arXiv subjects
Publications and source records attributed to K. Chen.
KAGRA is a newly build gravitational-wave observatory, a laser interferometer with 3 km arm length, located in Kamioka, Gifu, Japan. In this paper in the series of KAGRA-featured articles, we discuss the science targets of KAGRA projects, considering not only the baseline KAGRA (current design) but also its future upgrade candidates (KAGRA+) for the near to middle term (~5 years).
Owing to the experimental discovery of magnetic skyrmions stabilized by the Dzyaloshinskii-Moriya and/or dipolar interactions in thin films, there is a recent upsurge of interest in magnetic skyrmions with antiferromagnetic spins in order to overcome the fundamental limitations inherent with skyrmions in ferromagnetic materials. Here, we report on the observation of compact ferrimagnetic skyrmions for the class of amorphous alloys consisting of 4f rare-earth and 3d transition-metal elements with perpendicular magnetic anisotropy, using a DyCo$_3$ film, that are identified by combining x-ray magnetic scattering, scanning transmission x-ray microscopy, and Hall transport technique. These skyrmions, with antiparallel aligned Dy and Co magnetic moments and a characteristic core radius of about 40~nm, are formed during the nucleation and annihilation of the magnetic maze-like domain pattern exhibiting a topological Hall effect contribution. Our findings provide a promising route for fundamental research in the field of ferrimagnetic/antiferromagnetic spintronics towards practical applications.
We apply the independent component analysis (ICA) to the real data from a gravitational wave detector for the first time. Specifically we use the iKAGRA data taken in April 2016, and calculate the correlations between the gravitational wave strain channel and 35 physical environmental channels. Using a couple of seismic channels which are found to be strongly correlated with the strain, we perform ICA. Injecting a sinusoidal continuous signal in the strain channel, we find that ICA recovers correct parameters with enhanced signal-to-noise ratio, which demonstrates usefulness of this method. Among the two implementations of ICA used here, we find the correlation method yields the optimal result for the case environmental noises act on the strain channel linearly.
We have investigated the electronic structure of iridates in the double perovskite crystal structure containing either Ir$^{4+}$ or Ir$^{5+}$ using hard x-ray photoelectron spectroscopy. The experimental valence band spectra can be well reproduced using tight binding calculations including only the Ir $5d$, O $2p$ and O $2s$ orbitals with parameters based on the downfolding of the density-functional band structure results. We found that regardless of the A and B cations, the A$_2$BIrO$_6$ iridates have essentially zero O $2p$ to Ir $5d$ charge transfer energies. Hence, double perovskite iridates turn out to be extremely covalent systems with the consequence being that the magnetic exchange interactions become very long-ranged, thereby hampering the materialization of the long-sought Kitaev physics. Nevertheless, it still would be possible to realize a spin-liquid system using the iridates with a proper tuning of the various competing exchange interactions.
Charge transfer induced interfacial ferromagnetism and its impact on the exchange bias effect in La$_{0.7}$Sr$_{0.3}$MnO$_3$/NdNiO$_3$ correlated oxide heterostructures were investigated by soft x-ray absorption and x-ray magnetic circular dichroism spectra in a temperature range from 10 to 300 K. Besides the antiferromagnetic Ni$_3^+$ cations which are naturally part of the NdNiO$_3$ layer, Ni$_2^+$ ions are formed at the interface due to a charge transfer mechanism involving the Mn element of the adjacent layer. They exhibit a ferromagnetic behavior due to the exchange coupling to the Mn$_4^+$ ions in the La$_{0.7}$Sr$_{0.3}$MnO$_3$ layer. This can be seen as detrimental to the strength of the unidirectional anisotropy since a significant part of the interface does not contribute to the pinning of the ferromagnetic layer. By analyzing the line shape changes of the x-ray absorption at the Ni L$_{2,3}$ edges, the metal-insulator transition of the NdNiO$_3$ layer is resolved in an element specific manner. This phase transition is initiated at about 120 K, way above the paramagnetic to antiferromagnetic transition of NdNiO$_3$ layer which measured to be 50 K. Exchange bias and enhanced coercive fields were observed after field cooling the sample through the Neel temperature of the NdNiO$_3$ layer. Different from La$_{0.7}$Sr$_{0.3}$MnO$_3$/LaNiO$_3$, the exchange bias observed in La$_{0.7}$Sr$_{0.3}$MnO$_3$/NdNiO$_3$ is due to the antiferromagnetism of NdNiO$_3$ and the frustration at the interface. These results suggest that reducing the interfacial orbital hybridization may be used as a tunable parameter for the strength of the exchange bias effect in all-oxide heterostructures which exhibit a charge transfer mechanism.
