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Engineering 2D material exciton lineshape with graphene/h-BN encapsulation
Authors:
Steffi Y. Woo,
Fuhui Shao,
Ashish Arora,
Robert Schneider,
Nianjheng Wu,
Andrew J. Mayne,
Ching-Hwa Ho,
Mauro Och,
Cecilia Mattevi,
Antoine Reserbat-Plantey,
Alvaro Moreno,
Hanan Herzig Sheinfux,
Kenji Watanabe,
Takashi Taniguchi,
Steffen Michaelis de Vasconcellos,
Frank H. L. Koppens,
Zhichuan Niu,
Odile Stéphan,
Mathieu Kociak,
F. Javier García de Abajo,
Rudolf Bratschitsch,
Andrea Konečná,
Luiz H. G. Tizei
Abstract:
Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Remarkable advances have been achieved through alloying, chemical and electrical doping, and applied strain. However, the integration of TMDs with other 2D materials in van der Waals heterostructures (vdWHs…
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Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Remarkable advances have been achieved through alloying, chemical and electrical doping, and applied strain. However, the integration of TMDs with other 2D materials in van der Waals heterostructures (vdWHs) to tailor novel functionalities remains largely unexplored. Here, the near-field coupling between TMDs and graphene/graphite is used to engineer the exciton lineshape and charge state. Fano-like asymmetric spectral features are produced in WS$_{2}$, MoSe$_{2}$ and WSe$_{2}$ vdWHs combined with graphene, graphite, or jointly with hexagonal boron nitride (h-BN) as supporting or encapsulating layers. Furthermore, trion emission is suppressed in h-BN encapsulated WSe$_{2}$/graphene with a neutral exciton redshift (44 meV) and binding energy reduction (30 meV). The response of these systems to electron-beam and light probes is well-described in terms of 2D optical conductivities of the involved materials. Beyond fundamental insights into the interaction of TMD excitons with structured environments, this study opens an unexplored avenue toward shaping the spectral profile of narrow optical modes for application in nanophotonic devices.
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Submitted 13 November, 2023;
originally announced November 2023.
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Substrate influence on transition metal dichalcogenide monolayer exciton absorption linewidth broadening
Authors:
Fuhui Shao,
Steffi Y. Woo,
Nianjheng Wu,
Robert Schneider,
Andrew J. Mayne,
Steffen Michaelis de Vasconcellos,
Ashish Arora,
Benjamin J. Carey,
Johann A. Preuß,
Noémie Bonnet,
Cecilia Mattevi,
Kenji Watanabe,
Takashi Taniguchi,
Zhichuan Niu,
Rudolf Bratschitsch,
Luiz H. G. Tizei
Abstract:
The excitonic states of transition metal dichacolgenide (TMD) monolayers are heavily influenced by their external dielectric environment based on the substrate used. In this work, various wide bandgap dielectric materials, namely hexagonal boron nitride (\textit{h}-BN) and amorphous silicon nitride (Si$_3$N$_4$), under different configurations as support or encapsulation material for WS$_2$ monola…
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The excitonic states of transition metal dichacolgenide (TMD) monolayers are heavily influenced by their external dielectric environment based on the substrate used. In this work, various wide bandgap dielectric materials, namely hexagonal boron nitride (\textit{h}-BN) and amorphous silicon nitride (Si$_3$N$_4$), under different configurations as support or encapsulation material for WS$_2$ monolayers are investigated to disentangle the factors contributing to inhomogeneous broadening of exciton absorption lines in TMDs using electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). In addition, monolayer roughness in each configuration was determined from tilt series of electron diffraction patterns by assessing the broadening of diffraction spots by comparison with simulations. From our experiments, the main factors that play a role in linewidth broadening can be classified in increasing order of importance by: monolayer roughness, surface cleanliness, and substrate-induced charge trapping. Furthermore, because high-energy electrons are used as a probe, electron beam-induced damage on bare TMD monolayer is also revealed to be responsible for irreversible linewidth increases. \textit{h}-BN not only provides clean surfaces of TMD monolayer, and minimal charge disorder, but can also protect the TMD from irradiation damage. This work provides a better understanding of the mechanisms by which \textit{h}-BN remains, to date, the most compatible material for 2D material encapsulation, facilitating the realization of intrinsic material properties to their full potential.
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Submitted 9 February, 2022;
originally announced February 2022.
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Chemical tuning of spin clock transitions in molecular monomers based on nuclear spin-free Ni(II)
Authors:
Marcos Rubín-Osanz,
François Lambert,
Feng Shao,
Eric Rivière,
Régis Guillot,
Nicolas Suaud,
Nathalie Guihéry,
David Zueco,
Anne-Laure Barra,
Talal Mallah,
Fernando Luis
Abstract:
We report the existence of a sizeable quantum tunnelling splitting between the two lowest electronic spin levels of mononuclear Ni complexes. The level anti-crossing, or magnetic clock transition, associated with this gap has been directly monitored by heat capacity experiments. The comparison of these results with those obtained for a Co derivative, for which tunnelling is forbidden by symmetry,…
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We report the existence of a sizeable quantum tunnelling splitting between the two lowest electronic spin levels of mononuclear Ni complexes. The level anti-crossing, or magnetic clock transition, associated with this gap has been directly monitored by heat capacity experiments. The comparison of these results with those obtained for a Co derivative, for which tunnelling is forbidden by symmetry, shows that the clock transition leads to an effective suppression of intermolecular spin-spin interactions. In addition, we show that the quantum tunnelling splitting admits a chemical tuning via the modification of the ligand shell that determines the crystal field and the magnetic anisotropy. These properties are crucial to realize model spin qubits that combine the necessary resilience against decoherence, a proper interfacing with other qubits and with the control circuitry and the ability to initialize them by cooling.
