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 Select Hole Injection Enhancement of MoO$_{3}$/NPB/Al Composite Anode Yanjing Tang, Xianxi Yu, Shaobo Liu, Anran Yu, Jiajun Qin, Ruichen Yi, Yuan Pei, Chunqin Zhu, Xiaoyuan Hou Chin. Phys. Lett.    2019, 36 (12): .   DOI: 10.1088/0256-307X/36/12/127201 Abstract   HTML   PDF (812KB) An ultra-thin molybdenum(VI) oxide (MoO$_{3})$ modification layer can significantly improve hole injection from an electrode even though the MoO$_{3}$ layer does not contact the electrode. We find that as the thickness of the organic layer between MoO$_{3}$ and the electrode increases, the hole injection first increases and it then decreases. The optimum thickness of 5 nm corresponds to the best current improvement 70%, higher than that in the device where MoO$_{3}$ directly contacts the Al electrode. According to the 4,4-bis[N-(1-naphthyl)-N-phenyl-amino] biphenyl (NPB)/MoO$_{3}$ interface charge transfer mechanism and the present experimental results, we propose a mechanism that mobile carriers generated at the interface and accumulated inside the device change the distribution of electric field inside the device, resulting in an increase of the probability of hole tunneling through the injection barrier from the electrode, which also explains the phenomenon of hole injection enhanced by MoO$_{3}$/NPB/Al composite anode. Based on this mechanism, different organic materials other than NPB were applied to form the composite electrode with MoO$_{3}$. Similar current enhancement effects are also observed.
 Select The Unconventional Influence of a Nearby Molecule onto Transport of Single C$_{60}$ Molecule Transistor Xiao Guo, Wen-jie Liang Chin. Phys. Lett.    2019, 36 (12): 127301.   DOI: 10.1088/0256-307X/36/12/127301 Abstract   HTML   PDF (587KB) We study the transport property of single C$_{60}$ molecular transistors with special focus on the situation that other molecules are in vicinity. The devices are prepared using electromigration and thermal deposition techniques. Pure single C$_{60}$ molecule transistors show typical coulomb blockade behavior at low temperature. When we increase the coverage of molecules slightly by extending the deposition time, the transport spectrum of devices displays a switching behavior in the general coulomb blockade pattern. We attribute this unconventional phenomenon to the influence from a nearby C$_{60}$ molecule. By analyzing this transport behavior quantitatively based on the parallel-double-quantum-dot model, the interaction from the nearby molecule is proved to be of capacity and tunneling coupling. Thermal stimulation is also applied to the device to investigate the effect of local charging environment variation on intermolecular interaction.
 Select $^{19}$F NMR Study of the Bilayer Iron-Based Superconductor KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ Yu-Ting Shao, Wen-Shan Hong, Shi-Liang Li, Zheng Li, Jian-Lin Luo Chin. Phys. Lett.    2019, 36 (12): .   DOI: 10.1088/0256-307X/36/12/127401 Abstract   HTML   PDF (681KB) We report a $^{19}\!$F nuclear magnetic resonance (NMR) study on single-crystal KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ ($T_{\rm c} \sim 33.3$ K). The $^{19}$F NMR spectral shape of KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ is weakly dependent on temperature and the Knight shift is small, which implies weak coupling between the CaF layer and the FeAs layer. The temperature dependence of 1/$^{19}\!T_{1}$ shows a hump below $T_{\rm c}$, however the 1/$^{75}\!T_{1}$ decreases just below $T_{\rm c}$, which implies that there are strong in-plane magnetic fluctuations in the CaF layers than in the FeAs layers. This may be caused by the motion of vortices. The absence of the coherence peak suggests unconventional superconductivity in KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$.
 Select High Resolution Microwave B-Field Imaging Using a Micrometer-Sized Diamond Sensor Wen-Hao He, Ming-Ming Dong, Zhen-Zhong Hu, Qi-Han Zhang, Bo Yang, Ying Liu, Xiao-Long Fan, Guan-Xiang Du Chin. Phys. Lett.    2019, 36 (12): .   DOI: 10.1088/0256-307X/36/12/127601 Abstract   HTML   PDF (751KB) We propose a diamond-based micron-scale sensor and perform high-resolution $B$-field imaging of the near-field distribution of coplanar waveguides. The sensor consists of diamond crystals attached to the tip of a tapered fiber with a physical size on the order of submicron. The amplitude of the $B$-field component $B$ is obtained by measuring the Rabi oscillation frequency. The result of Rabi sequence is fitted with a decayed sinusoidal. We apply the modulation-locking technique that demonstrates the vector-resolved field mapping of the micromachine coplanar waveguide structure (CPW). $B$-field line scan was performed on the CPW with a scan step size of 1.25 μm. To demonstrate vector resolved rf field sensing, a full field line scan acts (was performed) along four NV axes at a height of 50 μm above the device surface. The simulations are compared with the experimental results by vector-resolved measurement. This technique allows the measurement of weak microwave signals with a minimum resolvable modulation depth of 20 ppm. The sensor will have great interest in micron-scale resolved microwave $B$-field measurements, such as electromagnetic compatibility testing of microwave integrated circuits and characterization of integrated microwave components.