Modern ground-based gravitational wave (GW) detectors require a complex interferometer configuration with multiple coupled optical cavities. Since achieving the resonances of the arm cavities is the most challenging among the lock acquisition processes, the scheme called arm length stabilization (ALS) had been employed for lock acquisition of the arm cavities. We designed a new type of the ALS, which is compatible with the interferometers having long arms like the next generation GW detectors. The features of the new ALS are that the control configuration is simpler than those of previous ones and that it is not necessary to lay optical fibers for the ALS along the kilometer-long arms of the detector. Along with simulations of its noise performance, an experimental test of the new ALS was performed utilizing a single arm cavity of KAGRA. This paper presents the first results of the test where we demonstrated that lock acquisition of the arm cavity was achieved using the new ALS and residual noise was measured to be $8.2\,\mathrm{Hz}$ in units of frequency, which is smaller than the linewidth of the arm cavity and thus low enough to lock the full interferometer of KAGRA in a repeatable and reliable manner.
We report on our investigation on the magnetism of the iridate double perovskite Sr$_2$CoIrO$_6$, a nominally Ir$^{5+}$ Van Vleck $J_{eff}=0$ system. Using x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy at the Ir-$L_{2,3}$ edges, we found a nearly zero orbital contribution to the magnetic moment and thus an apparent breakdown of the $J_{eff}=0$ ground state. By carrying out also XAS and XMCD experiments at the Co-$L_{2,3}$ edges and by performing detailed full atomic multiplet calculations to simulate all spectra, we discovered that the compound consists of about 90% Ir$^{5+}$ ($J_{eff}=0$) and Co$^{3+}$ ($S=2$) and 10% Ir$^{6+}$ ($S=3/2$) and Co$^{2+}$ ($S=3/2$). The magnetic signal of this minority Ir$^{6+}$ component is almost equally strong as that of the main Ir$^{5+}$ component. We infer that there is a competition between the Ir$^{5+}$-Co$^{3+}$ and the Ir$^{6+}$-Co$^{2+}$ configurations in this stoichiometric compound.
MXenes are a rapidly growing family of 2D materials that exhibit a highly versatile structure and composition, allowing for significant tuning of the material properties. These properties are, however, ultimately limited by the surface terminations, which are typically a mixture of species, including F and O that are inherent to the MXene processing. Other and robust terminations are lacking. Here, we apply high-resolution scanning transmission electron microscopy (STEM), corresponding image simulations and first-principles calculations to investigate the surface terminations on MXenes synthesized from MAX phases through Lewis acidic melts. The results show that atomic Cl terminates the synthesized MXenes, with mere residual presence of other termination species. Furthermore, in situ STEM-electron energy loss spectroscopy (EELS) heating experiments show that the Cl terminations are stable up to 750 °C. Thus, we present an attractive new termination that widely expands the MXenes functionalization space and enable new applications.