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Submitted 4 March, 2021;
originally announced March 2021.
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Nanoscale modification of WS$_2$ trion emission by its local electromagnetic environment
Authors:
Noémie Bonnet,
Hae Yeon Lee,
Fuhui Shao,
Steffi Y. Woo,
Jean-Denis Blazit,
Kenji Watanabe,
Takashi Taniguchi,
Alberto Zobelli,
Odile Stéphan,
Mathieu Kociak,
Silvija Gradecak-Garaj,
Luiz H. G. Tizei
Abstract:
Structural, electronic, and chemical nanoscale modifications of transition metal dichalcogenide monolayers alter their optical properties, including the generation of single photon emitters. A key missing element for complete control is a direct spatial correlation of optical response to nanoscale modifications, due to the large gap in spatial resolution between optical spectroscopy and nanometer…
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Structural, electronic, and chemical nanoscale modifications of transition metal dichalcogenide monolayers alter their optical properties, including the generation of single photon emitters. A key missing element for complete control is a direct spatial correlation of optical response to nanoscale modifications, due to the large gap in spatial resolution between optical spectroscopy and nanometer resolved techniques, such as transmission electron microscopy or scanning tunneling microscopy. Here, we bridge this gap by obtaining nanometer resolved optical properties using electron spectroscopy, specifically electron energy loss spectroscopy (EELS) for absorption and cathodoluminescence (CL) for emission, which were directly correlated to chemical and structural information. In an h-BN/WS$_2$/h-BN heterostructure, we observe local modulation of the trion (X$^{-}$) emission due to tens of nanometer wide dielectric patches, while the exciton, X$_A$, does not follow the same modulation. Trion emission also increases in regions where charge accumulation occurs, close to the carbon film supporting the heterostructures. Finally, localized exciton emission (L) detection is not correlated to strain variations above 1 $\%$, suggesting point defects might be involved in their formations.
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Submitted 12 February, 2021; v1 submitted 11 February, 2021;
originally announced February 2021.
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Controlling spin current polarization through non-collinear antiferromagnetism
Authors:
T. Nan,
C. X. Quintela,
J. Irwin,
G. Gurung,
D. F. Shao,
J. Gibbons,
N. Campbell,
K. Song,
S. Y. Choi,
L. Guo,
R. D. Johnson,
P. Manuel,
R. V. Chopdekar,
I. Hallsteinsen,
T. Tybell,
P. J. Ryan,
J. W. Kim,
Y. S. Choi,
P. G. Radaelli,
D. C. Ralph,
E. Y. Tsymba,
M. S. Rzchowski,
C. B. Eom
Abstract:
The spin-Hall effect describes the interconversion of charge currents and spin currents, enabling highly efficient manipulation of magnetization for spintronics. Symmetry conditions generally restrict polarizations of these spin currents to be orthogonal to both the charge and spin flows. Spin polarizations can deviate from such direction in nonmagnetic materials only when the crystalline symmetry…
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The spin-Hall effect describes the interconversion of charge currents and spin currents, enabling highly efficient manipulation of magnetization for spintronics. Symmetry conditions generally restrict polarizations of these spin currents to be orthogonal to both the charge and spin flows. Spin polarizations can deviate from such direction in nonmagnetic materials only when the crystalline symmetry is reduced11. Here we experimentally show control of the spin polarization direction by using a non-collinear antiferromagnet Mn$_{3}$GaN, in which the triangular spin structure creates a low magnetic symmetry state while maintaining a high crystalline symmetry. We demonstrate that epitaxial Mn3GaN/Permalloy heterostructures can generate unique types of spinHall torques at room temperature corresponding to unconventional spin polarizations collinear to spin currents or charge currents which are forbidden in any sample with two-fold rotational symmetry. Our results demonstrate an approach based on spin-structure design for controlling spinorbit torque, paving the way for further progress in the emergent field of antiferromagnetic spintronics.
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Submitted 29 December, 2019;
originally announced December 2019.