 Select Experimental Observations Indicating the Topological Nature of the Edge States on HfTe$_{5}$ Rui-Zhe Liu, Xiong Huang, Ling-Xiao Zhao, Li-Min Liu, Jia-Xin Yin, Rui Wu, Gen-Fu Chen, Zi-Qiang Wang, Shuheng H. Pan Chin. Phys. Lett.    2019, 36 (11): .   DOI: 10.1088/0256-307X/36/11/117301 Abstract   HTML   PDF (1646KB) The topological edge states of two-dimensional topological insulators with large energy gaps furnish ideal conduction channels for dissipationless current transport. Transition metal tellurides $X$Te$_{5}$ ($X$=Zr, Hf) are theoretically predicted to be large-gap two-dimensional topological insulators, and the experimental observations of their bulk insulating gap and in-gap edge states have been reported, but the topological nature of these edge states still remains to be further elucidated. Here, we report our low-temperature scanning tunneling microscopy/spectroscopy study on single crystals of HfTe$_{5}$. We demonstrate a full energy gap of $\sim$80 meV near the Fermi level on the surface monolayer of HfTe$_{5}$ and that such an insulating energy gap gets filled with finite energy states when measured at the monolayer step edges. Remarkably, such states are absent at the edges of a narrow monolayer strip of one-unit-cell in width but persist at both step edges of a unit-cell wide monolayer groove. These experimental observations strongly indicate that the edge states of HfTe$_{5}$ monolayers are not trivially caused by translational symmetry breaking, instead they are topological in nature protected by the 2D nontrivial bulk properties.
 Select Fe-Doped All-Boron Fullerene B$_{40}$ with Tunable Electronic and Magnetic Properties as Single Molecular Devices An-Zhi Xie, Tian-Zhen Wen, Ji-Ling Li Chin. Phys. Lett.    2019, 36 (11): .   DOI: 10.1088/0256-307X/36/11/117302 Abstract   HTML   PDF (962KB) Systematic theoretical calculations are performed to investigate the dopant effect of Fe on stability, electronic and magnetic properties of the newly synthesized all-boron fullerene B$_{40}$. The results reveal that as a typical ferromagnetic element, Fe atoms can either be chemically externally adsorbed on, or internally encapsulated in the cage of B$_{40}$, with the binding energies ranging from 3.07 to 5.31 eV/atom. By introducing the dopant states from the doped Fe atom, the energy gaps of the Fe-doped B$_{40}$-based metallofullerenes are decreased. Our spin-polarized calculations indicate that Fe-doped metallofullerenes have attractive magnetic properties: with alternative binary magnetic moments between 4.00$\mu_{_{\rm B}}$ and 2.00$\mu_{_{\rm B}}$, depending on the resident sites of the doped Fe atom. The findings of the tunable electronic properties and binary magnetic moments of the Fe-doped B$_{40}$-based metallofullerenes imply that this type of metallofullerene may be applied in single molecular devices.
 Select Experimental Evidence of Topological Surface States in Mg$_{3}$Bi$_{2}$ Films Grown by Molecular Beam Epitaxy Tong Zhou, Xie-Gang Zhu, Mingyu Tong, Yun Zhang, Xue-Bing Luo, Xiangnan Xie, Wei Feng, Qiuyun Chen, Shiyong Tan, Zhen-Yu Wang, Tian Jiang, Yuhua Tang, Xin-Chun Lai, Xuejun Yang Chin. Phys. Lett.    2019, 36 (11): .   DOI: 10.1088/0256-307X/36/11/117303 Abstract   HTML   PDF (3386KB) Nodal line semimetal (NLS) is a new quantum state hosting one-dimensional closed loops formed by the crossing of two bands. The so-called type-II NLS means that these two crossing bands have the same sign in their slopes along the radial direction of the loop, which requires that the crossing bands are either right-tilted or left-tilted at the same time. According to the theoretical prediction, Mg$_{3}$Bi$_{2}$ is an ideal candidate for studying the type-II NLS by tuning its spin-orbit coupling (SOC). High-quality Mg$_{3}$Bi$_{2}$ films are grown by molecular beam epitaxy (MBE). By in-situ angle resolved photoemission spectroscopy (ARPES), a pair of surface resonance bands around the $\bar{{{\it \Gamma}}}$ point are clearly seen. This shows that Mg$_{3}$Bi$_{2}$ films grown by MBE are Mg(1)-terminated by comparing the ARPES spectra with the first principles calculations results. Moreover, the temperature dependent weak anti-localization effect in Mg$_{3}$Bi$_{2}$ films is observed under magneto-transport measurements, which shows clear two-dimensional (2D) $e$–$e$ scattering characteristics by fitting with the Hikami–Larkin–Nagaoka model. Therefore, by combining with ARPES, magneto-transport measurements and the first principles calculations, this work proves that Mg$_{3}$Bi$_{2}$ is a semimetal with topological surface states. This paves the way for Mg$_{3}$Bi$_{2}$ to be used as an ideal material platform to study the exotic features of type-II nodal line semimetals and the topological phase transition by tuning its SOC.
