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1.
Nat Commun ; 13(1): 1623, 2022 Mar 25.
Artigo em Inglês | MEDLINE | ID: mdl-35338125

RESUMO

The thermoelectric effects of topological semimetals have attracted tremendous research interest because many topological semimetals are excellent thermoelectric materials and thermoelectricity serves as one of their most important potential applications. In this work, we reveal the transient photothermoelectric response of Dirac semimetallic Cd3As2, namely the photo-Seebeck effect and photo-Nernst effect, by studying the terahertz (THz) emission from the transient photocurrent induced by these effects. Our excitation polarization and power dependence confirm that the observed THz emission is due to photothermoelectric effect instead of other nonlinear optical effect. Furthermore, when a weak magnetic field (~0.4 T) is applied, the response clearly indicates an order of magnitude enhancement on transient photothermoelectric current generation compared to the photo-Seebeck effect. Such enhancement supports an ambipolar transport nature of the photo-Nernst current generation in Cd3As2. These results highlight the enhancement of thermoelectric performance can be achieved in topological Dirac semimetals based on the Nernst effect, and our transient studies pave the way for thermoelectric devices applicable for high field circumstance when nonequilibrium state matters. The large THz emission due to highly efficient photothermoelectric conversion is comparable to conventional semiconductors through optical rectification and photo-Dember effect.

2.
Adv Mater ; : e2200145, 2022 Mar 26.
Artigo em Inglês | MEDLINE | ID: mdl-35338784

RESUMO

The discovery of 2D van der Waals (vdW) magnetic materials is of great significance to explore intriguing 2D magnetic physics and develop innovative spintronic devices. In this work, a new quasi-1D vdW layered compound CrZr4 Te14 is successfully synthesized. Owing to the existence of 1D [CrTe2 ] and [ZrTe3 ] chains along the b-axis, CrZr4 Te14 crystals show strong anisotropy of phonon vibrations, electrical transport, and magnetism. Density functional theory calculations reveal the ferromagnetic (FM) coupling within the [CrTe2 ] chain, while the interchain and interlayer couplings are both weakly antiferromagnetic (AF). Notably, a large intrinsic negative magnetoresistance (nMR) of -56% is achieved at 2 K under 9 T, and the in-plane anisotropic factor of nMR can reach up to 8.2 in the CrZr4 Te14 device. The 1D FM chains and anisotropic nMR effect make CrZr4 Te14 an interesting platform for exploring novel polarization-sensitive spintronics.

3.
ACS Nano ; 15(12): 19513-19521, 2021 Dec 28.
Artigo em Inglês | MEDLINE | ID: mdl-34894654

RESUMO

Recently, two-dimensional magnetic material has attracted attention worldwide due to its potential application in magnetic memory devices. The previous concept of domain walls driven by current pulses is a disordered motion. Further investigation of the mechanism is urgently lacking. Here, Fe3GeTe2, a typical high-Curie temperature (TC) two-dimensional magnetic material, is chosen to explore the magnetic domain dynamics by in situ Lorentz transmission electron microscopy experiments. It has been found that the stripe domain could be driven, compressed, and expanded by the pulses with a critical current density. Revealed by micromagnetic simulations, all the domain walls cannot move synchronously due to the competition between demagnetization energy and spin-transfer torque effect. In consideration of the reflection of high-frequency pulses, the disordered motion could be well explained together. The multiple stable states of the magnetic structure due to the weak exchange interaction in a two-dimensional magnet provides complex dynamic processes. Based on plenty of experiments, a cluster of domain walls could be more steady and move more synchronously under the drive of pulse current. The complication of domain wall motions presents a challenge in race track memory devices and two-dimensional magnetic material will be a better choice for application research.