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Nonvolatile ferroelectric field control of the anomalous Hall effect in BiFeO3/SrRuO3 bilayer
Authors:
Z. Y. Ren,
Z. Yuan,
L. F. Wang,
F. Shao,
P. F. Liu,
J. Teng,
K. K. Meng,
X. G. Xu,
J. Miao,
Y. Jiang
Abstract:
In this work, the BiFeO3 (BFO)/SrRuO3 (SRO) heterostructure was fabricated and the anomalous Hall effect (AHE) was investigated the in BFO/SRO. It is found the nonmonotonic anomalous Hall resistivity behavior in BFO/SRO is originated from the inhomogeneous SRO layer instead of the topological Hall effect. It is surprised that the AHE in BFO/SRO structure can be manipulated by ferroelectric polariz…
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In this work, the BiFeO3 (BFO)/SrRuO3 (SRO) heterostructure was fabricated and the anomalous Hall effect (AHE) was investigated the in BFO/SRO. It is found the nonmonotonic anomalous Hall resistivity behavior in BFO/SRO is originated from the inhomogeneous SRO layer instead of the topological Hall effect. It is surprised that the AHE in BFO/SRO structure can be manipulated by ferroelectric polarization of BFO. Moreover, an inhomogeneous phenomenological model has been applied on those structure. Furthermore, the modification of band structure in SRO under ferroelectric polarization was discussed by first principle calculation. The ferroelectric-manipulated AHE suggests a new pathway to realize nonvolatile, reversible and low energy-consuming voltage-controlled spintronic devices.
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Submitted 6 October, 2019;
originally announced October 2019.
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Manipulating superconductivity of $1T$-TiTe$_2$ by high pressure
Authors:
R. C. Xiao,
W. J. Lu,
D. F. Shao,
J. Y. Li,
M. J. Wei,
H. Y. Lv,
P. Tong,
X. B. Zhu,
Y. P. Sun
Abstract:
Superconductivity of transition metal dichalcogenide $1T$-TiTe$_2$ under high pressure was investigated by the first-principles calculations. Our results show that the superconductivity of $1T$-TiTe$_2$ exhibits very different behavior under the hydrostatic and uniaxial pressure. The hydrostatic pressure is harmful to the superconductivity, while the uniaxial pressure is beneficial to the supercon…
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Superconductivity of transition metal dichalcogenide $1T$-TiTe$_2$ under high pressure was investigated by the first-principles calculations. Our results show that the superconductivity of $1T$-TiTe$_2$ exhibits very different behavior under the hydrostatic and uniaxial pressure. The hydrostatic pressure is harmful to the superconductivity, while the uniaxial pressure is beneficial to the superconductivity. Superconducting transition temperature $T_C$ at ambient pressure is 0.73 K, and it reduces monotonously under the hydrostatic pressure to 0.32 K at 30 GPa. While the $T_C$ increases dramatically under the uniaxial pressure along $c$ axis. The established $T_C$ of 6.34 K under the uniaxial pressure of 17 GPa, below which the structural stability maintains, is above the liquid helium temperature of 4.2 K. The increase of density of states at Fermi level, the redshift of $F(ω)$/$α^2F(ω)$ and the softening of the acoustic modes with pressure are considered as the main reasons that lead to the enhanced superconductivity under uniaxial pressure. In view of the previously predicted topological phase transitions of $1T$-TiTe$_2$ under the uniaxial pressure [Phys. Rev. B 88, 155317 (2013)], we consider $1T$-TiTe$_2$ as a possible candidate in transition metal chalcogenides for exploring topological superconductivity.
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Submitted 14 January, 2017; v1 submitted 7 January, 2017;
originally announced January 2017.
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Enhanced superconductivity by strain and carrier-doping in borophene: A first principles prediction
Authors:
R. C. Xiao,
D. F. Shao,
W. J. Lu,
H. Y. Lv,
J. Y. Li,
Y. P. Sun
Abstract:
We predict by first principles calculations that the recently prepared borophene is a pristine two-dimensional (2D) monolayer superconductor, in which the superconductivity can be significantly enhanced by strain and charge carrier doping. The intrinsic metallic ground state with high density of states at Fermi energy and strong Fermi surface nesting lead to sizeable electron-phonon coupling, maki…
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We predict by first principles calculations that the recently prepared borophene is a pristine two-dimensional (2D) monolayer superconductor, in which the superconductivity can be significantly enhanced by strain and charge carrier doping. The intrinsic metallic ground state with high density of states at Fermi energy and strong Fermi surface nesting lead to sizeable electron-phonon coupling, making the freestanding borophene superconduct with $T_c$ close to 19.0 K. The tensile strain can increase $T_c$ to 27.4 K, while the hole doping can notably increase $T_c$ to 34.8 K. The results indicate that the borophene grown on substrates with large lattice parameters or under photoexcitation can show enhanced superconductivity with $T_c$ far more above liquid hydrogen temperature of 20.3 K, which will largely broaden the applications of such novel material.
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Submitted 21 April, 2016;
originally announced April 2016.
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Strain-induced enhancement of thermoelectric performance in a ZrS2 monolayer
Authors:
H. Y. Lv,
W. J. Lu,
D. F. Shao,
H. Y. Lu,
Y. P. Sun
Abstract:
The increase of a thermoelectric material's figure of merit (ZT value) is limited by the interplay of the transport coefficients. Here we report the greatly enhanced thermoelectric performance of a ZrS2 monolayer by the biaxial tensile strain, due to the simultaneous increase of the Seebeck coefficient and decrease of the thermal conductivity. Based on the first-principles calculations combined wi…
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The increase of a thermoelectric material's figure of merit (ZT value) is limited by the interplay of the transport coefficients. Here we report the greatly enhanced thermoelectric performance of a ZrS2 monolayer by the biaxial tensile strain, due to the simultaneous increase of the Seebeck coefficient and decrease of the thermal conductivity. Based on the first-principles calculations combined with the Boltzmann transport theory, we predict the band gap of the ZrS2 monolayer can be effectively engineered by the strain and the Seebeck coefficient is significantly increased. The thermal conductivity is reduced by the applied tensile strain due to the phonon softening. At the strain of 6%, the maximal ZT value of 2.4 is obtained for the p-type doped ZrS2 monolayer at 300 K, which is 4.3 times larger than that of the unstrained system.
