arXiv subjects

Xianhui Chen

Publications and source records attributed to Xianhui Chen.

89 records · Page 5Linked to original sources

arXiv2015

Ultrafast pulse generation with black phosphorus

Black phosphorus has been recently rediscovered as a new and interesting two-dimensional material due to its unique electronic and optical properties. Here, we study the linear and nonlinear optical properties of black phosphorus thin films, indicating that both linear and nonlinear optical properties are anisotropic and can be tuned by the film thickness. Then we employ the nonlinear optical property of black phosphorus for ultrafast (pulse duration down to ~786 fs in mode-locking) and large-energy (pulse energy up to >18 nJ in Q-switching) pulse generation in fiber lasers at the near-infrared telecommunication band ~1.5 μm. Our results underscore relatively large optical nonlinearity in black phosphorus and its prospective for ultrafast pulse generation, paving the way to black phosphorus based nonlinear and ultrafast photonics applications (e.g., ultrafast all-optical switches/modulators, frequency converters etc.).

physics.optics↗

arXiv2015

Strong similarities between the local electronic structure of insulating iron pnictide and lightly doped cuprate

One of the major puzzles regarding unconventional superconductivity is how some of the most interesting superconductors are related to an insulating phase that lies in close proximity. Here we report scanning tunneling microscopy studies of the local electronic structure of Cu doped NaFeAs across the superconductor to insulator transition. We find that in the highly insulating regime the electronic spectrum develops an energy gap with diminishing density of state at the Fermi level. The overall lineshape and strong spatial variations of the spectra are strikingly similar to that of lightly doped cuprates close to the parent Mott insulator. We propose that the suppression of itinerant electron state and strong impurity potential induced by Cu dopants lead to this insulating iron pnictide.

cond-mat.supr-con↗

arXiv2015

Monolayer Excitonic Laser

Recently, two-dimensional (2D) materials have opened a new paradigm for fundamental physics explorations and device applications. Unlike gapless graphene, monolayer transition metal dichalcogenide (TMDC) has new optical functionalities for next generation ultra-compact electronic and opto-electronic devices. When TMDC crystals are thinned down to monolayers, they undergo an indirect to direct bandgap transition, making it an outstanding 2D semiconductor. Unique electron valley degree of freedom, strong light matter interactions and excitonic effects were observed. Enhancement of spontaneous emission has been reported on TMDC monolayers integrated with photonic crystal and distributed Bragg reflector microcavities. However, the coherent light emission from 2D monolayer TMDC has not been demonstrated, mainly due to that an atomic membrane has limited material gain volume and is lack of optical mode confinement. Here, we report the first realization of 2D excitonic laser by embedding monolayer tungsten disulfide (WS2) in a microdisk resonator. Using a whispering gallery mode (WGM) resonator with a high quality factor and optical confinement, we observed bright excitonic lasing in visible wavelength. The Si3N4/WS2/HSQ sandwich configuration provides a strong feedback and mode overlap with monolayer gain. This demonstration of 2D excitonic laser marks a major step towards 2D on-chip optoelectronics for high performance optical communication and computing applications.

cond-mat.mtrl-sci↗

arXiv2014

Structure and composition of the superconducting phase in alkali iron selenide K$_y$Fe$_{1.6+x}$Se$_2$

We use neutron diffraction to study the temperature evolution of the average structure and local lattice distortions in insulating and superconducting potassium iron selenide K$_y$Fe$_{1.6+x}$Se$_2$. In the high temperature paramagnetic state, both materials have a single phase with crystal structure similar to that of the BaFe$_2$As$_2$ family of iron pnictides. While the insulating K$_y$Fe$_{1.6+x}$Se$_2$ forms a $\sqrt{5}\times\sqrt{5}$ iron vacancy ordered block antiferromagnetic (AF) structure at low-temperature, the superconducting compounds spontaneously phase separate into an insulating part with $\sqrt{5}\times\sqrt{5}$ iron vacancy order and a superconducting phase with chemical composition of K$_z$Fe$_{2}$Se$_2$ and BaFe$_2$As$_2$ structure. Therefore, superconductivity in alkaline iron selenides arises from alkali deficient K$_z$Fe$_{2}$Se$_2$ in the matrix of the insulating block AF phase.