4.
Nat Commun ; 12(1): 6580, 2021 Nov 12.
Artigo em Inglês | MEDLINE | ID: mdl-34772912

RESUMO

Superconductor-ferromagnet interfaces in two-dimensional heterostructures present a unique opportunity to study the interplay between superconductivity and ferromagnetism. The realization of such nanoscale heterostructures in van der Waals (vdW) crystals remains largely unexplored due to the challenge of making atomically-sharp interfaces from their layered structures. Here, we build a vdW ferromagnetic Josephson junction (JJ) by inserting a few-layer ferromagnetic insulator Cr2Ge2Te6 into two layers of superconductor NbSe2. The critical current and corresponding junction resistance exhibit a hysteretic and oscillatory behavior against in-plane magnetic fields, manifesting itself as a strong Josephson coupling state. Also, we observe a central minimum of critical current in some JJ devices as well as a nontrivial phase shift in SQUID structures, evidencing the coexistence of 0 and π phase in the junction region. Our study paves the way to exploring sensitive probes of weak magnetism and multifunctional building-blocks for phase-related superconducting circuits using vdW heterostructures.

5.
Phys Rev Lett ; 126(14): 147401, 2021 Apr 09.
Artigo em Inglês | MEDLINE | ID: mdl-33891459

RESUMO

Tunable terahertz plasmons are essential for reconfigurable photonics, which have been demonstrated in graphene through gating, though with relatively weak responses. Here we demonstrate strong terahertz plasmons in graphite thin films via infrared spectroscopy, with dramatic tunability by even a moderate temperature change or an in situ bias voltage. Meanwhile, through magnetoplasmon studies, we reveal that massive electrons and massless Dirac holes make comparable contributions to the plasmon response. Our study not only sets up a platform for further exploration of two-component plasmas, but also opens an avenue for terahertz modulation through electrical bias or all-optical means.

6.
Nat Commun ; 12(1): 386, 2021 Jan 15.
Artigo em Inglês | MEDLINE | ID: mdl-33452268

RESUMO

Plasmons in two-dimensional (2D) materials beyond graphene have recently gained much attention. However, the experimental investigation is limited due to the lack of suitable materials. Here, we experimentally demonstrate localized plasmons in a correlated 2D charge-density-wave (CDW) material: 2H-TaSe2. The plasmon resonance can cover a broad spectral range from the terahertz (40 µm) to the telecom (1.55 µm) region, which is further tunable by changing thickness and dielectric environments. The plasmon dispersion flattens at large wave vectors, resulted from the universal screening effect of interband transitions. More interestingly, anomalous temperature dependence of plasmon resonances associated with CDW excitations is observed. In the CDW phase, the plasmon peak close to the CDW excitation frequency becomes wider and asymmetric, mimicking two coupled oscillators. Our study not only reveals the universal role of the intrinsic screening on 2D plasmons, but also opens an avenue for tunable plasmons in 2D correlated materials.

7.
Nano Lett ; 21(1): 288-297, 2021 Jan 13.
Artigo em Inglês | MEDLINE | ID: mdl-33346673

RESUMO

The motion of Abrikosov vortices is the dominant origin of dissipation in type II superconductors subjected to a magnetic field, which leads to a finite electrical resistance. It is generally believed that the increase in the magnetic field results in the aggravation of energy dissipation through the increase in vortex density. Here, we show a distinctive re-entrance of the dissipationless state in quasi-one-dimensional superconducting Ta2PdS5 nanostrips. Utilizing magnetotransport measurements, we unveil a prominent magnetoresistance drop with the increase in the magnetic field below the superconducting transition temperature, manifesting itself as a giant re-entrance to the superconducting phase. Time-dependent Ginzburg-Landau calculations show that this is originated from the suppression of the vortex motion by the increased energy barrier on the edges. Interestingly, both our experiments and simulations demonstrate that this giant re-entrance of superconductivity occurs only in certain geometrical regimes because of the finite size of the vortex.