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Submitted 2 March, 2016; v1 submitted 27 February, 2016;
originally announced February 2016.
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Nature of charge density waves and superconductivity in 1\emph{T}-TaSe$_{2-x}$Te$_x$
Authors:
Y. Liu,
D. F. Shao,
L. J. Li,
W. J. Lu,
X. D. Zhu,
P. Tong,
R. C. Xiao,
L. S. Ling,
C. Y. Xi,
L. Pi,
H. F. Tian,
H. X. Yang,
J. Q. Li,
W. H. Song,
X. B. Zhu,
Y. P. Sun
Abstract:
Transition-metal dichalcogenides (TMDs) $MX_2$ ($M$ = Ti, Nb, Ta; $X$ = S, Se, Te) exhibit a rich set of charge density wave (CDW) orders, which usually coexist and/or compete with superconductivity. The mechanisms of CDWs and superconductivity in TMDs are still under debate. Here we perform an investigation on a typical TMD system, 1\emph{T}-TaSe$_{2-x}$Te$_x$ ($0 \leq x \leq 2$). Doping-induced…
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Transition-metal dichalcogenides (TMDs) $MX_2$ ($M$ = Ti, Nb, Ta; $X$ = S, Se, Te) exhibit a rich set of charge density wave (CDW) orders, which usually coexist and/or compete with superconductivity. The mechanisms of CDWs and superconductivity in TMDs are still under debate. Here we perform an investigation on a typical TMD system, 1\emph{T}-TaSe$_{2-x}$Te$_x$ ($0 \leq x \leq 2$). Doping-induced disordered distribution of Se/Te suppresses CDWs in 1\emph{T}-TaSe$_2$. A domelike superconducting phase with the maximum $T_\textrm{c}^{\textrm{onset}}$ of 2.5 K was observed near CDWs. The superconducting volume is very small inside the CDW phase and becomes very large instantly when the CDW phase is fully suppressed. The observations can be understood based on the strong \emph{\textbf{q}}-dependent electron-phonon coupling-induced periodic-lattice-distortion (PLD) mechanism of CDWs. The volume variation of superconductivity implies the emergence of domain walls in the suppressing process of CDWs. Our concluded scenario makes a fundamental understanding about CDWs and related superconductivity in TMDs.
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Submitted 21 July, 2016; v1 submitted 25 February, 2016;
originally announced February 2016.
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Superconductivity enhancement in the S-doped Weyl semimetal candidate MoTe2
Authors:
F. C. Chen,
X. Luo,
R. C. Xiao,
W. J. Lu,
B. Zhang,
H. X. Yang,
J. Q. Li,
Q. L. Pei,
D. F. Shao,
R. R. Zhang,
L. S. Ling,
C. Y. Xi,
W. H. Song,
Y. P. Sun
Abstract:
Two-dimensional (2D) transition-metal dichalcogenide (TMDs) MoTe2 has attracted much attention due to its predicted Weyl semimetal (WSM) state and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that the superconductivity in MoTe2 single crystal can be much enhanced by the partial substitution of the Te ions by the S ones. The maximum of the superconducting temperat…
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Two-dimensional (2D) transition-metal dichalcogenide (TMDs) MoTe2 has attracted much attention due to its predicted Weyl semimetal (WSM) state and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that the superconductivity in MoTe2 single crystal can be much enhanced by the partial substitution of the Te ions by the S ones. The maximum of the superconducting temperature TC of MoTe1.8S0.2 single crystal is about 1.3 K. Compared with the parent MoTe2 single crystal (TC=0.1 K), nearly 13-fold in TC is improved in MoTe1.8S0.2 one. The superconductivity has been investigated by the resistivity and magnetization measurements. MoTe2-xSx single crystals belong to weak coupling superconductors and the improvement of the superconductivity may be related to the enhanced electron-phonon coupling induced by the S-ion substitution. A dome-shape superconducting phase diagram is obtained in the S-doped MoTe2 single crystals. MoTe2-xSx materials may provide a new platform for our understanding of superconductivity phenomena and topological physics in TMDs.
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Submitted 28 December, 2015; v1 submitted 27 December, 2015;
originally announced December 2015.