 Select Transient Photoconductivity in LaRhO$_{3}$ Thin Film Zhi Meng, Lei Shen, Zongwei Ma, Muhammad Adnan Aslam, Liqiang Xu, Xueli Xu, Wang Zhu, Long Cheng, Yuecheng Bian, Li Pi, Chun Zhou, Zhigao Sheng Chin. Phys. Lett.    2019, 36 (11): .   DOI: 10.1088/0256-307X/36/11/117801 Abstract   HTML   PDF (672KB) High-quality epitaxial LaRhO$_{3}$ (LRO) thin films on SrTiO$_{3}$ (110) single-crystalline substrates are fabricated by pulsed laser deposition and their photoconductivity properties are studied. The transient photoconductivity (TPC) effect is found in this semiconductor LRO film at room temperature. The magnitude of TPC increases almost linearly with the laser power intensities and the photon energies in visible light range. Moreover, the difference in the TPC results under two airflow conditions confirms that both intrinsic photoinduced carrier accumulation and extrinsic photoinduced heating effects contribute to the magnitude of TPC effect.
 Select Pressure-Induced Metallization Accompanied by Elongated S–S Dimer in Charge Transfer Insulator NiS$_{2}$ Hao Wu, Yong-Hui Zhou, Yi-Fang Yuan, Chun-Hua Chen, Ying Zhou, Bo-Wen Zhang, Xu-Liang Chen, Chuan-Chuan Gu, Chao An, Shu-Yang Wang, Meng-Yao Qi, Ran-Ran Zhang, Li-Li Zhang, Xin-Jian Li, Zhao-Rong Yang Chin. Phys. Lett.    2019, 36 (10): .   DOI: 10.1088/0256-307X/36/10/107101 Abstract   HTML   PDF (1071KB) The insulator-metal transition triggered by pressure in charge transfer insulator NiS$_{2}$ is investigated by combining high-pressure electrical transport, synchrotron x-ray diffraction and Raman spectroscopy measurements up to 40–50 GPa. Upon compression, we show that the metallization firstly appears in the low temperature region at $\sim$3.2 GPa and then extends to room temperature at $\sim$8.0 GPa. During the insulator-metal transition, the bond length of S–S dimer extracted from the synchrotron x-ray diffraction increases with pressure, which is supported by the observation of abnormal red-shift of the Raman modes between 3.2 and 7.1 GPa. Considering the decreasing bonding-antibonding splitting due to the expansion of S–S dimer, the charge gap between the S-$pp\pi^*$ band and the upper Hubbard band of Ni-3$d$ $e_{\rm g}$ state is remarkably decreased. These results consistently indicate that the elongated S–S dimer plays a predominant role in the insulator-metal transition under high pressure, even though the $p$-$d$ hybridization is enhanced simultaneously, in accordance with a scenario of charge-gap-controlled type.
 Select Charge Transport Properties of the Majorana Zero Mode Induced Noncollinear Spin Selective Andreev Reflection Xin Shang, Hai-Wen Liu, Ke Xia Chin. Phys. Lett.    2019, 36 (10): .   DOI: 10.1088/0256-307X/36/10/107102 Abstract   HTML   PDF (1723KB) We study the charge transport properties of the spin-selective Andreev reflection (SSAR) effect between a spin polarized scanning tunneling microscope (STM) tip and a Majorana zero mode (MZM). Considering both the MZM and the excited states, we calculate the conductance and the shot noise power of the noncollinear SSAR using scattering theory. We find that the excited states give rise to inside peaks. Moreover, we numerically calculate the shot noise power and the Fano factor of the SSAR effect. Our calculation shows that the shot noise power and the Fano factor are related to the angle between the spin polarization direction of the STM tip and that of the MZM, which provide additional characteristics to detect the MZM via SSAR.
 Select Strong Exciton-Plasmon Coupling and Hybridization of Organic-Inorganic Exciton-Polaritons in Plasmonic Nanocavity Ping Jiang, Chao Li, Yuan-Yuan Chen, Gang Song, Yi-Lin Wang, Li Yu Chin. Phys. Lett.    2019, 36 (10): .   DOI: 10.1088/0256-307X/36/10/107301 Abstract   HTML   PDF (938KB) We investigate strong exciton-plasmon coupling and plasmon-mediated hybridization between the Frenkel (F) and Wannier–Mott (WM) excitons of an organic-inorganic hybrid system consisting of a silver ring separated from a monolayer WS$_{2}$ by J-aggregates. The extinction spectra of the hybrid system calculated by employing the coupled oscillator model are consistent with the results simulated by the finite-difference time-domain method. The calculation results show that strong couplings among F excitons, WM excitons, and localized surface plasmon resonances (LSPRs) lead to the appearance of three plexciton branches in the extinction spectra. The weighting efficiencies of the F exciton, WM exciton and LSPR modes in three plexciton branches are used to analyze the exciton-polaritons in the system. Furthermore, the strong coupling between two different excitons and LSPRs is manipulated by tuning F or WM exciton resonances.