cond-mat.supr-con↗

arXiv2014

Iron based high transition temperature superconductors

In a superconductor electrons form pairs and electric transport becomes dissipation-less at low temperatures. Recently discovered iron based superconductors have the highest superconducting transition temperature next to copper oxides. In this article, we review material aspects and physical properties of iron based superconductors. We discuss the dependence of transition temperature on the crystal structure, the interplay between antiferromagnetism and superconductivity by examining neutron scattering experiments, and the electronic properties of these compounds obtained by angle resolved photoemission spectroscopy in link with some results from scanning tunneling microscopy/spectroscopy measurements. Possible microscopic model for this class of compounds is discussed from a strong coupling point of view.

cond-mat.mtrl-sci↗

arXiv2014

Doping dependence of the anisotropic quasiparticle interference in NaFe1-xCoxAs iron-based superconductors

We use scanning tunneling microscopy to investigate the doping dependence of quasiparticle interference (QPI) in NaFe1-xCoxAs iron-based superconductors. The goal is to study the relation between nematic fluctuations and Cooper pairing. In the parent and underdoped compounds, where four-fold rotational symmetry is broken macroscopically, the QPI patterns reveal strong rotational anisotropy. At optimal doping, however, the QPI patterns are always four-fold symmetric. We argue this implies small nematic susceptibility and hence insignificant nematic fluctuation in optimally doped iron pnictides. Since Tc is the highest this suggests nematic fluctuation is not a prerequistite for strong Cooper pairing.

cond-mat.supr-con↗

arXiv2014

Emergence of a coherent in-gap state in SmB6 Kondo insulator revealed by scanning tunneling spectroscopy

We use scanning tunneling microscopy to investigate the (001) surface of cleaved SmB6 Kondo insulator. Variable temperature dI/dV spectroscopy up to 60 K reveals a gap-like density of state suppression around the Fermi level, which is due to the hybridization between the itinerant Sm 5d band and localized Sm 4f band. At temperatures below 40 K, a sharp coherence peak emerges within the hybridization gap near the lower gap edge. We propose that the in-gap resonance state is due to a collective excitation in magnetic origin with the presence of spin-orbital coupling and mixed valence fluctuations. These results shed new lights on the electronic structure evolution and transport anomaly in SmB6.

cond-mat.mtrl-sci↗

arXiv2012

Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides

Motivated by the triumph and limitation of graphene for electronic applications, atomically thin layers of group VI transition metal dichalcogenides are attracting extensive interest as a class of graphene-like semiconductors with a desired band-gap in the visible frequency range. The monolayers feature a valence band spin splitting with opposite sign in the two valleys located at corners of 1st Brillouin zone. This spin-valley coupling, particularly pronounced in tungsten dichalcogenides, can benefit potential spintronics and valleytronics with the important consequences of spin-valley interplay and the suppression of spin and valley relaxations. Here we report the first optical studies of WS2 and WSe2 monolayers and multilayers. The efficiency of second harmonic generation shows a dramatic even-odd oscillation with the number of layers, consistent with the presence (absence) of inversion symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show the crossover from an indirect band gap semiconductor at mutilayers to a direct-gap one at monolayers. The PL spectra and first-principle calculations consistently reveal a spin-valley coupling of 0.4 eV which suppresses interlayer hopping and manifests as a thickness independent splitting pattern at valence band edge near K points. This giant spin-valley coupling, together with the valley dependent physical properties, may lead to rich possibilities for manipulating spin and valley degrees of freedom in these atomically thin 2D materials.

cond-mat.mes-hall↗

arXiv2012

Visualizing the microscopic coexistence of spin density wave and superconductivity in underdoped NaFe1-xCoxAs

Although the origin of high temperature superconductivity in the iron pnictides is still under debate, it is widely believed that magnetic interactions or fluctuations play an important role in triggering Cooper pairing. Because of the relevance of magnetism to pairing, the question of whether long range spin magnetic order can coexist with superconductivity microscopically has attracted strong interests. The available experimental methods used to answer this question are either bulk probes or local ones without control of probing position, thus the answers range from mutual exclusion to homogeneous coexistence. To definitively answer this question, here we use scanning tunneling microscopy to investigate the local electronic structure of an underdoped NaFe1-xCoxAs near the spin density wave (SDW) and superconducting (SC) phase boundary. Spatially resolved spectroscopy directly reveal both the SDW and SC gap features at the same atomic location, providing compelling evidence for the microscopic coexistence of the two phases. The strengths of the SDW and SC features are shown to anti correlate with each other, indicating the competition of the two orders. The microscopic coexistence clearly indicates that Cooper pairing occurs when portions of the Fermi surface (FS) are already gapped by the SDW order. The regime TC < T < TSDW thus show a strong resemblance to the pseudogap phase of the cuprates where growing experimental evidences suggest a FS reconstruction due to certain density wave order. In this phase of the pnictides, the residual FS has a favorable topology for magnetically mediated pairing when the ordering moment of the SDW is small.

