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H. Zhang

Publications and source records attributed to H. Zhang.

At least 217 records · Page 12Linked to original sources
arXiv2011

Electric field-induced quantum interference control in a semiconductor: A new manifestation of the Franz-Keldysh effect

In (100)-oriented GaAs illuminated at normal incidence by a laser and its second harmonic, interference between one- and two-photon absorption results in ballistic current injection, but not modulation of the overall carrier injection rate. Results from a pump-probe experiment on a transversely biased sample show that a constant electric field enables coherent control of the carrier injection rate. We ascribe this to the nonlinear optical Franz-Keldysh effect and calculate it for a two-band parabolic model. The mechanism is relevant to centrosymmetric semiconductors as well.

cond-mat.other↗
arXiv2011

Bulk sensitive x-ray absorption spectroscopy free of self-absorption effects

We demonstrate a new method of x-ray absorption spectroscopy (XAS) that is bulk sensitive, like traditional fluorescence yield measurements, but is not affected by self-absorption or saturation effects. This measure of XAS is achieved by scanning the incident photon energy through an absorption edge and using an energy sensitive photon detector to measure the partial fluorescence yield (PFY). The x-ray emission from any element or core-hole excitation that is not resonant with the absorption edge under investigation is selected from the PFY. It is found that the inverse of this PFY spectrum, which we term inverse partial fluorescence yield (IPFY), is linearly proportional to the x-ray absorption cross-section without any corrections due to saturation or self-absorption effects. We demonstrate this technique on the Cu L and Nd M absorption edges of the high-Tc cuprate LNSCO by measuring the O K PFY and comparing the total electron yield, total fluorescence yield and IPFY spectra.

cond-mat.str-el↗
arXiv2011

Unveiling the quantum critical point of an Ising chain

Quantum phase transitions occur at zero temperature upon variation of some nonthermal control parameters. The Ising chain in a transverse field is probably the most-studied model undergoing such a transition, from ferromagnetic to paramagnetic state1,2. This model can be exactly solved by using a Jordan-Wigner transformation, which transforms the spins into noninteracting spinless fermions1. At the quantum critical point, the magnetic excitations can carry arbitrarily low energy and dominate the low temperature properties. Here we report the unveiling of such quantum critical point in quasi-one-dimensional Ising ferromagnet CoNb2O6 by ultra-low-temperature thermal conductivity measurements. We find that in the paramagnetic state, phonons are scattered by the magnetic excitations above certain temperature Ts, which corresponds to a gap. As predicted by the theoretical model1, this gap linearly goes to zero with decreasing the transverse field, thus determining the quantum critical point of the Ising chain.

cond-mat.str-el↗
arXiv2010

Imaging charge density fluctuations in graphene using Coulomb blockade spectroscopy

Using scanning tunneling microscopy, we have imaged local charge density fluctuations in monolayer graphene. By placing a small gold nanoparticle on the end of the STM tip, a charge sensor is created. By raster scanning the tip over the surface and using Coulomb blockade spectroscopy, we map the local charge on the graphene. We observe a series of electron and hole doped puddles with a characteristic length scale of about 20 nm. Theoretical calculations for the correlation length of the puddles based on the number of impurities are in agreement with our measurements.

cond-mat.mes-hall↗
arXiv2010

Lithography-free Fabrication of High Quality Substrate-supported and Freestanding Graphene devices

We present a lithography-free technique for fabrication of clean, high quality graphene devices. This technique is based on evaporation through hard Si shadow masks, and eliminates contaminants introduced by lithographical processes. We demonstrate that devices fabricated by this technique have significantly higher mobility values than those by standard electron beam lithography. To obtain ultra-high mobility devices, we extend this technique to fabricate suspended graphene samples with mobility as high as 120,000 cm^2/Vs.

cond-mat.mes-hall↗
arXiv2010

Quantum criticality and nodal superconductivity in the FeAs-based superconductor KFe2As2