 Select Cooper Molecules: Second Pairing of Cooper Pairs in Gapless Superconductor CeCoIn$_5$ Jiang Hong Man, Ze Cheng Chin. Phys. Lett.    2019, 36 (10): .   DOI: 10.1088/0256-307X/36/10/107401 Abstract   HTML   PDF (497KB) We establish a quantum field theory of phase transitions in gapless superconductor CeCoIn$_5$. It is found that uniform Cooper pair gases with pure gradient interactions with negative coefficient can undergo a Bardeen–Cooper–Schrieffer (BCS) condensation below a critical temperature. In the BCS condensation state, bare Cooper pairs with opposite wave vectors are bound into Cooper molecules, and uncoupled bare Cooper pairs are transformed into a new kind of quasiparticle, i.e., the dressed particles. The Cooper molecule system is a condensate or a superfluid, and the dressed particle system is a normal fluid. The critical temperature is derived analytically. The critical temperature of the superconductor CeCoIn$_5$ is obtained to be $T_{\rm c}=2.289$ K, which approaches the experimental data. The transition from the BCS condensation state to the normal state is a first-order phase transition.
 Select Broken Time-Reversal Symmetry in Superconducting Partially Filled Skutterudite Pr$_{1-\delta}$Pt$_{4}$Ge$_{12}$ Jia-Wei Zang, Jian Zhang, Zi-Hao Zhu, Zhao-Feng Ding, Kevin Huang, Xiao-Ran Peng, Adrian D. Hillier, Lei Shu Chin. Phys. Lett.    2019, 36 (10): .   DOI: 10.1088/0256-307X/36/10/107402 Abstract   HTML   PDF (812KB) Time reversal symmetry (TRS) is a key symmetry for classification of unconventional superconductors, and the violation of TRS often results in a wealth of novel properties. Here we report the synthesis and superconducting properties of the partially filled skutterudite Pr$_{1-\delta}$Pt$_{4}$Ge$_{12}$. The results from x-ray diffraction and magnetization measurements show that the [Pt$_{4}$Ge$_{12}$] cage-forming structure survives and bulk superconductivity is preserved below the superconducting transition temperature $T_{\rm c}=7.80$ K. The temperature dependence of both the upper critical field and the electronic specific heat can be described in terms of a two-gap model, providing strong evidence of multi-band superconductivity. TRS breaking is observed using zero field muon-spin relaxation experiments, and the magnitude of the spontaneous field is nearly half of that in PrPt$_{4}$Ge$_{12}$.
 Select Neutron Powder Diffraction Study on the Non-Superconducting Phases of ThFeAsN$_{1-x}$O$_x$ ($x=0.15, 0.6$) Iron Pnictide Hui-Can Mao, Bing-Feng Hu, Yuan-Hua Xia, Xi-Ping Chen, Cao Wang, Zhi-Cheng Wang, Guang-Han Cao, Shi-Liang Li, Hui-Qian Luo Chin. Phys. Lett.    2019, 36 (10): .   DOI: 10.1088/0256-307X/36/10/107403 Abstract   HTML   PDF (999KB) We use neutron powder diffraction to study the non-superconducting phases of ThFeAsN$_{1-x}$O$_x$ with $x=0.15$, 0.6. In our previous results of the superconducting phase ThFeAsN with $T_{\rm c}=30$ K, no magnetic transition is observed by cooling down to 6 K, and possible oxygen occupancy at the nitrogen site is shown in the refinement [Europhys. Lett. 117 (2017) 57005]. Here in the oxygen doped system ThFeAsN$_{1-x}$O$_x$, two superconducting regions ($0\leqslant x \leqslant 0.1$ and $0.25\leqslant x \leqslant 0.55$) are identified by transport experiments [J. Phys.: Condens. Matter 30 (2018) 255602]. However, within the resolution of our neutron powder diffraction experiment, neither the intermediate doping $x=0.15$ nor the heavily overdoped compound $x=0.6$ shows any magnetic order from 300 K to 4 K. Therefore, while it shares the common phenomenon of two superconducting domes as most 1111-type iron-based superconductors, the magnetically ordered parent compound may not exist in this nitride family.
 Select Superconductivity of the FeSe/SrTiO$_{3}$ Interface in View of BCS–BEC Crossover Shuyuan Zhang, Guangyao Miao, Jiaqi Guan, Xiaofeng Xu, Bing Liu, Fang Yang, Weihua Wang, Xuetao Zhu, Jiandong Guo Chin. Phys. Lett.    2019, 36 (10): .   DOI: 10.1088/0256-307X/36/10/107404 Abstract   HTML   PDF (2479KB) In paired Fermi systems, strong many-body effects exhibit in the crossover regime between the Bardeen–Cooper–Schrieffer (BCS) and the Bose–Einstein condensation (BEC) limits. The concept of the BCS–BEC crossover, which is studied intensively in the research field of cold atoms, has been extended to condensed matters. Here by analyzing the typical superconductors within the BCS–BEC phase diagram, we find that FeSe-based superconductors are prone to shift their positions in the BCS–BEC crossover regime by charge doping or substrate substitution, since their Fermi energies and the superconducting gap sizes are comparable. Especially at the interface of single-layer FeSe on SrTiO$_{3}$ substrate, the superconductivity is relocated closer to the crossover unitary than other doped FeSe-based materials, indicating that the pairing interaction is effectively modulated. We further show that hole-doping can drive the interfacial system into the phase with possible pre-paired electrons, demonstrating its flexible tunability within the BCS–BEC crossover regime.
