of the EuS layer is canted towards a perpendicular direction at the interface. These results are evidence of induced magnetism at the SnTe-EuS interfaces resulting in broken time-reversal symmetry in the TCI.
△ Less
Submitted 23 April, 2015;
originally announced April 2015.
-
Breaking time reversal symmetry in topological insulators
Authors:
Cui-Zu Chang,
Peng Wei,
Jagadeesh. S. Moodera
Abstract:
A wide class of materials that were discovered to carry a topologically protected phase order has led to a highly active area of research called topological insulators. This phenomenon has radically changed our thinking because of their robust quantum coherent behavior showing two-dimensional Dirac-type metallic surface states and simultaneously insulating bulk states. The Dirac SSs are induced by…
▽ More
A wide class of materials that were discovered to carry a topologically protected phase order has led to a highly active area of research called topological insulators. This phenomenon has radically changed our thinking because of their robust quantum coherent behavior showing two-dimensional Dirac-type metallic surface states and simultaneously insulating bulk states. The Dirac SSs are induced by the strong spin-orbit coupling as well as protected by the time reversal symmetry. Breaking TRS in a TI with ferromagnetic perturbation can lead to many exotic quantum phenomena, such as the quantum anomalous Hall effect, topological magnetoelectric effect, as well as image magnetic monopole. This article presents an overview of the current status of TRS breaking in TIs and outlines the prospects for future studies
△ Less
Submitted 18 October, 2014;
originally announced October 2014.
-
Quantum Coherent Transport in SnTe Topological Crystalline Insulator Thin Films
Authors:
Badih A. Assaf,
Ferhat Katmis,
Peng Wei,
Biswarup Satpati,
Zhan Zhang,
Steven P. Bennett,
Vincent G. Harris,
Jagadeesh S. Moodera,
Don Heiman
Abstract:
Topological crystalline insulators (TCI) are unique systems where a band inversion that is protected by crystalline mirror symmetry leads to a multiplicity of topological surface states. Binary SnTe is an attractive lead-free TCI compound; the present work on high-quality thin films provides a route for increasing the mobility and reducing the carrier density of SnTe without chemical doping. Resul…
▽ More
Topological crystalline insulators (TCI) are unique systems where a band inversion that is protected by crystalline mirror symmetry leads to a multiplicity of topological surface states. Binary SnTe is an attractive lead-free TCI compound; the present work on high-quality thin films provides a route for increasing the mobility and reducing the carrier density of SnTe without chemical doping. Results of quantum coherent magnetotransport measurements reveal a multiplicity of Dirac surface states that are unique to TCI. Modeling of the weak antilocalization shows variations in the extracted number of carrier valleys that reflect the role of coherent intervalley scattering in coupling different Dirac states on the degenerate TCI surface.
△ Less
Submitted 3 December, 2014; v1 submitted 7 March, 2014;
originally announced March 2014.
-
Spin-filtered Edge States with an Electrically Tunable Gap in a Two-Dimensional Topological Crystalline Insulator
Authors:
Junwei Liu,
Timothy H. Hsieh,
Peng Wei,
Wenhui Duan,
Jagadeesh Moodera,
Liang Fu
Abstract:
Three-dimensional topological crystalline insulators were recently predicted and observed in the SnTe class of IV-VI semiconductors, which host metallic surface states protected by crystal symmetries. In this work, we study thin films of these materials and expose their potential for device applications. We demonstrate that thin films of SnTe and Pb(1-x)Sn(x)Se(Te) grown along the (001) direction…
▽ More
Three-dimensional topological crystalline insulators were recently predicted and observed in the SnTe class of IV-VI semiconductors, which host metallic surface states protected by crystal symmetries. In this work, we study thin films of these materials and expose their potential for device applications. We demonstrate that thin films of SnTe and Pb(1-x)Sn(x)Se(Te) grown along the (001) direction are topologically nontrivial in a wide range of film thickness and carry conducting spin-filtered edge states that are protected by the (001) mirror symmetry via a topological invariant. Application of an electric field perpendicular to the film will break the mirror symmetry and generate a band gap in these edge states. This functionality motivates us to propose a novel topological transistor device, in which charge and spin transport are maximally entangled and simultaneously controlled by an electric field. The high on/off operation speed and coupling of spin and charge in such a device may lead to electronic and spintronic applications for topological crystalline insulators.
△ Less
Submitted 31 October, 2013; v1 submitted 3 October, 2013;
originally announced October 2013.