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Manipulating charge-density-wave in $1T$-TaS$_{2}$ by charge carrier doping: A first-principles investigation
Authors:
D. F. Shao,
R. C. Xiao,
W. J. Lu,
H. Y. Lv,
J. Y. Li,
X. B. Zhu,
Y. P. Sun
Abstract:
The transition metal dichalcogenide (TMD) $1T$-TaS$_{2}$ exhibits a rich set of charge density wave (CDW) orders. Recent investigations suggested that using light or electric field can manipulate the commensurate (C) CDW ground state. Such manipulations are considered to be determined by the charge carrier doping. Here we simulate by first-principles calculations the carrier doping effect on CCDW…
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The transition metal dichalcogenide (TMD) $1T$-TaS$_{2}$ exhibits a rich set of charge density wave (CDW) orders. Recent investigations suggested that using light or electric field can manipulate the commensurate (C) CDW ground state. Such manipulations are considered to be determined by the charge carrier doping. Here we simulate by first-principles calculations the carrier doping effect on CCDW in $1T$-TaS$_{2}$. We investigate the charge doping effects on the electronic structures and phonon instabilities of $1T$ structure and analyze the doping induced energy and distortion ratio variations in CCDW structure. We found that both in bulk and monolayer $1T$-TaS$_{2}$, CCDW is stable upon electron doping, while hole doping can significantly suppress the CCDW, implying different mechanisms of such reported manipulations. Light or positive perpendicular electric field induced hole doping increases the energy of CCDW, so that the system transforms to NCCDW or similar metastable state. On the other hand, even the CCDW distortion is more stable upon in-plain electric field induced electron injection, some accompanied effects can drive the system to cross over the energy barrier from CCDW to nearly commensurate (NC) CDW or similar metastable state. We also estimate that hole doping can introduce potential superconductivity with $T_{c}$ of $6\sim7$ K. Controllable switching of different states such as CCDW/Mott insulating state, metallic state, and even the superconducting state can be realized in $1T$-TaS$_{2}$, which makes the novel material have very promising applications in the future electronic devices.
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Submitted 2 September, 2016; v1 submitted 21 December, 2015;
originally announced December 2015.
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Superconductivity in CaSn3 single crystal with a AuCu3-type structure
Authors:
X. Luo,
D. F. Shao,
Q. L. Pei,
J. Y. Song,
L. Hu,
Y. Y. Han,
X. B. Zhu,
W. H. Song,
W. J. Lu,
Y. P. Sun
Abstract:
We report the superconductivity of the CaSn3 single crystal with a AuCu3-type structure, namely cubic space group Pm3m. The superconducting transition temperature TC=4.2 K is determined by the magnetic susceptibility, electrical resistivity, and heat capacity measurements. The magnetization versus magnetic field (M-H) curve at low temperatures shows the typical-II superconducting behavior. The est…
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We report the superconductivity of the CaSn3 single crystal with a AuCu3-type structure, namely cubic space group Pm3m. The superconducting transition temperature TC=4.2 K is determined by the magnetic susceptibility, electrical resistivity, and heat capacity measurements. The magnetization versus magnetic field (M-H) curve at low temperatures shows the typical-II superconducting behavior. The estimated lower and upper critical fields are about 125 Oe and 1.79 T, respectively. The penetration depth λ(0) and coherence length ξ(0) are calculated to be approximately 1147 nm and 136 nm by the Ginzburg-Landau equations. The estimated Sommerfeld coefficient of the normal state γ_N is about 2.9 mJ/mol K2. ΔC/γNTC =1.13 and λep=0.65 suggest that CaSn3 single crystal is a weakly coupled superconductor. Electronic band structure calculations show a complex multi-sheet Fermi surface formed by three bands and a low density of states (DOS) at the Fermi level, which is consistent with the experimental results. Based on the analysis of electron phonon coupling of AX3 compounds (A=Ca, La, and Y; X=Sn and Pb), we theoretically proposed a way to increase TC in the system.
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Submitted 19 November, 2015;
originally announced December 2015.
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Spin-orbit coupling enhanced superconductivity in Bi-rich compounds ABi$_{3}$ (A=Sr and Ba)
Authors:
D. F. Shao,
X. Luo,
W. J. Lu,
L. Hu,
X. D. Zhu,
W. H. Song,
X. B. Zhu,
Y. P. Sun
Abstract:
Recently, Bi-based compounds have attracted attentions because of the strong spin-orbit coupling (SOC). In this work, we figured out the role of SOC in ABi$_{3}$ (A=Sr and Ba) by theoretical investigation of the band structures, phonon properties, and electron-phonon coupling. Without SOC, strong Fermi surface nesting leads to phonon instabilities in ABi$_{3}$. SOC suppresses the nesting and stabi…
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Recently, Bi-based compounds have attracted attentions because of the strong spin-orbit coupling (SOC). In this work, we figured out the role of SOC in ABi$_{3}$ (A=Sr and Ba) by theoretical investigation of the band structures, phonon properties, and electron-phonon coupling. Without SOC, strong Fermi surface nesting leads to phonon instabilities in ABi$_{3}$. SOC suppresses the nesting and stabilizes the structure. Moreover, without SOC the calculation largely underestimates the superconducting transition temperatures ($T_{c}$), while with SOC the calculated $T_{c}$ are very close to those determined by measurements on single crystal samples. The SOC enhanced superconductivity in ABi$_{3}$ is due to not only the SOC induced phonon softening, but also the SOC related increase of electron-phonon coupling matrix elements. ABi$_{3}$ can be potential platforms to construct heterostructure of superconductor/topological insulator to realize topological superconductivity.
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Submitted 19 October, 2015;
originally announced October 2015.