 Select Roles of Nano-Domain Switching and Non-180$^{\circ}$ Domains in Enhancing Local Piezoelectric Responses of Highly (100)-Oriented Pb(Zr$_{0.60}$Ti$_{0.40}$)O$_{3}$ Thin Films Chen-Fei Jin, Si-Qi Zhang, Zhi-Qiang Shen, Wei-Li Li Chin. Phys. Lett.    2019, 36 (10): .   DOI: 10.1088/0256-307X/36/10/107701 Abstract   HTML   PDF (3483KB) Ferroelectric Pb(Zr$_{0.60}$Ti$_{0.40}$)O$_{3}$ thin films deposited on the niobium-doped SrTiO$_{3}$ and Pt (111)/Ti/SiO$_{2}$/Si substrates are fabricated by a sol-gel method. X-ray diffraction indicates that the films have a 'cube-on-cube' growth with highly (100) preferred orientation and good surface qualities. Using piezoelectric force microscopy, we investigate domain structures and butterfly amplitude loops of ferroelectric thin films. The results indicate that the film deposited on Nb:SrTiO$_{3}$ has both kinds of 180$^{\circ}$ polarizations perpendicular or parallel to the surface while the film deposited on Pt/Ti/SiO$_{2}$/Si has irregular phase differences. Excellent piezoelectric polarization are observed in the films on niobium-doped SrTiO$_{3}$ with local $d_{33}^{\ast}$ values around 45 pm/V three times more than that of the films around 13 pm/V deposited on Pt (111)/Ti/SiO$_{2}$/Si. Our findings emphasize that nano-domain switching ability and non-180$^{\circ}$ domains will contribute significantly to enhance piezoelectric responses of ferroelectric thin films.
 Select Computational Simulation of Sodium Doublet Line Intensities in Multibubble Sonoluminescence Jin-Fu Liang, Yu An, Wei-Zhong Chen Chin. Phys. Lett.    2019, 36 (10): .   DOI: 10.1088/0256-307X/36/10/107801 Abstract   HTML   PDF (696KB) We perform a computational simulation of the fluid dynamics of sodium doublet (Na-D) line emissions from one sonoluminescing bubble among the cavitation bubbles in argon-saturated Na hydroxide (NaOH) aqueous solutions. Our simulation includes the distributions of acoustic pressures and the dynamics of cavitation bubbles by numerically solving the cavitation dynamic equation and bubble-pulsation equation. The simulation results demonstrate that when the maximum temperature inside a luminescing bubble is relatively low, two emission peaks from excited Na are prominent within the emission spectra, at wavelengths of 589.0 and 589.6 nm. As the maximum temperature of the bubble increases, the two peaks merge into one peak and the full width at half maximum of this peak increases. These calculations match with the observations of Na-D line emissions from MBSL occurring in aqueous solutions of NaOH under an argon gas.
 Select Hybridization Effects Revealed by Angle-Resolved Photoemission Spectroscopy in Heavy-Fermion Ce$_{2}$IrIn$_{8}$ Haijiang Liu, Yuanji Xu, Yigui Zhong, Jianyu Guan, Lingyuan Kong, Junzhang Ma, Yaobo Huang, Qiuyun Chen, Genfu Chen, Ming Shi, Yi-feng Yang, Hong Ding Chin. Phys. Lett.    2019, 36 (9): .   DOI: 10.1088/0256-307X/36/9/097101 Abstract   HTML   PDF (980KB) We utilize high-resolution resonant angle-resolved photoemission spectroscopy (ARPES) to study the band structure and hybridization effect of the heavy-fermion compound Ce$_{2}$IrIn$_{8}$. We observe a nearly flat band at the binding energy of 7 meV below the coherent temperature $T_{\rm coh}\sim 40$ K, which characterizes the electrical resistance maximum and indicates the onset temperature of hybridization. However, the Fermi vector and the Fermi surface volume have little change around $T_{\rm coh}$, which challenges the widely believed evolution from a high-temperature small Fermi surface to a low-temperature large Fermi surface. Our experimental results of the band structure fit well with the density functional theory plus dynamic mean-field theory calculations.