In the context of the studies of the ATLAS High Luminosity LHC programme, radiation tolerant pixel detectors in CMOS technologies are investigated. To evaluate the effects of substrate resistivity on CMOS sensor performance, the H35DEMO demonstrator, containing different diode and amplifier designs, was produced in ams H35 HV-CMOS technology using four different substrate resistivities spanning from $\mathrm{80}$ to $\mathrm{1000~Ω\cdot cm}$. A glueing process using a high-precision flip-chip machine was developed in order to capacitively couple the sensors to FE-I4 Readout ASIC using a thin layer of epoxy glue with good uniformity over a large surface. The resulting assemblies were measured in beam test at the Fermilab Test Beam Facilities with 120 GeV protons and CERN SPS H8 beamline using 80 GeV pions. The in-time efficiency and tracking properties measured for the different sensor types are shown to be compatible with the ATLAS ITk requirements for its pixel sensors.
The Address in Real Time Data Driver Card (ADDC) is designed to transmit the trigger data in the Micromegas detector of the ATLAS New Small Wheel (NSW) upgrade. The ART signals are generated by the front end ASIC, named VMM chip, to indicate the address of the first above-threshold event. A custom ASIC (ART ASIC) is designed to receive the ART signals from the VMM chip and do the hit-selection processing. Processed data from ART ASIC will be transmitted out of the NSW detector through GBTx serializer, VTTx optical transmitter module and fiber optical links. The ART signal is critical for the ATLAS experiment trigger selection thus the functionality and stability of the ADDC is very important. To ensure extensive testing of the ADDC, an FMC based testing platform and special firmware/software are developed. This test platform works with the commercial Xilinx VC707 FPGA develop kit, even without the other electronics of the NSW it can test all the functionality of the ADDC and also long term stability. This paper will introduce the design, testing procedure and results from the ADDC and the FMC testing platform.
Yb$_2$Si$_2$Al may be a prototype for exploring different aspects of the Shastry-Sutherland lattice, formed by planes of orthogonally coupled Yb ions. Measurements of the magnetic susceptibility find incoherently fluctuating Yb$^{3+}$ moments coexisting with a weakly correlated metallic state that is confirmed by measurements of the electrical resistivity. Increasing signs of Kondo coherence are found with decreasing temperature, including an enhanced Sommerfeld coefficient and Kadowaki-Woods ratio that signal that the metallic state found at the lowest temperatures is a Fermi liquid where correlations have become significantly stronger. A pronounced peak in the electronic and magnetic specific heat indicates that the coupling of the Yb moments to the conduction electrons leads to an effective Kondo temperature that is approximately 30 K. The valence of Yb$_2$Si$_2$Al has been investigated with electron spectroscopy methods. Yb$_2$Si$_2$Al is found to be strongly intermediate valent ($v_F=2.68(2)$ at 80 K). Taken together, these experimental data are consistent with a scenario where a coherent Kondo lattice forms in Yb$_2$Si$_2$Al from an incoherently fluctuating ensemble of Yb moments with incomplete Kondo compensation, and strong intermediate valence character.
We report a combined experimental and theoretical x-ray magnetic circular dichroism (XMCD) spectroscopy study at the Ir-$L_{2,3}$ edges on the Ir$^{5+}$ ions of the layered hybrid solid state oxide Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$ with the K$_2$NiF$_4$ structure. From theoretical simulation of the experimental Ir-$L_{2,3}$ XMCD spectrum, we found a deviation from a pure $J_{eff}=0$ ground state with an anisotropic orbital-to-spin moment ratio ($L_x/2S_x$ = 0.43 and $L_z/2S_z$ = 0.78). This deviation is mainly due to multiplet interactions being not small compared to the cubic crystal field and due to the presence of a large tetragonal crystal field associated with the crystal structure. Nevertheless, our calculations show that the energy gap between the singlet ground state and the triplet excited state is still large and that the magnetic properties of the Ir$^{5+}$ ions can be well described in terms of singlet Van Vleck paramagnetism.
