arXiv subjects

J. Lu

Publications and source records attributed to J. Lu.

At least 73 records · Page 4Linked to original sources

arXiv2018

Phase change materials for nano-polaritonics: a case study of hBN/VO2 heterostructures

Polaritonic excitation and control in van der Waals (vdW) materials exhibit superior merits than conventional materials and thus hold new promise for exploring light matter interactions. In this work, we created vdW heterostructures combining hexagonal boron nitride (hBN) and a representative phase change material - vanadium dioxide (VO2). Using infrared nano-spectroscopy and nano-imaging, we demonstrated the dynamic tunability of hyperbolic phonon polaritons in hBN/VO2 heterostructures by temperature control in a precise and reversible fashion. The dynamic tuning of the polaritons stems from the change of local dielectric properties of the VO2 sublayer through insulator to metal transition by the temperature control. The high susceptibility of polaritons to electronic phase transitions opens possibilities for applications of vdW materials in combination with correlated phase change materials.

cond-mat.mes-hall↗

arXiv2018

Off-axis point spread function characterisation in laser-guide star adaptive optics systems

Adaptive optics (AO) restore the angular resolution of ground-based telescopes, but at the cost of delivering a time- and space-varying point spread function (PSF) with a complex shape. PSF knowledge is crucial for breaking existing limits on the measured accuracy of photometry and astrometry in science observations. In this paper, we concentrate our analyses on anisoplanatism signature only onto PSF: for large-field observations (20") with single- conjugated AO, PSFs are strongly elongated due to anisoplanatism that manifests itself as three different terms for Laser-guide star (LGS) systems: angular, focal and tilt. We propose a generalized model that relies on a point-wise decomposition of the phase and encompasses the non-stationarity of LGS systems. We demonstrate it is more accurate and less computationally demanding than existing models: it agrees with end-to-end physical-optics simulations to within 0.1% of PSF measurables, such as Strehl-ratio, FWHM and fraction of variance unexplained.Secondly, we study off-axis PSF modelling is with respect to $C_n^2(h)$ profile (heights and fractional weights). For 10m class telescope, PSF morphology is estimated at 1% level as long as we model the atmosphere with at least 7 layers whose heights and weights are known respectively with 200m and 10% precision. As a verification test we used the Canada's NRC-Herzberg HeNOS testbed data, featuring four lasers. We highlight capability of retrieving off-axis PSF characteristics within 10% of fraction of variance unexplained, which complies with the expected range from the sensitivity analysis. Our new off-axis PSF modelling method lays the ground-work for testing on-sky in the near future.

astro-ph.IM↗

arXiv2017

Low temperature physical properties of Co-35Ni-20Mo-10Cr alloy MP35N

Multiphase Co-35Ni-20Mo-10Cr alloy MP35N is a high strength alloy with excellent corrosion resistance. Its applications span chemical, medical, and food processing industries. Thanks to its high modulus and high strength, it found applications in reinforcement of ultra-high field pulsed magnets. Recently, it has also been considered for reinforcement in superconducting wires used in ultra-high field superconducting magnets. For these applications, accurate measurement of its physical properties at cryogenic temperatures is very important. In this paper, physical properties including electrical resistivity, specific heat, thermal conductivity, and magnetization of as-received and aged samples are measured from 2 to 300 K. The electrical resistivity of the aged sample is slightly higher than the as-received sample, both showing a weak linear temperature dependence in the entire range of 2 - 300 K. The measured specific heat Cp of 0.43 J/g-K at 295 K agrees with a theoretical prediction, but is significantly smaller than the values in the literature. The thermal conductivity between 2 and 300 K is in good agreement with the literature which is only available above 77 K. Magnetic property of MP35N changes significantly with aging. The as-received sample exhibits Curie paramagnetism with a Curie constant C = 0.175 K. While the aged sample contains small amounts of a ferromagnetic phase even at room temperature. The measured MP35N properties will be useful for the engineering design of pulsed magnets and superconducting magnets using MP35N as reinforcement.

cond-mat.mtrl-sci↗

arXiv2017

Influence of heat treatment excursion on critical current and residual resistivity ratio of ITER Nb3Sn strands