 Select The 2D InSe/WS$_2$ Heterostructure with Enhanced Optoelectronic Performance in the Visible Region Lu-Lu Yang, Jun-Jie Shi, Min Zhang, Zhong-Ming Wei, Yi-Min Ding, Meng Wu, Yong He, Yu-Lang Cen, Wen-Hui Guo, Shu-Hang Pan, Yao-Hui Zhu Chin. Phys. Lett.    2019, 36 (9): .   DOI: 10.1088/0256-307X/36/9/097301 Abstract   HTML   PDF (1530KB) Two-dimensional (2D) InSe and WS$_2$ exhibit promising characteristics for optoelectronic applications. However, they both have poor absorption of visible light due to wide bandgaps: 2D InSe has high electron mobility but low hole mobility, while 2D WS$_2$ is on the contrary. We propose a 2D heterostructure composed of their monolayers as a solution to both problems. Our first-principles calculations show that the heterostructure has a type-II band alignment as expected. Consequently, the bandgap of the heterostructure is reduced to 2.19 eV, which is much smaller than those of the monolayers. The reduction in bandgap leads to a considerable enhancement of the visible-light absorption, such as about fivefold (threefold) increase in comparison to monolayer InSe (WS$_2$) at the wavelength of 490 nm. Meanwhile, the type-II band alignment also facilitates the spatial separation of photogenerated electron-hole pairs; i.e., electrons (holes) reside preferably in the InSe (WS$_2$) layer. As a result, the two layers complement each other in carrier mobilities of the heterostructure: the photogenerated electrons and holes inherit the large mobilities from the InSe and WS$_2$ monolayers, respectively.
 Select Superconducting Single-Layer T-Graphene and Novel Synthesis Routes Qinyan Gu, Dingyu Xing, Jian Sun Chin. Phys. Lett.    2019, 36 (9): 097401.   DOI: 10.1088/0256-307X/36/9/097401 Abstract   HTML   PDF (15832KB) Single-layer superconductors are ideal materials for fabricating superconducting nano devices. However, up to date, very few single-layer elemental superconductors have been predicted and especially no one has been successfully synthesized yet. Here, using crystal structure search techniques and ab initio calculations, we predict that a single-layer planar carbon sheet with 4- and 8-membered rings called T-graphene is a new intrinsic elemental superconductor with superconducting critical temperature ($T_{\rm c}$) up to around 20.8 K. More importantly, we propose a synthesis route to obtain such a single-layer T-graphene, that is, a T-graphene potassium intercalation compound (C$_4$K with $P4/mmm$ symmetry) is firstly synthesized at high pressure ($>$11.5 GPa) and then quenched to ambient condition; and finally, the single-layer T-graphene can be either exfoliated using the electrochemical method from the bulk C$_4$K, or peeled off from bulk T-graphite C$_4$, where C$_4$ can be obtained from C$_4$K by evaporating the K atoms. Interestingly, we find that the calculated $T_{\rm c}$ of C$_4$K is about 30.4 K at 0 GPa, which sets a new record for layered carbon-based superconductors. The present findings add a new class of carbon-based superconductors. In particular, once the single-layer T-graphene is synthesized, it can pave the way for fabricating superconducting devices together with other 2D materials using the layer-by-layer growth techniques.
 Select Machine Learning and Micromagnetic Studies of Magnetization Switching Jing-Yue Miao Chin. Phys. Lett.    2019, 36 (9): .   DOI: 10.1088/0256-307X/36/9/097501 Abstract   HTML   PDF (1286KB) Magnetization switching is one of the most fundamental topics in the field of magnetism. Machine learning (ML) models of random forest (RF), support vector machine (SVM), deep neural network (DNN) methods are built and trained to classify the magnetization reversal and non-reversal cases of single-domain particle, and the classification performances are evaluated by comparison with micromagnetic simulations. The results show that the ML models have achieved great accuracy and the DNN model reaches the best area under curve (AUC) of 0.997, even with a small training dataset, and RF and SVM models have lower AUCs of 0.964 and 0.836, respectively. This work validates the potential of ML applications in studies of magnetization switching and provides the benchmark for further ML studies in magnetization switching.
 Select Magnetic and Magnetocaloric Properties of Polycrystalline and Oriented Mn$_{2-\delta}$Sn Kun Li, Fanggui Wang, Youfang Lai, Mingzhu Xue, Xin Li, Jinbo Yang, Changsheng Wang, Jingzhi Han, Shunquan Liu, Wenyun Yang, Yingchang Yang, Honglin Du Chin. Phys. Lett.    2019, 36 (9): .   DOI: 10.1088/0256-307X/36/9/097502 Abstract   HTML   PDF (784KB) Mn-based Heusler alloys have attracted significant research attention as half-metallic materials because of their giant magnetocrystalline anisotropy and magnetocaloric properties. We investigate the crystal structure and magnetic properties of polycrystalline, [101]-oriented, and [100]-oriented Mn$_{2-\delta}$Sn prepared separately by arc melting, the Bridgeman method, and the flux method. All of these compounds crystallize in a Ni$_{2}$In-type structure. In the Mn$_{2-\delta}$Sn lattice, Mn atoms occupy all of the 2$a$ and a fraction of the 2$d$ sites. Site disorder exists between Mn and Sn atoms in the 2$c$ sites. In addition, these compounds undergo a re-entrant spin-glass-like transition at low temperatures, which is caused by frustration and randomness within the spin system. The magnetic properties of these systems depend on the crystal directions, which means that the magnetic interactions differ significantly along different directions. Furthermore, these materials exhibit a giant magnetocaloric effect near the Curie temperature. The largest value of maximum of magnetic entropy change ($-\Delta S_{\rm M})$ occurs perpendicular to the [100] direction. Specifically, at 252 K, maximum $-\Delta S_{\rm M}$ is 2.91 and 3.64 J$\cdot$kg$^{-1}$K$^{-1}$ for a magnetic field of 5 and 7 T, respectively. The working temperature span over 80 K and the relative cooling power reaches 302 J/kg for a magnetic field of 7 T, which makes the Mn$_{2-\delta}$Sn compound a promising candidate for a magnetic refrigerator.