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Strain-controlled switch between ferromagnetism and antiferromagnetism in 1T-CrX2 (X = Se, Te) monolayers
Authors:
H. Y. Lv,
W. J. Lu,
D. F. Shao,
Y. Liu,
Y. P. Sun
Abstract:
We report on the strain-induced switch between ferromagnetic (FM) and antiferromagnetic (AFM) orderings in 1T-CrX2 (X = Se, Te) monolayers based on the first-principles calculations. The CrSe2 and CrTe2 monolayers without strains are found to be AFM and FM, respectively. Under the biaxial tensile strain, the CrSe2 monolayer tends to be FM when the strain is larger than 2%. The FM state is further…
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We report on the strain-induced switch between ferromagnetic (FM) and antiferromagnetic (AFM) orderings in 1T-CrX2 (X = Se, Te) monolayers based on the first-principles calculations. The CrSe2 and CrTe2 monolayers without strains are found to be AFM and FM, respectively. Under the biaxial tensile strain, the CrSe2 monolayer tends to be FM when the strain is larger than 2%. The FM state is further stabilized when the strain is increased. Moreover, the CrSe2 monolayer changes to be half-metallic when the tensile strain is larger than 10%. While for the CrTe2 monolayer, the critical strain at which the transition between the FM and AFM states occurs is compressive, of -1%. Relatively small tensile strains of 4% and 2%, respectively, can enhance the Curie temperature of CrSe2 and CrTe2 monolayers above the room temperature. The strain-induced switch between the FM and AFM states in CrSe2 (CrTe2) monolayer can be understood by the competition between the AFM Cr-Cr direct exchange and FM Cr-Se(Te)-Cr superexchange interactions. The tunable and attractive magnetic and electronic properties controlled by the flexible strain are desirable for the future nanoelectronic applications.
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Submitted 19 October, 2015; v1 submitted 24 September, 2015;
originally announced September 2015.
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Perfect charge compensation in WTe2 for the extraordinary magnetoresistance: From bulk to monolayer
Authors:
H. Y. Lv,
W. J. Lu,
D. F. Shao,
Y. Liu,
S. G. Tan,
Y. P. Sun
Abstract:
The electronic structure of WTe2 bulk and layers are investigated by using the first principles calculations. The perfect electron-hole (n-p) charge compensation and high carrier mobilities are found in WTe2 bulk, which may result in the large and non-saturating magnetoresistance (MR) observed very recently in the experiment [Ali et al., Nature 514, 205 (2014)]. The monolayer and bilayer of WTe2 p…
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The electronic structure of WTe2 bulk and layers are investigated by using the first principles calculations. The perfect electron-hole (n-p) charge compensation and high carrier mobilities are found in WTe2 bulk, which may result in the large and non-saturating magnetoresistance (MR) observed very recently in the experiment [Ali et al., Nature 514, 205 (2014)]. The monolayer and bilayer of WTe2 preserve the semimetallic property, with the equal hole and electron carrier concentrations. Moreover, the very high carrier mobilities are also found in WTe2 monolayer, indicating that the WTe2 monolayer would have the same extraordinary MR effect as the bulk, which could have promising applications in nanostructured magnetic devices.
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Submitted 29 December, 2014;
originally announced December 2014.
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Disorder suppressed charge-density-wave and its origin in 1T-TaSe2-xTex
Authors:
Y. Liu,
D. F. Shao,
W. J. Lu,
L. J. Li,
H. Y. Lv,
X. D. Zhu,
S. G. Tan,
B. Yuan,
L. Zu,
X. C. Kan,
W. H. Song,
Y. P. Sun
Abstract:
In the sake of connecting the charge-density-wave (CDW) of TaSe$_2$ and single-\emph{\textbf{q}} CDW-type distortion of TaTe$_2$, we present an overall electronic phase diagram of 1\emph{T}-TaSe$_{2-x}$Te$_x$ ($0 \leq x \leq 2$). In the experimentally prepared single crystals, the CDW is completely suppressed as $0.5 < x < 1.5$, while superconductivity emerges as $0.2 < x < 1.2$. Theoretically, si…
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In the sake of connecting the charge-density-wave (CDW) of TaSe$_2$ and single-\emph{\textbf{q}} CDW-type distortion of TaTe$_2$, we present an overall electronic phase diagram of 1\emph{T}-TaSe$_{2-x}$Te$_x$ ($0 \leq x \leq 2$). In the experimentally prepared single crystals, the CDW is completely suppressed as $0.5 < x < 1.5$, while superconductivity emerges as $0.2 < x < 1.2$. Theoretically, similar to 1\emph{T}-TaSe$_2$ and 1\emph{T}-TaTe$_2$, the hypothetic 1\emph{T}-TaSeTe with ordered Se/Ta/Te stacking shows instability in the phonon dispersion, indicating the presence of CDW in the ideally ordered sample. The contradictory between experimental and theoretical results suggests that the CDW is suppressed by disorder in 1\emph{T}-TaSe$_{2-x}$Te$_x$. The formation and suppression of CDW are found to be independent with Fermi surface nesting based on the generated electron susceptibility calculations. The calculation of phonon linewidth suggests the strong \textbf{\emph{q}}-dependent electron-phonon coupling induced period-lattice-distortion (PLD) should be related to our observation: The doping can largely distort the TaX$_6$ (X = Se, Te) octahedra, which are disorderly distributed. The resulted puckered Ta-Ta layers are not compatible with the two-dimensional PLD. Therefore, CDW is suppressed in 1\emph{T}-TaSe$_{2-x}$Te$_x$. Our results offer an indirect evidence that PLD, which can be influenced by strong disorder, is the origin of CDW in the system.