8.
Nat Commun ; 11(1): 5634, 2020 Nov 06.
Artigo em Inglês | MEDLINE | ID: mdl-33159059

RESUMO

The rise of two-dimensional (2D) crystalline superconductors has opened a new frontier of investigating unconventional quantum phenomena in low dimensions. However, despite the enormous advances achieved towards understanding the underlying physics, practical device applications like sensors and detectors using 2D superconductors are still lacking. Here, we demonstrate nonreciprocal antenna devices based on atomically thin NbSe2. Reversible nonreciprocal charge transport is unveiled in 2D NbSe2 through multi-reversal antisymmetric second harmonic magnetoresistance isotherms. Based on this nonreciprocity, our NbSe2 antenna devices exhibit a reversible nonreciprocal sensitivity to externally alternating current (AC) electromagnetic waves, which is attributed to the vortex flow in asymmetric pinning potentials driven by the AC driving force. More importantly, a successful control of the nonreciprocal sensitivity of the antenna devices has been achieved by applying electromagnetic waves with different frequencies and amplitudes. The device's response increases with increasing electromagnetic wave amplitude and exhibits prominent broadband sensing from 5 to 900 MHz.

9.
Nano Lett ; 20(10): 7004-7010, 2020 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-32897723

RESUMO

Stimulated by novel properties in topological insulators, experimentally realizing quantum phases of matter and employing control over their properties have become a central goal in condensed matter physics. ß-silver telluride (Ag2Te) is predicted to be a new type narrow-gap topological insulator. While enormous efforts have been plunged into the topological nature in silver chalcogenides, sophisticated research on low-dimensional nanostructures remains unexplored. Here, we report the record-high bulk carrier mobility of 298 600 cm2/(V s) in high-quality Ag2Te nanoplates and the coexistence of the surface and bulk state from systematic Shubnikov-de Haas oscillations measurements. By tuning the correlation between the top and bottom surfaces, we can effectively enhance the contribution of the surface to the total conductance up to 87% at 130 V. These results are instrumental to the high-mobility physics study and even suitable to explore exotic topological phenomena in this material system.

10.
ACS Nano ; 14(8): 9512-9520, 2020 Aug 25.
Artigo em Inglês | MEDLINE | ID: mdl-32538609

RESUMO

Recently, 2D ferromagnetic materials have aroused wide interest for their magnetic properties and potential applications in spintronic and topological devices. However, their actual applications have been severely hindered by intricate challenges such as the unclear spin arrangement. In particular, the evolution of spin texture driven by high-density electron current, which is an essential condition for fabricating devices, remains unclear. Herein, the current-pulse-driven spin textures in 2D ferromagnetic material Fe3GeTe2 have been thoroughly investigated by in situ Lorentz transmission electron microscopy. The dynamic experiments reveal that the stripe domain structure in the AB and AC planes can be broken and rearranged by the high-density current. In particular, the density of domain walls can be modulated, which offers an avenue to achieve a high-density domain structure. This phenomenon is attributed to the weak interlayer exchange interaction in 2D metallic ferromagnetic materials and the strong disturbance from the high-density current. Therefore, a bubble domain structure and random magnetization in Fe3GeTe2 can be acquired by synchronous current pulses and magnetic fields. These achievements reveal domain structure transitions driven by the current in 2D metallic magnetic materials and provide references for the practical applications.

11.
Nat Commun ; 11(1): 2451, 2020 May 15.
Artigo em Inglês | MEDLINE | ID: mdl-32415119

RESUMO

Harmonic generation is a general characteristic of driven nonlinear systems, and serves as an efficient tool for investigating the fundamental principles that govern the ultrafast nonlinear dynamics. Here, we report on terahertz-field driven high-harmonic generation in the three-dimensional Dirac semimetal Cd3As2 at room temperature. Excited by linearly-polarized multi-cycle terahertz pulses, the third-, fifth-, and seventh-order harmonic generation is very efficient and detected via time-resolved spectroscopic techniques. The observed harmonic radiation is further studied as a function of pump-pulse fluence. Their fluence dependence is found to deviate evidently from the expected power-law dependence in the perturbative regime. The observed highly non-perturbative behavior is reproduced based on our analysis of the intraband kinetics of the terahertz-field driven nonequilibrium state using the Boltzmann transport theory. Our results indicate that the driven nonlinear kinetics of the Dirac electrons plays the central role for the observed highly nonlinear response.