-
Linear magnetoresistance in topological insulators: Quantum phase coherence effects at high temperatures
Authors:
Badih A. Assaf,
Thomas Cardinal,
Peng Wei,
Ferhat Katmis,
Jagadeesh S. Moodera,
Don Heiman
Abstract:
In addition to the weak antilocalization cusp observed in the magnetoresistance (MR) of topological insulators at low temperatures and low magnetic fields, we find that the high-field MR in Bi2Te2Se is linear in field. At fields up to B=14T the slope of this linear-like MR is nearly independent of temperature over the range T=7 to 150K. We find that the linear MR arises from the competition betwee…
▽ More
In addition to the weak antilocalization cusp observed in the magnetoresistance (MR) of topological insulators at low temperatures and low magnetic fields, we find that the high-field MR in Bi2Te2Se is linear in field. At fields up to B=14T the slope of this linear-like MR is nearly independent of temperature over the range T=7 to 150K. We find that the linear MR arises from the competition between a logarithmic phase coherence component and a quadratic component. The quantum phase coherence dominates up to high temperatures, where the coherence length remains longer than the mean free path of electrons.
△ Less
Submitted 4 January, 2013; v1 submitted 21 May, 2012;
originally announced May 2012.
-
Field-effect mobility enhanced by tuning the Fermi level into the band gap of Bi2Se3
Authors:
Peng Wei,
Zhiyong Wang,
Xinfei Liu,
Vivek Aji,
Jing Shi
Abstract:
By eliminating normal fabrication processes, we preserve the bulk insulating state of calcium-doped Bi2Se3 single crystals in suspended nanodevices, as indicated by the activated temperature dependence of the resistivity at low temperatures. We perform low-energy electron beam irradiation (<16 keV) and electrostatic gating to control the carrier density and therefore the Fermi level position in th…
▽ More
By eliminating normal fabrication processes, we preserve the bulk insulating state of calcium-doped Bi2Se3 single crystals in suspended nanodevices, as indicated by the activated temperature dependence of the resistivity at low temperatures. We perform low-energy electron beam irradiation (<16 keV) and electrostatic gating to control the carrier density and therefore the Fermi level position in the nanodevices. In slightly p-doped Bi2-xCaxSe3 devices, continuous tuning of the Fermi level from the bulk valence band to the band-gap reveals dramatic enhancement (> a factor of 10) in the field-effect mobility, which suggests suppressed backscattering expected for the Dirac fermion surface states in the gap of topological insulators.
△ Less
Submitted 2 July, 2012; v1 submitted 9 April, 2012;
originally announced April 2012.
-
Modified electrical transport probe design for standard magnetometer
Authors:
Badih A. Assaf,
Thomas Cardinal,
Peng Wei,
Ferhat Katmis,
Jagadeesh S. Moodera,
Don Heiman
Abstract:
Making electrical transport measurements on a material is often a time consuming process that involves testing a large number of samples. It is thus inconvenient to wire up and rewire samples on to a sample probe. We therefore present a method of modifying Quantum Design's MPMS SQUID magnetometer transport probe that simplifies the process of sample mounting. One of the difficulties to overcome is…
▽ More
Making electrical transport measurements on a material is often a time consuming process that involves testing a large number of samples. It is thus inconvenient to wire up and rewire samples on to a sample probe. We therefore present a method of modifying Quantum Design's MPMS SQUID magnetometer transport probe that simplifies the process of sample mounting. One of the difficulties to overcome is the small diameter of the sample space. A small socket is designed and mounted on the probe so that various samples mounted on individual headers can be readily exchanged in the socket. We also present some test results on the topological insulator Bi2Te2Se using the modified probe.
△ Less
Submitted 3 March, 2012;
originally announced March 2012.
-
Tuning carrier type and density in Bi2Se3 by Ca-doping
Authors:
Zhiyong Wang,
Tao Lin,
Peng Wei,
Xinfei Liu,
Randy Dumas,
Kai Liu,
Jing Shi
Abstract:
The carrier type and density in Bi2Se3 single crystals are systematically tuned by introducing a calcium (Ca) dopant. A carrier density of ~1x1017 cm-3 which corresponds to ~25 meV in the Fermi energy is obtained in both n- and p-type materials. Electrical transport properties show that the insulating behavior is achieved in low carrier density crystals. In addition, both the band gap and reduced…
▽ More
The carrier type and density in Bi2Se3 single crystals are systematically tuned by introducing a calcium (Ca) dopant. A carrier density of ~1x1017 cm-3 which corresponds to ~25 meV in the Fermi energy is obtained in both n- and p-type materials. Electrical transport properties show that the insulating behavior is achieved in low carrier density crystals. In addition, both the band gap and reduced effective mass of carriers are determined.
△ Less
Submitted 3 July, 2010;
originally announced July 2010.