 Select Modelling Debye Dielectric Relaxation in Monohydroxy Alcohols Li-Na Wang, Xing-Yu Zhao, Yi-Neng Huang Chin. Phys. Lett.    2019, 36 (9): .   DOI: 10.1088/0256-307X/36/9/097701 Abstract   HTML   PDF (645KB) The Debye relaxation of dielectric spectroscopy exists extensively in monohydroxy alcohols. We model the relaxation based on the infinite-pseudospin-chain Ising model and the Glauber dynamics, and the corresponding complex permittivity is obtained. The model results are in good agreement with the experimental data of 3,7-dimethyl-1-octanol, 2-ethyl-1-hexanol and 5-methyl-2-hexanol in a wide temperature range. Moreover, in the model parameters, the sum of the mean-field interaction energy and two times the orientation is nearly twice the hydrogen bond energy, which further states the rationality of this model.
 Select Quasihole Tunneling in Disordered Fractional Quantum Hall Systems Min Lu, Na Jiang, Xin Wan Chin. Phys. Lett.    2019, 36 (8): .   DOI: 10.1088/0256-307X/36/8/087301 Abstract   HTML   PDF (554KB) Fractional quantum Hall systems are often described by model wave functions, which are the ground states of pure systems with short-range interaction. A primary example is the Laughlin wave function, which supports Abelian quasiparticles with fractionalized charge. In the presence of disorder, the wave function of the ground state is expected to deviate from the Laughlin form. We study the disorder-driven collapse of the quantum Hall state by analyzing the evolution of the ground state and the single-quasihole state. In particular, we demonstrate that the quasihole tunneling amplitude can signal the fractional quantum Hall phase to insulator transition.
 Select Superconductivity in Topological Semimetal $\theta$-TaN at High Pressure Ya-Ting Jia, Jian-Fa Zhao, Si-Jia Zhang, Shuang Yu, Guang-Yang Dai, Wen-Min Li, Lei Duan, Guo-Qiang Zhao, Xian-Cheng Wang, Xu Zheng, Qing-Qing Liu, You-Wen Long, Zhi Li, Xiao-Dong Li, Hong-Ming Weng, Run-Ze Yu, Ri-Cheng Yu, Chang-Qing Jin Chin. Phys. Lett.    2019, 36 (8): .   DOI: 10.1088/0256-307X/36/8/087401 Abstract   HTML   PDF (1401KB) Recently, $\theta$-TaN was proposed to be a topological semimetal with a new type of triply degenerate nodal points. Here, we report studies of pressure dependence of transport, Raman spectroscopy and synchrotron x-ray diffraction on $\theta$-TaN up to 61 GPa. We find that $\theta$-TaN becomes superconductive above 24.6 GPa with $T_{\rm c}$ at 3.1 K. The $\theta$-TaN is of n-type carrier nature with carrier density about $1.1\times 10^{20}$/cm$^{3}$ at 1.2 GPa and 20 K, while the carrier density increases with the pressure and saturates at about 40 GPa in the measured range. However, there is no crystal structure transition with pressure up to 39 GPa, suggesting the topological nature of the pressure induced superconductivity.
 Select Magneto-Transport and Shubnikov–de Haas Oscillations in the Type-II Weyl Semimetal Candidate NbIrTe$_{4}$ Flake Xiang-Wei Huang, Xiao-Xiong Liu, Peng Yu, Pei-Ling Li, Jian Cui, Jian Yi, Jian-Bo Deng, Jie Fan, Zhong-Qing Ji, Fan-Ming Qu, Xiu-Nian Jing, Chang-Li Yang, Li Lu, Zheng Liu, Guang-Tong Liu Chin. Phys. Lett.    2019, 36 (7): .   DOI: 10.1088/0256-307X/36/7/077101 Abstract   HTML   PDF (1820KB) We report on magnetoresistance, Hall effect, and quantum Shubnikov–de Haas oscillation (SdH) experiments in NbIrTe$_4$ single crystals, which was recently predicted to be a type-II Weyl semimetal. NbIrTe$_4$ manifests a non-saturating and parabolic magnetoresistance at low temperatures. The magneto-transport measurements show that NbIrTe$_4$ is a multiband system. The analysis of the SdH oscillations reveals four distinct oscillation frequencies. Combined with the density-functional theory calculations, we show that they come from two types of Fermi surfaces: electron pocket E$_1$ and hole pocket H$_2$.