Heat treatment is critically important to the performance of Nb3Sn superconducting strands. For very large Nb3Sn magnet coils, such as the International Thermonuclear Experimental Reactor (ITER) Central Solenoid (CS) coils, heat treatment carries risk of temperature and time excursion, which may result in performance degradation. Therefore, it is prudent to study the effect of possible excursion on Nb3Sn performance. In this study, Nb3Sn strands used for ITER CS coils are heat treated at different temperatures for different times. Their critical current, residual resistance ratio and hysteresis losses are measured. It is found that in the range we studied, critical current and hysteresis losses do not change significantly. Residual resistance ratio, however, decreases with increasing heat treatment temperature and time. This is attributed to the diffusion of metallic elements from the plated Cr layer to the copper stabilizer. Based on a model of metallic elements diffusion, a numerical code is developed to predict residual resistance ratio as a function of heat treatment temperature and time.

cond-mat.supr-con↗

arXiv2016

Comparative measurements of ITER Nb3Sn strands between two laboratories

ITER Nb3Sn strand quality verification tests require large quantities of precise measurements. Therefore regular cross-checking between testing laboratories is critically important. In this paper, we present results from a cross-checking test of 140 samples between the National High Magnetic Field Laboratory, USA and the University of Twente, the Netherlands. The tests comprise measurements at 4.2 K on critical current, RRR and hysteresis loss, while at room temperature the chromium layer thickness, Cu/nonCu ratio, filament twist pitch, and diameter were determined. Our results show very good agreement between the two labs. The reasons for small random discrepancies are discussed.

physics.ins-det↗

arXiv2016

Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO$_3$

The dynamical processes associated with electric field manipulation of the polarization in a ferroelectric remain largely unknown but fundamentally determine the speed and functionality of ferroelectric materials and devices. Here we apply sub-picosecond duration, single-cycle terahertz pulses as an ultrafast electric field bias to prototypical BaTiO$_3$ ferroelectric thin films with the atomic-scale response probed by femtosecond x-ray scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large amplitude displacements of the titanium atoms along the ferroelectric polarization axis, comparable to that of the built-in displacements associated with the intrinsic polarization and incoherent across unit cells. This effect is associated with a dynamic rotation of the ferroelectric polarization switching on and then off on picosecond timescales. These transient polarization modulations are followed by long-lived vibrational heating effects driven by resonant excitation of the ferroelectric soft mode, as reflected in changes in the c-axis tetragonality. The ultrafast structural characterization described here enables direct comparison with first-principles-based molecular dynamics simulations, with good agreement obtained.

cond-mat.mtrl-sci↗

arXiv2015

Effect of inhomogeneities and substrate on the dynamics of the metal-insulator transition in VO$_2$ thin films

We study the thermal relaxation dynamics of VO$_2$ films after the ultrafast photo-induced metal-insulator transition for two VO$_2$ film samples grown on Al$_2$O$_3$ and TiO$_2$ substrates. We find two orders of magnitude difference in the recovery time (a few ns for the VO$_2$/Al$_2$O$_3$ sample vs. hundreds of ns for the VO$_2$/TiO$_2$ sample). We present a theoretical model that accurately describes the MIT thermal properties and interpret the experimental measurements. We obtain quantitative results that show how the microstructure of the VO$_2$ film and the thermal conductivity of the interface between the VO$_2$ film and the substrate affect long time-scale recovery dynamics. We also obtain a simple analytic relationship between the recovery time-scale and some of the film parameters.

cond-mat.mtrl-sci↗

arXiv2015

Substrate-induced microstructure effects on the dynamics of the photo-induced Metal-insulator transition in VO$_2$ thin films

We investigate the differences in the dynamics of the ultrafast photo-induced metal-insulator transition (MIT) of two VO$_2$ thin films deposited on different substrates, TiO$_2$ and Al$_2$O$_3$, and in particular the temperature dependence of the threshold laser fluence values required to induce various MIT stages in a wide range of sample temperatures (150 K - 320 K). We identified that, although the general pattern of MIT evolution was similar for the two samples, there were several differences. Most notably, the threshold values of laser fluence required to reach the transition to a fully metallic phase in the VO$_2$ film on the TiO$_2$ substrate were nearly constant in the range of temperatures considered, whereas the VO$_2$/Al$_2$O$_3$ sample showed clear temperature dependence. Our analysis qualitatively connects such behavior to the structural differences in the two VO$_2$ films.