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Submitted 20 May, 2015; v1 submitted 14 December, 2014;
originally announced December 2014.
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Enhanced thermoelectric performance of phosphorene by strain-induced band convergence
Authors:
H. Y. Lv,
W. J. Lu,
D. F. Shao,
Y. P. Sun
Abstract:
The newly emerging monolayer phosphorene was recently predicted to be a promising thermoelectric material. In this work, we propose to further enhance the thermoelectric performance of phosphorene by the strain-induced band convergence. The effect of the uniaxial strain on the thermoelectric properties of phosphorene was investigated by using the first-principles calculations combined with the sem…
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The newly emerging monolayer phosphorene was recently predicted to be a promising thermoelectric material. In this work, we propose to further enhance the thermoelectric performance of phosphorene by the strain-induced band convergence. The effect of the uniaxial strain on the thermoelectric properties of phosphorene was investigated by using the first-principles calculations combined with the semi-classical Boltzmann theory. When the zigzag-direction strain is applied, the Seebeck coefficient and electrical conductivity in zigzag direction can be greatly enhanced simultaneously at the critical strain of 5% where the band convergence is achieved. The largest ZT value of 1.65 at 300 K is then achieved conservatively estimated by using the bulk lattice thermal conductivity. When the armchair-direction strain of 8% is applied, the room-temperature ZT value can reach 2.12 in the armchair direction of phosphorene. Our results indicate that strain induced band convergence could be an effective method to enhance the thermoelectric performance of phosphorene.
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Submitted 19 June, 2014;
originally announced June 2014.
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Electron-doped phosphorene: A potential monolayer superconductor
Authors:
D. F. Shao,
W. J. Lu,
H. Y. Lv,
Y. P. Sun
Abstract:
We predict by first-principles calculations that the electron-doped phosphorene is a potential BCS-like superconductor. The stretching modes at the Brillouin-zone center are remarkably softened by the electron-doping, which results in the strong electron-phonon coupling. The superconductivity can be introduced by a doped electron density ($n_{2D}$) above $1.3 \times10^{14}$ cm$^{-2}$, and may exis…
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We predict by first-principles calculations that the electron-doped phosphorene is a potential BCS-like superconductor. The stretching modes at the Brillouin-zone center are remarkably softened by the electron-doping, which results in the strong electron-phonon coupling. The superconductivity can be introduced by a doped electron density ($n_{2D}$) above $1.3 \times10^{14}$ cm$^{-2}$, and may exist over the liquid helium temperature when $n_{2D}>2.6 \times10^{14}$ cm$^{-2}$. The maximum critical temperature is predicted to be higher than 10 K. The superconductivity of phosphorene will significantly broaden the applications of this novel material.
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Submitted 18 December, 2014; v1 submitted 1 May, 2014;
originally announced May 2014.
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Large thermoelectric power factors in black phosphorus and phosphorene
Authors:
H. Y. Lv,
W. J. Lu,
D. F. Shao,
Y. P. Sun
Abstract:
The electronic properties of the layered black phosphorus (black-P) and its monolayer counterpart phosphorene are investigated by using the first-principles calculations based on the density functional theory (DFT). The room-temperature electronic transport coefficients are evaluated within the semi-classical Boltzmann theory. The electrical conductivity exhibits anisotropic behavior while the See…
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The electronic properties of the layered black phosphorus (black-P) and its monolayer counterpart phosphorene are investigated by using the first-principles calculations based on the density functional theory (DFT). The room-temperature electronic transport coefficients are evaluated within the semi-classical Boltzmann theory. The electrical conductivity exhibits anisotropic behavior while the Seebeck coefficient is almost isotropic. At the optimal doping level and room temperature, bulk black-P and phosphorene are found to have large thermoelectric power factors of 118.4 and 138.9 μWcm-1K-2, respectively. The maximum dimensionless figure of merit (ZT value) of 0.22 can be achieved in bulk black-P by appropriate n-type doping, primarily limited by the reducible lattice thermal conductivity. For the phosphorene, the ZT value can reach 0.30 conservatively estimated by using the bulk lattice thermal conductivity. Our results suggest that both bulk black-P and phosphorene are potentially promising thermoelectric materials.
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Submitted 21 April, 2014;
originally announced April 2014.
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CuSe-based layered compound Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$ as a quasi-two-dimensional metal
Authors:
S. G. Tan,
D. F. Shao,
W. J. Lu,
B. Yuan,
Y. Liu,
J. Yang,
W. H. Song,
Hechang Lei,
Y. P. Sun
Abstract:
We have investigated the physical properties of a new layered oxyselenide Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$, which crystallizes in an unusual intergrowth structure with Cu$_{2}$Se$_{2}$ and Bi$_{2}$YO$_{4}$ layers. Electric transport measurement indicates that Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$ behaves metallic. Thermal transport and Hall measurements show that the type of the carriers is hole-like a…
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We have investigated the physical properties of a new layered oxyselenide Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$, which crystallizes in an unusual intergrowth structure with Cu$_{2}$Se$_{2}$ and Bi$_{2}$YO$_{4}$ layers. Electric transport measurement indicates that Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$ behaves metallic. Thermal transport and Hall measurements show that the type of the carriers is hole-like and it may be a potential thermoelectric material at high temperatures. First principle calculations are in agreement with experimental results and show that Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$ is a quasi-2D metal. Further theoretical investigation suggests the ground states of the Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$-type can be tuned by designing the blocking layers, which will enrich the physical properties of these compounds.