12.
Nat Commun ; 11(1): 1259, 2020 Mar 06.
Artigo em Inglês | MEDLINE | ID: mdl-32144239

RESUMO

The experimental discovery of Weyl semimetals offers unprecedented opportunities to study Weyl physics in condensed matters. Unique electromagnetic response of Weyl semimetals such as chiral magnetic effect has been observed and presented by the axial θ E · B term in electromagnetic Lagrangian (E and B are the electric and magnetic field, respectively). But till now, the experimental progress in this direction in Weyl semimetals is restricted to the DC regime. Here we report experimental access to the dynamic regime in Weyl semimetal NbAs by combining the internal deformation potential of coupled phonons with applied static magnetic field. While the dynamic E · B field is realized, it produces an anomalous phonon activity with a characteristic angle-dependence. Our results provide an effective approach to achieve the dynamic regime beyond the widely-investigated DC limit which enables the coupling between the Weyl fermions and the electromagnetic wave for further study of novel light-matter interactions in Weyl semimetals.

13.
Phys Rev Lett ; 125(26): 267205, 2020 Dec 31.
Artigo em Inglês | MEDLINE | ID: mdl-33449751

RESUMO

The recent discovery of intrinsic ferromagnetism in two-dimensional (2D) van der Waals (vdW) crystals has opened up a new arena for spintronics, raising an opportunity of achieving tunable intrinsic 2D vdW magnetism. Here, we show that the magnetization and the magnetic anisotropy energy (MAE) of few-layered Fe_{3}GeTe_{2} (FGT) is strongly modulated by a femtosecond laser pulse. Upon increasing the femtosecond laser excitation intensity, the saturation magnetization increases in an approximately linear way and the coercivity determined by the MAE decreases monotonically, showing unambiguously the effect of the laser pulse on magnetic ordering. This effect observed at room temperature reveals the emergence of light-driven room-temperature (300 K) ferromagnetism in 2D vdW FGT, as its intrinsic Curie temperature T_{C} is ∼200 K. The light-tunable ferromagnetism is attributed to the changes in the electronic structure due to the optical doping effect. Our findings pave a novel way to optically tune 2D vdW magnetism and enhance the T_{C} up to room temperature, promoting spintronic applications at or above room temperature.

14.
Natl Sci Rev ; 7(9): 1468-1475, 2020 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-34691543

RESUMO

WTe2, as a type-II Weyl semimetal, has 2D Fermi arcs on the (001) surface in the bulk and 1D helical edge states in its monolayer. These features have recently attracted wide attention in condensed matter physics. However, in the intermediate regime between the bulk and monolayer, the edge states have not been resolved owing to its closed band gap which makes the bulk states dominant. Here, we report the signatures of the edge superconductivity by superconducting quantum interference measurements in multilayer WTe2 Josephson junctions and we directly map the localized supercurrent. In thick WTe2 ([Formula: see text], the supercurrent is uniformly distributed by bulk states with symmetric Josephson effect ([Formula: see text]). In thin WTe2 (10 nm), however, the supercurrent becomes confined to the edge and its width reaches up to [Formula: see text]and exhibits non-symmetric behavior [Formula: see text]. The ability to tune the edge domination by changing thickness and the edge superconductivity establishes WTe2 as a promising topological system with exotic quantum phases and a rich physics.