physics.optics↗

arXiv2013

Neutron collimator design of neutron radiography based on the BNCT facility

For the research of CCD neutron radiography, a neutron collimator was designed based on the exit of thermal neutron of the Boron Neutron Capture Therapy (BNCT) reactor. Based on the Geant4 simulations, the preliminary choice of the size of the collimator was determined. The materials were selected according to the literature data. Then, a collimator was constructed and tested on site. The results of experiment and simulation show that the thermal neutron flux at the end of theneutron collimator is greater than 10^6 n/cm^2/s, the maximum collimation ratio (L/D) is 58, the Cd-ratio(Mn) is 160 and the diameter of collimator end is 10 cm. This neutron collimator is considered to be applicable for neutron radiography.

physics.ins-det↗

arXiv2013

Room-temperature delayed giant magnetodielectric effects observed in Bi4Fe2TiO12 film

This letter reports our experimental results on magnetic-field-induced aftereffects in solution derived Bi4Fe2TiO12 film, in which e' decreases as much as 600 times after treated in a 5T magnetic field as that of the pristine one. The phenomena are explained by an unusual spin-orbital coupling in film, where strong magnetic field provides drive force for spin polarized electrons of electrode to enter film, so as to form a ferromagnetic orbital state, increase the relaxation time of defect dipoles and then produce giant magnetodielectric effects. This process is reversible just by applying a opposite 0.5T magnetic field. Our findings have potential applications on magnetic fabrication of nanopatterns somehow to evade nanohazard, as well as on integraged phononic or photonic crystals on chip.

cond-mat.mtrl-sci↗

arXiv2013

Substrate Effect on Optical Properties of Insulator-Metal Transition in VO2 Thin Films

In this paper we used Raman spectroscopy to investigate the optical properties of vanadium dioxide (VO2) thin films during the thermally induced insulating to metallic phase transition. We observed a significant difference in transition temperature in similar VO2 films grown on quartz and sapphire substrates: the film grown on quartz displayed the phase transition at a lower temperature (Tc=50C) compared a film grown on sapphire (Tc=68C). We also investigated differences in the detected Raman signal for different wavelengths and polarizations of the excitation laser. We found that for either substrate, a longer wavelength (in our case 785 nm) yielded the clearest VO2 Raman spectra, with no polarization dependence.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic transitions and magnetodielectric effect in the antiferromagnet SrNdFeO$_4$

We investigated the magnetic phase diagram of single crystals of SrNdFeO$_{4}$ by measuring the magnetic properties, the specific heat and the dielectric permittivity. The system has two magnetically active ions, Fe$^{3+}$ and Nd$^{3+}$. The Fe$^{3+}$ spins are antiferromagnetically ordered below 360 K with the moments lying in the ab-plane, and undergo a reorientation transition at about 35-37 K to an antiferromagnetic order with the moments along the c-axis. A short-range, antiferromagnetic ordering of Nd$^{3+}$ along the c-axis was attributed to the reorientation of Fe$^{3+}$ followed by a long-range ordering at lower temperature [S. Oyama {\it et al.} J. Phys.: Condens. Matter. {\bf 16}, 1823 (2004)]. At low temperatures and magnetic fields above 8 T, the Nd$^{3+}$ moments are completely spin-polarized. The dielectric permittivity also shows anomalies associated with spin configuration changes, indicating that this compound has considerable coupling between spin and lattice. A possible magnetic structure is proposed to explain the results.

cond-mat.mtrl-sci↗

arXiv2012

Magneto-electric effect in NdCrTiO5

We have measured the dielectric constant and the pyroelectric current of orthorhombic (space group $Pbam$) NdCrTiO$_5$ polycrystalline samples. The dielectric constant and the pyroelectric current show features associated with ferroelectric transitions at the antiferromagnetic transition temperature ($T_{\text{N}}$ = 21 K). The effect of magnetic fields is to enhance the features almost linearly up to the maximum measured field (7 T) with a spontaneous polarization value of $\sim 3.5 μ$C/m$^2$. Two possible scenarios, the linear magnetoelectric effect and multiferroicity (antiferromagnetism + ferroelectricity), are discussed as possible explanations for the observations.

cond-mat.mtrl-sci↗

arXiv2009

Correlations of structural, magnetic, and dielectric properties of undoped and doped CaCu3Ti4O12