The in-plane resistivity $ρ$ and thermal conductivity $κ$ of FeAs-based superconductor KFe$_2$As$_2$ single crystal were measured down to 50 mK. We observe non-Fermi-liquid behavior $ρ(T) \sim T^{1.5}$ at $H_{c_2}$ = 5 T, and the development of a Fermi liquid state with $ρ(T) \sim T^2$ when further increasing field. This suggests a field-induced quantum critical point, occurring at the superconducting upper critical field $H_{c_2}$. In zero field there is a large residual linear term $κ_0/T$, and the field dependence of $κ_0/T$ mimics that in d-wave cuprate superconductors. This indicates that the superconducting gaps in KFe$_2$As$_2$ have nodes, likely d-wave symmetry. Such a nodal superconductivity is attributed to the antiferromagnetic spin fluctuations near the quantum critical point.

cond-mat.supr-con↗
arXiv2010

Direct bandgap optical transitions in Si nanocrystals

The effect of quantum confinement on the direct bandgap of spherical Si nanocrystals has been modelled theoretically. We conclude that the energy of the direct bandgap at the $Γ$-point decreases with size reduction: quantum confinement enhances radiative recombination across the direct bandgap and introduces its "red" shift for smaller grains. We postulate to identify the frequently reported efficient blue emission (F-band) from Si nanocrystals with this zero-phonon recombination. In a dedicated experiment, we confirm the "red" shift of the F-band, supporting the proposed identification.

cond-mat.mtrl-sci↗
arXiv2009

Dark Pulse Emission of A Fiber Laser

We report on the dark pulse emission of an all-normal dispersion erbium-doped fiber laser with a polarizer in cavity. We found experimentally that apart from the bright pulse emission, under appropriate conditions the fiber laser could also emit single or multiple dark pulses. Based on numerical simulations we interpret the dark pulse formation in the laser as a result of dark soliton shaping.

physics.optics↗
arXiv2009

Induced solitons formed by cross polarization coupling in a birefringent cavity fiber laser

We report on the experimental observation of induced solitons in a passively mode-locked fiber ring laser with birefringence cavity. Due to the cross coupling between the two orthogonal polarization components of the laser, it was found that if a soliton was formed along one cavity polarization axis, a weak soliton was also induced along the orthogonal polarization axis, and depending on the net cavity birefringence, the induced soliton could either have the same or different center wavelengths to that of the inducing soliton. Moreover, the induced soliton always had the same group velocity as that of the inducing soliton. They form a vector soliton in the cavity. Numerical simulations confirmed the experimental observations.

physics.optics↗
arXiv2009

Thermal conductivity study of KFe$_2$As$_2$ single crystal: clear evidence for unconventional superconducting gap with nodes

The in-plane resistivity $ρ$ and thermal conductivity $κ$ of extremely overdoped KFe$_2$As$_2$ ($T_c$ = 3.0 K) single crystal were studied. It is found that $ρ\sim T^{1.5}$ at low temperature, a typical non-Fermi liquid behavior of electrons scattered by antiferromagnetic spin fluctuations. In zero field, we observed a large residual linear term $κ_0/T$, about one third of the normal-state value. In low magnetic fields, $κ_0/T(H)$ increases very fast. Such a behavior of $κ_0/T$ mimics the d-wave cuprate superconductors, therefore provides clear evidence for nodes in the superconducting gap of KFe$_2$As$_2$. Based on the Fermi surface topology of KFe$_2$As$_2$, it is believed that the dominant intraband pairing via antiferromagnetic spin fluctuations results in the unconventional superconducting gap with nodes.

cond-mat.supr-con↗
arXiv2009

Vector dark domain wall solitons in a fiber ring laser

We observe a novel type of vector dark soliton in a fiber ring laser. The vector dark soliton consists of stable localized structures separating the two orthogonal linear polarization eigenstates of the laser emission and is visible only when the total laser emission is measured. Moreover, polarization domain splitting and moving polarization domain walls (PDWs) were also experimentally observed.