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Submitted 30 May, 2014; v1 submitted 31 March, 2014;
originally announced March 2014.
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Prediction of superconductivity of $3d$ transition-metal based antiperovskites via magnetic phase diagram
Authors:
D. F. Shao,
W. J. Lu,
P. Tong,
S. Lin,
J. C. Lin,
Y. P. Sun
Abstract:
We theoretically studied the electronic structure, magnetic properties, and lattice dynamics of a series of $3d$ transition-metal antiperovskite compounds AXM$_{3}$ by density function theory. Based on the Stoner criterion, we drew the magnetic phase diagram of carbon-based antiperovskites ACM$_{3}$. In the phase diagram, compounds with non-magnetic ground state but locating near the ferromagnetic…
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We theoretically studied the electronic structure, magnetic properties, and lattice dynamics of a series of $3d$ transition-metal antiperovskite compounds AXM$_{3}$ by density function theory. Based on the Stoner criterion, we drew the magnetic phase diagram of carbon-based antiperovskites ACM$_{3}$. In the phase diagram, compounds with non-magnetic ground state but locating near the ferromagnetic boundary are suggested to yield sizeable electron-phonon coupling and behave superconductivity. To approve this deduction, we systematically calculated the phonon spectra and electron-phonon coupling of a series of Cr-based antiperovskites ACCr$_{3}$ and ANCr$_{3}$. The results show that AlCCr$_{3}$, GaCCr$_{3}$, and ZnNCr$_{3}$ could be moderate coupling BCS superconductors. The influence of spin fluctuation on superconductivity are discussed. Furthermore, other potential superconducting AXM$_{3}$ including some new Co-base and Fe-based antiperovskite superconductors are predicted from the magnetic phase diagram.
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Submitted 22 November, 2013; v1 submitted 17 November, 2013;
originally announced November 2013.
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Protein loops, solitons and side-chain visualization with applications to the left-handed helix region
Authors:
Martin Lundgren,
Antti J. Niemi,
Fan Sha
Abstract:
Folded proteins have a modular assembly. They are constructed from regular secondary structures like alpha-helices and beta-strands that are joined together by loops. Here we develop a visualization technique that is adapted to describe this modular structure. In complement to the widely employed Ramachandran plot that is based on toroidal geometry, our approach utilizes the geometry of a two-sphe…
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Folded proteins have a modular assembly. They are constructed from regular secondary structures like alpha-helices and beta-strands that are joined together by loops. Here we develop a visualization technique that is adapted to describe this modular structure. In complement to the widely employed Ramachandran plot that is based on toroidal geometry, our approach utilizes the geometry of a two-sphere. Unlike the more conventional approaches that only describea given peptide unit, ours is capable of describing the entire backbone environment including the neighboring peptide units. It maps the positions of each atom to the surface of the two-sphere exactly how these atoms are seen by an observer who is located at the position of the central C-alpha atom. At each level of side-chain atoms we observe a strong correlation between the positioning of the atom and the underlying local secondary structure with very little if any variation between the different amino acids. As a concrete example we analyze the left-handed helix region of non-glycyl amino acids. This region corresponds to an isolated and highly localized residue independent sector in the direction of the C-beta carbons on the two-sphere. We show that the residue independent localization extends to C-gamma and C-delta carbons, and to side-chain oxygen and nitrogen atoms in the case of asparagine and aspartic acid. When we extend the analysis to the side-chain atoms of the neighboring residues, we observe that left-handed beta-turns display a regular and largely amino acid independent structure that can extend to seven consecutive residues. This collective pattern is duu to the presence of a backbone soliton. We show how one can use our visualization techniques to analyze and classify the different solitons in terms of selection rules that we describe in detail.
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Submitted 2 September, 2012;
originally announced September 2012.
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On universal aspects of the left-handed helix region
Authors:
Martin Lundgren,
Antti J. Niemi,
Fan Sha
Abstract:
We inspect the geometry of proteins by identifying their backbones as framed polygons. We find that the left-handed helix region of the Ramachandran map for non-glycyl residues corresponds to an isolated and highly localized sector in the orientation of the $C_β$ carbons, when viewed in a Frenet frame that is centered at the corresponding $C_α$ carbons. We show that this localization in the orient…
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We inspect the geometry of proteins by identifying their backbones as framed polygons. We find that the left-handed helix region of the Ramachandran map for non-glycyl residues corresponds to an isolated and highly localized sector in the orientation of the $C_β$ carbons, when viewed in a Frenet frame that is centered at the corresponding $C_α$ carbons. We show that this localization in the orientation persists to $C_γ$ and $C_δ$ carbons. Furthermore, when we extend our analysis to the neighboring residues we conclude that the left-handed helix region reflects a very regular and apparently residue independent collective interplay of at least seven consecutive amino acids.
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Submitted 12 April, 2011;
originally announced April 2011.