cond-mat.supr-con↗

arXiv2012

Superconductivity in Yb$_x$($Me$)$_y$HfNCl ($Me$ = NH$_3$ and THF)

The intercalated layered nitride $β$-HfNCl has attracted much attention due to the high superconducting transition temperature up to 25.5 K. Electrons can be introduced into $β$-$M$NCl ($M$=Zr and Hf) through alkali-metals intercalation to realize the superconductivity. Here, we report the observation of superconductivity in rare-earth metals cointercalated compounds Yb$_x$($Me$)$_y$HfNCl with $Me$ = NH$_3$ and tetrahydrofuran (THF), which were synthesized by the liquid ammonia method at room temperature. The superconducting transition temperature is about 23 K and 24.6 K for Yb$_{0.2}$(NH$_3$)$_y$HfNCl and Yb$_{0.3}$(NH$_3$)$_y$HfNCl, respectively. Replacing the NH$_3$ with a larger molecule THF, superconducting transition temperature increases to 25.2 K in Yb$_{0.2}$(THF)$_y$HfNCl, which is almost the same as the highest $T_{\rm c}$ reported in the alkali-metals intercalated HfNCl superconductors. The $T_{\rm c}$ of Yb$_{0.2}$(THF)$_y$HfNCl is apparently suppressed by pressure up to 0.5 GPa, while the pressure effect on $T_{\rm c}$ becomes very small above 0.5 GPa. The liquid ammonia method is proved to be an effective synthetic method to intercalate metal ions into HfNCl. Our results suggest that the superconductivity in these layered intercalated superconductors nearly does not rely on the intercalated metal ions, even magnetic ion.

cond-mat.supr-con↗

arXiv2012

Evolution from unconventional spin density wave to superconductivity and a novel gap-like phase in NaFe1-xCoxAs

Similar to the cuprate high TC superconductors, the iron pnictide superconductors also lie in close proximity to a magnetically ordered phase. A central debate concerning the superconducting mechanism is whether the local magnetic moments play an indispensable role or the itinerant electron description is sufficient. A key step for resolving this issue is to acquire a comprehensive picture regarding the nature of various phases and interactions in the iron compounds. Here we report the doping, temperature, and spatial evolutions of the electronic structure of NaFe1-xCoxAs studied by scanning tunneling microscopy. The spin density wave gap in the parent state is observed for the first time, which shows a strongly asymmetric lineshape that is incompatible with the conventional Fermi surface nesting scenario. The optimally doped sample exhibits a single, symmetric energy gap, but in the overdoped regime another asymmetric gap-like feature emerges near the Fermi level. This novel gap-like phase coexists with superconductivity in the ground state, persists deep into the normal state, and shows strong spatial variations. The characteristics of the three distinct low energy states, in conjunction with the peculiar high energy spectra, suggest that the coupling between the local moments and itinerant electrons is the fundamental driving force for the phases and phase transitions in the iron pnictides.

cond-mat.supr-con↗

arXiv2011

Imaging the coexistence of superconductivity and a charge density modulation in K0.73Fe1.67Se2 superconductor

We report scanning tunneling microscopy studies of the local structural and electronic properties of the iron selenide superconductor K0.73Fe1.67Se2 with TC = 32K. On the atomically resolved FeSe surface, we observe well-defined superconducting gap and the microscopic coexistence of a charge density modulation with root2*root2 periodicity with respect to the original Se lattice. We propose that a possible origin of the pattern is the electronic superstructure caused by the block antiferromagnetic ordering of the iron moments. The widely expected iron vacancy ordering is not observed, indicating that it is not a necessary ingredient for superconductivity in the intercalated iron selenides.

cond-mat.supr-con↗

arXiv2010

Quasiparticle interference of C2-symmetric surface states in LaOFeAs parent compound

We present scanning tunneling microscopy studies of the LaOFeAs parent compound of iron pnictide superconductors. Topographic imaging reveals two types of atomically flat surfaces, corresponding to the exposed LaO layer and FeAs layer respectively. On one type of surface, we observe strong standing wave patterns induced by quasiparticle interference of two-dimensional surface states. The distribution of scattering wavevectors exhibits pronounced two-fold symmetry, consistent with the nematic electronic structure found in the Ca(Fe1-xCox)2As2 parent state.