nlin.PS↗
arXiv2009

Large energy mode locking of an erbium-doped fiber laser with atomic layer graphene

We report on large energy pulse generation in an erbium-doped fiber laser passively mode-locked with atomic layer graphene. Stable mode locked pulses with single pulse energy up to 7.3 nJ and pulse width of 415 fs have been directly generated from the laser. Our results show that atomic layer graphene could be a promising saturable absorber for large energy mode locking.

physics.optics↗
arXiv2009

Observation of polarization domain wall solitons in weakly birefringent cavity fiber lasers

We report on the experimental observation of two types of phase-locked vector soliton in weakly birefringent cavity erbium-doped fiber lasers. While a phase-locked dark-dark vector soliton was only observed in fiber lasers of positive dispersion, a phase-locked dark-bright vector soliton was obtained in fiber lasers of either positive or negative dispersion. Numerical simulations confirmed the experimental observations, and further showed that the observed vector solitons are the two types of phase-locked polarization domain-wall solitons theoretically predicted.

physics.optics↗
arXiv2009

Multi-wavelength dissipative soliton operation of an erbium-doped fiber laser

We report on the generation of multi-wavelength dissipative soliton (DS) in an all normal dispersion fiber laser passively mode-locked with a semiconductor saturable absorber mirror (SESAM). We show that depending on the strength of the cavity birefringence, stable single-, dual- and triple-wavelength DSs can be formed in the laser. The multi-wavelength soliton operation of the laser was experimentally investigated, and the formation mechanisms of the multi-wavelength DSs are discussed.

physics.optics↗
arXiv2009

Is the energy generation rate of nuclear reactions in hot accretion flows important?

The temperature of hot accretion flows around black holes is sufficiently high for the ignition of nuclear reactions. This is potentially an important nucleosynthesis mechanism in the universe. As the first step in studying this problem, we need to measure physical quantities such as density and temperature of the accretion flow. In usual studies of the hot accretion flow, viscous dissipation is considered to be the only heating mechanism, while the heating caused by nuclear reactions is not considered. In this paper, we investigate whether the energy generation rate of nuclear reaction is important compared to the viscous heating. Our calculation indicates that the former is at most one percent of the latter and thus is not important. The dynamics of accretion flow can be therefore calculated in the usual way, without the need to consider heating due to nuclear reactions.

astro-ph.HE↗
arXiv2009

Dark Soliton Fiber Laser

We report on the experimental observation of stable dark solitons in an all normal dispersion fiber laser. We found experimentally that dark soliton formation is a generic feature of the fiber laser under strong continuous wave (CW) emission. However, only under appropriate pump strength and negative cavity feedback, stable single or multiple dark soliton could be achieved. Furthermore, we show that the features of the observed dark solitons could be well understood based on the nonlinear Schrodinger equation (NLSE).

physics.optics↗
arXiv2009

Measurement of x-ray absorption spectra of overdoped high-temperature cuprate superconductors: Inapplicability of the single-band Hubbard model

X-ray absorption spectra on the overdoped high-temperature superconductors Tl_2Ba_2CuO_{6+delta} (Tl-2201) and La_{2-x}Sr_xCuO_{4+delta} (LSCO) reveal a striking departure in the electronic structure from that of the underdoped regime. The upper Hubbard band, identified with strong correlation effects, is not observed on the oxygen K edge, while the lowest-energy prepeak gains less intensity than expected above p ~ 0.21. This suggests a breakdown of the Zhang-Rice singlet approximation and a loss of correlation effects or a significant shift in the most fundamental parameters of the system, rendering single-band Hubbard models inapplicable. Such fundamental changes suggest that the overdoped regime may offer a distinct route to understanding in the cuprates.

cond-mat.supr-con↗