15.
Natl Sci Rev ; 7(4): 745-754, 2020 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-34692093

RESUMO

Mechanically exfoliated two-dimensional ferromagnetic materials (2D FMs) possess long-range ferromagnetic order and topologically nontrivial skyrmions in few layers. However, because of the dimensionality effect, such few-layer systems usually exhibit much lower Curie temperature (T C) compared to their bulk counterparts. It is therefore of great interest to explore effective approaches to enhance their T C, particularly in wafer-scale for practical applications. Here, we report an interfacial proximity-induced high-T C 2D FM Fe3GeTe2 (FGT) via A-type antiferromagnetic material CrSb (CS) which strongly couples to FGT. A superlattice structure of (FGT/CS)n, where n stands for the period of FGT/CS heterostructure, has been successfully produced with sharp interfaces by molecular-beam epitaxy on 2-inch wafers. By performing elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally discovered that T C of 4-layer Fe3GeTe2 can be significantly enhanced from 140 K to 230 K because of the interfacial ferromagnetic coupling. Meanwhile, an inverse proximity effect occurs in the FGT/CS interface, driving the interfacial antiferromagnetic CrSb into a ferrimagnetic state as evidenced by double-switching behavior in hysteresis loops and the XMCD spectra. Density functional theory calculations show that the Fe-Te/Cr-Sb interface is strongly FM coupled and doping of the spin-polarized electrons by the interfacial Cr layer gives rise to the T C enhancement of the Fe3GeTe2 films, in accordance with our XMCD measurements. Strikingly, by introducing rich Fe in a 4-layer FGT/CS superlattice, T C can be further enhanced to near room temperature. Our results provide a feasible approach for enhancing the magnetic order of few-layer 2D FMs in wafer-scale and render opportunities for realizing realistic ultra-thin spintronic devices.

16.
Opt Lett ; 44(17): 4103-4106, 2019 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-31465339

RESUMO

In this Letter, we successfully introduce a long-lived non-radiative photocarrier decay component in a Dirac semimetal Cd3As2 thin film via Mn doping. The long-lived decay component is found to vary between 200 ps and 2.8 ns with different Mn concentrations and probing wavelengths. Most remarkably, the elongated transients persist over the important mid-infrared wavelengths (observed up to 4 µm). Saturable absorption measurement reveals stronger modulation effects for long-width pulses (∼80 ps) from the Mn-doped samples. Our results provide new insights into the effect of transition-metal doping on the ultrafast optical properties of Dirac semimetal Cd3As2 and establish Cd3As2 as a highly amendable material for mid-infrared photonic applications.

17.
J Phys Chem Lett ; 10(14): 3914-3921, 2019 Jul 18.
Artigo em Inglês | MEDLINE | ID: mdl-31248258

RESUMO

In this work, high-performance ultraviolet to long-wave infrared (UV-LIR) devices based on an N-type three-dimensional (3D) Dirac semimetal Cd3As2 and P-type organic (small molecules and polymers) heterojunction are prepared. Primarily, the photodetector shows a broadband photoresponse from 365 to 10600 nm. The optimized device responsivity is 729 mA/W, along with a fast response time of 282 µs and a high on-off ratio of 6268, which are 2 orders of magnitude higher than those previously reported for a 3D Dirac semimetal-based device. In the LIR region (10600 nm), the responsivity and on-off ratio can reach 81.3 mA/W and 100, respectively. In addition, the time-resolved femtosecond pump detection technology is used to reveal the relaxation time of Cd3As2/organic thin films (4.30 ps), indicating that Cd3As2/organic thin films have great potential for the manufacture of fast IR devices. These results demonstrate that the 3D Dirac semimetal/organic thin film heterojunction photodetectors will be a feasible solution for high-speed and broadband photodetectors in large-array imaging.