cond-mat.supr-con↗

arXiv2010

Physical origin of superconductivity in EuFe2As1.4P0.6 and EuFe2As2: pressure-induce valence change of europium

Superconductivity can be realized in Eu-containing pnictides by application of chemical (internal) and physical (external) pressure, the intrinsic physical mechanism of which attracts much attention in physics community. Here we present the experimental evidence for the valence change of europium in compounds of EuFe2As1.4P0.6 exposed to ambient pressure and EuFe2As2 to high pressure by x-ray absorption measurements on L3-Eu edge. We find that the absorption spectrum of EuFe2As1.4P0.6 at ambient pressure shows clear spectra weight transfer from a divalent to a trivalent state. Furthermore, application of pressure on EuFe2As2 using a diamond anvil cell shows a similar behavior of valence transition as EuFe2As1.4P0.6. These findings are the first observation of superconductivity mechanized by valence change in pnictides superconductors and elucidate the intrinsic physical origin of superconductivity in EuFe2As1.4P0.6 and compressed EuFe2As2.

cond-mat.supr-con↗

arXiv2009

Universal $\sqrt{2}\times\sqrt{2}$ structure and short-range charge order at the surfaces of BaFe$_{2-x}$Co$_{x}$As$_{2}$ compounds with various Co doping levels

The structure and electronic order at the cleaved (001) surfaces of the newly-discovered pnictide superconductors BaFe$_{2-x}$Co$_{x}$As$_{2}$ with x ranging from 0 to 0.32 are systematically investigated by scanning tunneling microscopy. A $\sqrt{2}\times\sqrt{2}$ surface structure is revealed for all the compounds, and is identified to be Ba layer with half Ba atoms lifted-off by combination with theoretical simulation. A universal short-range charge order is observed at this $\sqrt{2}\times\sqrt{2}$ surface associated with an energy gap of about 30 meV for all the compounds.

cond-mat.supr-con↗

arXiv2008

Identification of Nodal Kink in Electron-Doped (Nd$_{1.85}$Ce$_{0.15}$)CuO$_4$ Superconductor from Laser-Based Angle-Resolved Photoemission Spectroscopy

High-resolution laser-based angle-resolved photoemission measurements have been carried out on the electron-doped (Nd$_{1.85}$Ce$_{0.15}$)CuO$_4$ high temperature superconductor. We have revealed a clear kink at $\sim$60 meV in the dispersion along the (0,0)-($π$,$π$) nodal direction, accompanied by a peak-dip-hump feature in the photoemission spectra. This indicates that the nodal electrons are coupled to collective excitations (bosons) in electron-doped superconductors, with the phonons as the most likely candidate of the boson. This finding has established a universality of nodal electron coupling in both hole- and electron-doped high temperature cuprate superconductors.

cond-mat.supr-con↗

arXiv2007

Impact of the Fe Doping on Magnetism in Perovskite Cobaltites

We systematically studied the magnetic and transport properties for the polycrystalline samples of Fe-doped perovskite cobaltites: Pr$_{1-y}$Ca$_{y}$Co$_{1-x}$Fe$_x$O$_3$ ($y$=0.3, $x$=0-0.15; $y$=0.45, $x$=0-0.3) and Gd$_{0.55}$Sr$_{0.45}$Co$_{1-x}$Fe$_x$O$_{3}$ ($x$=0-0.3). Fe doping leads to an enhancement of the ferromagnetism in the systems of Pr$_{1-y}$Ca$_{y}$Co$_{1-x}$Fe$_x$O$_3$, while the ferromagnetism is suppressed with further increasing Fe content and spin-glass behavior is observed at high doping level of Fe. In contrast, the ferromagnetism is suppressed in the system Gd$_{0.55}$Sr$_{0.45}$Co$_{1-x}$Fe$_x$O$_{3}$ as long as Fe is doped, and no spin-glass behavior is observed in the sample with Fe doping up to 0.3. The competition between ferromagnetic interactions through Fe$^{3+}$-O-(LS)Co$^{4+}$ and antiferromagnetic interactions through Fe$^{3+}$-O-Fe$^{3+}$ and Fe$^{3+}$-O-(IS)Co$^{3+}$ is considered to be responsible for the behavior observed above. The average radius of the ions on A sites plays the key role in determining what type of interactions Fe doping mainly introduces.

cond-mat.str-el↗