-
Anomalous thermoelectric transport of Dirac particles in graphene
Authors:
Peng Wei,
Wenzhong Bao,
Yong Pu,
Chun Ning Lau,
Jing Shi
Abstract:
We report a thermoelectric study of graphene in both zero and applied magnetic fields. As a direct consequence of the linear dispersion of massless particles, we find that the Seebeck coefficient Sxx diverges with 1 /, where n2D is the carrier density. We observe a very large Nernst signal Sxy (~ 50 uV/K at 8 T) at the Dirac point, and an oscillatory dependence of both Sxx and Sxy on n2D at low…
▽ More
We report a thermoelectric study of graphene in both zero and applied magnetic fields. As a direct consequence of the linear dispersion of massless particles, we find that the Seebeck coefficient Sxx diverges with 1 /, where n2D is the carrier density. We observe a very large Nernst signal Sxy (~ 50 uV/K at 8 T) at the Dirac point, and an oscillatory dependence of both Sxx and Sxy on n2D at low temperatures. Our results underscore the anomalous thermoelectric transport in graphene, which may be used as a highly sensitive probe for impurity bands near the Dirac point.
△ Less
Submitted 2 April, 2009; v1 submitted 7 December, 2008;
originally announced December 2008.
-
Evidence For Heavy Hole and Light Hole Current Separation in P-Type Resonant Tunneling Diodes With Prewells
Authors:
R. M. Lewis,
H. P. Wei,
S. Y. Lin,
J. F. Klem
Abstract:
We investigate the transport of holes through $AlAs/In_{.10}Ga_{.90}As$ resonant tunneling diodes which utilize $In_xGa_{1-x}As$ prewells in the emitter with $x=0,.10,$ and $.20$. The data show an increase in peak current and bias at resonance and a concurrent increase in the peak-to-valley ratio with increasing x. We explain this enhancement in tunneling as due to confinement (or localiz- ation…
▽ More
We investigate the transport of holes through $AlAs/In_{.10}Ga_{.90}As$ resonant tunneling diodes which utilize $In_xGa_{1-x}As$ prewells in the emitter with $x=0,.10,$ and $.20$. The data show an increase in peak current and bias at resonance and a concurrent increase in the peak-to-valley ratio with increasing x. We explain this enhancement in tunneling as due to confinement (or localiz- ation) of charges in the prewell and the formation of direct heavy(light) hole to heavy(light) hole conduction channels as a consequence.
△ Less
Submitted 24 May, 2000;
originally announced May 2000.
-
Probing the plateau-insulator quantum phase transition in the quantum Hall regime
Authors:
R. T. F. van Schaijk,
A. de Visser,
S. Olsthoorn,
H. P. Wei,
A. M. M. Pruisken
Abstract:
We report quantum Hall experiments on the plateau-insulator transition in a low mobility In_{.53} Ga_{.47} As/InP heterostructure. The data for the longitudinal resistance ρ_{xx} follow an exponential law and we extract a critical exponent κ= .55 \pm .05 which is slightly different from the established value κ= .42 \pm .04 for the plateau transitions. Upon correction for inhomogeneity effects, w…
▽ More
We report quantum Hall experiments on the plateau-insulator transition in a low mobility In_{.53} Ga_{.47} As/InP heterostructure. The data for the longitudinal resistance ρ_{xx} follow an exponential law and we extract a critical exponent κ= .55 \pm .05 which is slightly different from the established value κ= .42 \pm .04 for the plateau transitions. Upon correction for inhomogeneity effects, which cause the critical conductance σ_{xx}^* to depend marginally on temperature, our data indicate that the plateau-plateau and plateau- insulator transitions are in the same universality class.
△ Less
Submitted 2 December, 1998;
originally announced December 1998.
-
Phonon Emission from a 2D Electron Gas: Evidence of Transition to the Hydrodynamic Regime
Authors:
Edmond Chow,
H. P. Wei,
S. M. Girvin,
M. Shayegan
Abstract:
Using as a thermometer the temperature dependent magneto-transport of a two-dimensional electron gas, we find that effective temperature scales with current as $T_{\rm e} \sim I^a$, where $a=0.4 \pm 2\%$ in the {\it Shubnikov de-Haas} regime, and $0.53 \pm 2\%$ in both the {\it integer and fractional} quantum Hall effect. This implies the phonon energy emission rate changes from the expected…
▽ More
Using as a thermometer the temperature dependent magneto-transport of a two-dimensional electron gas, we find that effective temperature scales with current as $T_{\rm e} \sim I^a$, where $a=0.4 \pm 2\%$ in the {\it Shubnikov de-Haas} regime, and $0.53 \pm 2\%$ in both the {\it integer and fractional} quantum Hall effect. This implies the phonon energy emission rate changes from the expected $P\sim T^5$ to $P\sim T^4$. We explain this, as well as the dramatic enhancement in phonon emission efficiency using a hydrodynamic model.
△ Less
Submitted 8 May, 1996;
originally announced May 1996.