18.
Nat Commun ; 10(1): 2217, 2019 05 17.
Artigo em Inglês | MEDLINE | ID: mdl-31101813

RESUMO

Cd3As2 is a three-dimensional Dirac semimetal with separated Dirac points in momentum space. In spite of extensive transport and spectroscopic studies on its exotic properties, the evidence of superconductivity in its surface states remains elusive. Here, we report the observation of proximity-induced surface superconductivity in Nb/Cd3As2 hybrid structures. Our four-terminal transport measurement identifies a pronounced proximity-induced pairing gap (gap size comparable to Nb) on the surfaces, which exhibits a flat conductance plateau in differential conductance spectra, consistent with our theoretical simulations. The surface supercurrent from Nb/Cd3As2/Nb junctions is also achieved with a Fraunhofer/SQUID-like pattern under out-of-plane/in-plane magnetic fields, respectively. The resultant mapping shows a predominant distribution on the top and bottom surfaces as the bulk carriers are depleted, which can be regarded as a higher dimensional analog of edge supercurrent in two-dimensional quantum spin Hall insulators. Our study provides the evidence of surface superconductivity in Dirac semimetals.

19.
J Phys Condens Matter ; 31(28): 285802, 2019 Jul 17.
Artigo em Inglês | MEDLINE | ID: mdl-30939461

RESUMO

We report high-precision magnetization ([Formula: see text]), magnetic susceptibility ([Formula: see text]), specific heat (C p (T, H)) and 'zero-field' electrical resistivity, [Formula: see text], data taken on Gd2Te3 single crystal over wide ranges of temperature and magnetic field (H), with either [Formula: see text]-axis or [Formula: see text]-plane. [Formula: see text] and [Formula: see text] unambiguously establish that the b-axis is the easy direction of magnetization whereas any direction in the ac-plane is a hard direction. The [Formula: see text]-type anomaly in 'zero-field' specific heat, C p (T, H = 0), and an abrupt drop in [Formula: see text] (characteristic of the paramagnetic (PM) - antiferromagnetic (AFM) phase transition) are observed at the Néel temperature, [Formula: see text] K. [Formula: see text] and C p (T,H) clearly demonstrate that [Formula: see text] shifts to lower temperatures with increasing H irrespective of whether H points in the easy or hard direction. When [Formula: see text], the [Formula: see text] isotherms at temperatures in the range 2.5 K [Formula: see text] [Formula: see text] K reveal the existence of a field-induced spin-flop (SF) transition at fields 4.0 T [Formula: see text] [Formula: see text] [Formula: see text] 4.5 T. The first principles electronic band structure and density of states calculations, based on the density functional theory, correctly predict an AFM ground state (stabilized primarily by the 4f  Gd3+ - 5p  Te2-- 4f  Gd3+ superexchange interactions) and the observed semi-metallic behavior for the Gd2Te3 compound. Moreover, these calculations yield the values [Formula: see text] [Formula: see text] for the ordered magnetic moment per Gd atom at T = 0, [Formula: see text] mJ mol-1 K-2 for the Sommerfeld coefficient for the electronic specific heat contribution and [Formula: see text] K for the Curie-Weiss temperature, respectively. These theoretical estimates conform well with the corresponding experimental values [Formula: see text] [Formula: see text], [Formula: see text] mJ mol-1 K-2 and [Formula: see text] K.

20.
Nat Mater ; 18(5): 482-488, 2019 05.
Artigo em Inglês | MEDLINE | ID: mdl-30886399

RESUMO

In two-dimensional (2D) systems, high mobility is typically achieved in low-carrier-density semiconductors and semimetals. Here, we discover that the nanobelts of Weyl semimetal NbAs maintain a high mobility even in the presence of a high sheet carrier density. We develop a growth scheme to synthesize single crystalline NbAs nanobelts with tunable Fermi levels. Owing to a large surface-to-bulk ratio, we argue that a 2D surface state gives rise to the high sheet carrier density, even though the bulk Fermi level is located near the Weyl nodes. A surface sheet conductance up to 5-100 S per □ is realized, exceeding that of conventional 2D electron gases, quasi-2D metal films, and topological insulator surface states. Corroborated by theory, we attribute the origin of the ultrahigh conductance to the disorder-tolerant Fermi arcs. The evidenced low-dissipation property of Fermi arcs has implications for both fundamental study and potential electronic applications.

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