 Select Single Crystal Growth and Magnetoresistivity of Topological Semimetal CoSi D. S. Wu, Z. Y. Mi, Y. J. Li, W. Wu, P. L. Li, Y. T. Song, G. T. Liu, G. Li, J. L. Luo Chin. Phys. Lett.    2019, 36 (7): .   DOI: 10.1088/0256-307X/36/7/077102 Abstract   HTML   PDF (1044KB) We report single crystal growth of CoSi, which has recently been recognized as a new type of topological semimetal hosting fourfold and sixfold degenerate nodes. The Shubnikov–de Haas quantum oscillation (QO) is observed on our crystals. There are two frequencies originating from almost isotropic bulk electron Fermi surfaces, in accordance with band structure calculations. The effective mass, scattering rate, and QO phase difference of the two frequencies are extracted and discussed.
 Select Spin Transport under In-plane Electric Fields with Different Orientations in Undoped InGaAs/AlGaAs Multiple Quantum Wells Xiao-di Xue, Yu Liu, Lai-pan Zhu, Wei Huang, Yang Zhang, Xiao-lin Zeng, Jing Wu, Bo Xu, Zhan-guo Wang, Yong-hai Chen, Wei-feng Zhang Chin. Phys. Lett.    2019, 36 (7): .   DOI: 10.1088/0256-307X/36/7/077201 Abstract   HTML   PDF (583KB) The spin-polarized photocurrent is used to study the in-plane electric field dependent spin transport in undoped InGaAs/AlGaAs multiple quantum wells. In the temperature range of 77–297 K, the spin-polarized photocurrent shows an anisotropic spin transport under different oriented in-plane electric fields. We ascribe this characteristic to two dominant mechanisms: the hot phonon effect and the Rashba spin-orbit effect which is influenced by the in-plane electric fields with different orientations. The formulas are proposed to fit our experiments, suggesting a guide of potential applications and devices.
 Select Pressure-Induced Ionic-Electronic Transition in BiVO$_{4}$ Shu-Peng Lyu, Jia Wang, Guo-Zhao Zhang, Yu-Fei Wang, Min Wang, Cai-Long Liu, Chun-Xiao Gao, Yong-Hao Han Chin. Phys. Lett.    2019, 36 (7): .   DOI: 10.1088/0256-307X/36/7/077202 Abstract   HTML   PDF (609KB) Electrical transport properties of bismuth vanadate (BiVO$_{4}$) are studied under high pressures with electrochemical impedance spectroscopy. A pressure-induced ionic-electronic transition is found in BiVO$_{4}$. Below 3.0 GPa, BiVO$_{4}$ has ionic conduction behavior. The ionic resistance decreases under high pressures due to the increasing migration rate of O$^{2-}$ ions. Above 3.0 GPa the channels for ion migration are closed. Transport mechanism changes from the ionic to the electronic behavior. First-principles calculations show that bandgap width narrows under high pressures, causing the continuous decrease of electrical resistance of BiVO$_{4}$.
 Select Ionic-Liquid-Gating Induced Protonation and Superconductivity in FeSe, FeSe$_{0.93}$S$_{0.07}$, ZrNCl, 1$T$-TaS$_2$ and Bi$_2$Se$_3$ Yi Cui, Ze Hu, Jin-Shan Zhang, Wen-Long Ma, Ming-Wei Ma, Zhen Ma, Cong Wang, Jia-Qiang Yan, Jian-Ping Sun, Jin-Guang Cheng, Shuang Jia, Yuan Li, Jin-Sheng Wen, He-Chang Lei, Pu Yu, Wei Ji, Wei-Qiang Yu Chin. Phys. Lett.    2019, 36 (7): 077401.   DOI: 10.1088/0256-307X/36/7/077401 Abstract   HTML   PDF (901KB) We report protonation in several compounds by an ionic-liquid-gating method, under optimized gating conditions. This leads to single superconducting phases for several compounds. Non-volatility of protons allows post-gating magnetization and transport measurements. The superconducting transition temperature $T_{\rm c}$ is enhanced to 43.5 K for FeSe$_{0.93}$S$_{0.07}$, and 41 K for FeSe after protonation. Superconducting transitions with $T_{\rm c} \sim 15$ K for ZrNCl, $\sim$7.2 K for 1$T$-TaS$_2$, and $\sim$3.8 K for Bi$_2$Se$_3$ are induced after protonation. Electric transport in protonated FeSe$_{0.93}$S$_{0.07}$ confirms high-temperature superconductivity. Our $^{1}$H nuclear magnetic resonance (NMR) measurements on protonated FeSe$_{1-x}$S$_{x}$ reveal enhanced spin-lattice relaxation rate $1/^{1}T_1$ with increasing $x$, which is consistent with the LDA calculations that H$^{+}$ is located in the interstitial sites close to the anions.
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