The present work reports synthesis, as well as a detailed and careful characterization of structural, magnetic, and dielectric properties of differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this purpose, neutron and x-ray powder diffraction, SQUID measurements, and dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO ceramics were investigated in great detail to document the influence of low-level doping with 3d metals on the antiferromagnetic structure and dielectric properties. In the light of possible magnetoelectric coupling in these doped ceramics, the dielectric measurements were also carried out in external magnetic fields up to 7 T, showing a minor but significant dependence of the dielectric constant on the applied magnetic field. Undoped CCTO is well-known for its colossal dielectric constant in a broad frequency and temperature range. With the present extended characterization of doped as well as undoped CCTO, we want to address the question why doping with only 1% Mn or 0.5% Fe decreases the room-temperature dielectric constant of CCTO by a factor of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni doping changes the dielectric properties only slightly. In addition, diffraction experiments and magnetic investigations were undertaken to check for possible correlations of the magnitude of the colossal dielectric constants with structural details or with magnetic properties like the magnetic ordering, the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that while the magnetic ordering temperature and the effective moment of all investigated CCTO ceramics are rather similar, there is a dramatic influence of doping and tempering time on the Curie-Weiss constant.

cond-mat.mtrl-sci↗

arXiv2009

Mild sonochemical exfoliation of bromine-intercalated graphite: a new route towards graphene

A method to produce suspensions of graphene sheets by combining solution-based bromine intercalation and mild sonochemical exfoliation is presented. Ultrasonic treatment of graphite in water leads to the formation of suspensions of graphite flakes. The delamination is dramatically improved by intercalation of bromine into the graphite before sonication. The bromine intercalation was verified by Raman spectroscopy as well as by x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations show an almost ten times lower interlayer binding energy after introducing Br2 into the graphite. Analysis of the suspended material by transmission and scanning electron microscopy (TEM and SEM) revealed a significant content of few-layer graphene with sizes up to 30 $mu$m, corresponding to the grain size of the starting material.

cond-mat.mtrl-sci↗

arXiv2009

Field-induced domain wall propagation velocity in magnetic nanowires

A thory of field-induced domain wall (DW) propagation is developed. The theory not only explains why a DW in a defect-free nanowire must propagate at a finite velocity, but also provides a proper definition of DW propagation velocity. This definition, valid for an arbitrary DW structure, allows one to compute the instantaneous DW velocity in a meaningful way even when the DW is not moving as a rigid body. A new velocity-field formula beyond the Walker breakdown field, which is in excellent agreement with both experiments and numerical simulations, is derived.

cond-mat.mes-hall↗

arXiv2009

The Galactic Center: A Laboratory for Fundamental Astrophysics and Galactic Nuclei

As the closest example of a galactic nucleus, the Galactic center presents an exquisite laboratory for learning about supermassive black holes (SMBH) and their environs. Detailed studies of stellar dynamics deep in the potential well of a galaxy, with exisiting and future large ground-based telescopes, offer several exciting directions in the coming decade. First, it will be possible to obtain precision measurements of the Galaxy's central potential, providing both a unique test of General Relativity (GR) and a detection of the extended dark matter distribution that is predicted to exist around the SMBH. Tests of gravity have not previously been possible on the mass scale of a SMBH. Similarly, only upper limits on the extended matter distribution on small scales currently exist; detection of dark matter on these scales is an important test of Lambda-CDM and the detection of stellar remnants would reveal a population that may dominate the stellar dynamics on the smallest scales. Second, our detailed view of the SMBH and its local gas and stellar environment provides insight into how SMBHs at the centers of galaxies form, grow and interact with their environs as well as on the exotic processes at work in the densest stellar clusters in the Universe. The key questions, still unanswered, of when and how SMBHs formed in the early universe, and the myriad ways in which feedback from SMBHs can affect structure formation, can be informed by directly observing the physical processes operating at the SMBH.

astro-ph.GA↗

arXiv2008

The theory of magnetic field induced domain-wall propagation in magnetic nanowires

A global picture of magnetic domain wall (DW) propagation in a nanowire driven by a magnetic field is obtained: A static DW cannot exist in a homogeneous magnetic nanowire when an external magnetic field is applied. Thus, a DW must vary with time under a static magnetic field. A moving DW must dissipate energy due to the Gilbert damping. As a result, the wire has to release its Zeeman energy through the DW propagation along the field direction. The DW propagation speed is proportional to the energy dissipation rate that is determined by the DW structure. An oscillatory DW motion, either the precession around the wire axis or the breath of DW width, should lead to the speed oscillation.

cond-mat.other↗