We present linear polarization-dependent soft x-ray absorption spectroscopy data at the Ce $M_{4,5}$ edges of Cd and Sn doped CeCoIn$_5$. The 4$f$ ground state wave functions have been determined for their superconducting, antiferromagnetic and paramagnetic ground states. The absence of changes in the wave functions in CeCo(In$_{1-x}$Cd$_x$)$_5$ suggests the 4$f$\,--\,conduction electron ($cf$) hybridization is not affected by globally Cd doping, thus supporting the interpretation of magnetic droplets nucleating long range magnetic order. This is contrasted by changes in the wave function due to Sn substitution. Increasing Sn in CeCo(In$_{1-y}$Sn$_y$)$_5$ compresses the 4$f$ orbitals into the tetragonal plane of these materials, suggesting enhanced $cf$ hybridization with the in-plane In(1) atoms and a homogeneous altering of the electronic structure. As these experiments show, the 4$f$ wave functions are a very sensitive probe of small changes in the hybridization of 4$f$ and conduction electrons, even conveying information about direction dependencies.
HV-CMOS pixel sensors are a promising option for the tracker upgrade of the ATLAS experiment at the LHC, as well as for other future tracking applications in which large areas are to be instrumented with radiation-tolerant silicon pixel sensors. We present results of testbeam characterisations of the $4^{\mathrm{th}}$ generation of Capacitively Coupled Pixel Detectors (CCPDv4) produced with the ams H18 HV-CMOS process that have been irradiated with different particles (reactor neutrons and 18 MeV protons) to fluences between $1\cdot 10^{14}$ and $5\cdot 10^{15}$ 1-MeV-n$_\textrm{eq}$/cm$^2$. The sensors were glued to ATLAS FE-I4 pixel readout chips and measured at the CERN SPS H8 beamline using the FE-I4 beam telescope. Results for all fluences are very encouraging with all hit efficiencies being better than 97% for bias voltages of $85\,$V. The sample irradiated to a fluence of $1\cdot 10^{15}$ n$_\textrm{eq}$/cm$^2$ - a relevant value for a large volume of the upgraded tracker - exhibited 99.7% average hit efficiency. The results give strong evidence for the radiation tolerance of HV-CMOS sensors and their suitability as sensors for the experimental HL-LHC upgrades and future large-area silicon-based tracking detectors in high-radiation environments.
We report on a combined soft x-ray absorption and magnetic circular dichroism (XMCD) study at the Co-$L_{3,2}$ on the hybrid 3$d$/5$d$ solid state oxide Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$ with the K$_2$NiF$_4$ structure. Our data indicate unambiguously a pure high spin state $(S=2)$ for the Co$^{3+}$ (3$d^6$) ions with a significant unquenched orbital moment $L_z/2S_z=0.25$ despite the sizeable elongation of the CoO$_6$ octahedra. Using quantitative model calculations based on parameters consistent with our spectra, we have investigated the stability of this high spin state with respect to the competing low spin and intermediate spin states.
Bulk sensitive hard x-ray photoelectron spectroscopy data of the Ce 3$p$ core level of CeRu$_4$Sn$_6$ are presented. Using a combination of full multiplet and configuration iteration model we were able to obtain an accurate lineshape analysis of the data, thereby taking into account correlations for the strong plasmon intensities. We conclude that CeRu$_4$Sn$_6$ is a moderately mixed valence compound with a weight of 8% for the Ce $f^0$ configuration in the ground state.
This paper describes the design and construction of the MicroBooNE liquid argon time projection chamber and associated systems. MicroBooNE is the first phase of the Short Baseline Neutrino program, located at Fermilab, and will utilize the capabilities of liquid argon detectors to examine a rich assortment of physics topics. In this document details of design specifications, assembly procedures, and acceptance tests are reported.
We investigated the ground state symmetry of the cubic hidden order compound CeB$_6$ by means of core level non-resonant inelastic x-ray scattering (NIXS). The information is obtained from the directional dependence of the scattering function that arises from higher than dipole transitions. Our new method confirms that the ground state is well described using a localized crystal-field model assuming a $Γ_8$ quartet ground state.