arXiv subjects
Publications and source records attributed to B. Shen.
Compilation of papers presented by the VERITAS Collaboration at the 39th International Cosmic Ray Conference (ICRC), held July 14 through July 24, 2025 in Geneva, Switzerland.
Motivated by recent studies of the cluster Mott insulator candidate compound Nb3Cl8, this study performs 93Nb and 35Cl nuclear magnetic resonance (NMR) measurements to investigate the electron correlations. Below the structural transition temperature Ts ~ 97 K, all satellites of the 93Nb NMR spectra split into three distinct peaks, which suggests symmetry lowering due to the structural transition and could be attributed to the change in the Nb-Nb bond-lengths of the Nb3 clusters. The spin-lattice relaxation rate 1/T1 divided by the temperature T, 1/T1T, increases upon cooling to Ts for all Cl sites, whereas only the Knight shift K of Cl located at the center of the Nb3 clusters exhibits a temperature dependence similar to that observed in magnetic susceptibility. These findings collectively demonstrate the existence of strong spin correlations between the Nb atoms in Nb3Cl8, which are closely associated with Mottness.
Trench defects in multi-quantum wells (MQWs) have been considered as flawed structures that severely degrade the internal quantum efficiency of light-emitting diodes (LEDs) in the past. In this research, trench defects are innovatively modulated into the structure to enhance the efficiency of red InGaN LEDs. Specifically, dual-color MQWs structures are grown with green MQWs at the bottom and red MQWs at the top. When high-density trench defects are introduced into the green MQWs, the upper red MQWs exhibit a significant wavelength redshift of 68 nm and approximately 6-fold luminescence enhancement compared to those without trench defects. The wavelength redshift is attributed to the increased indium incorporation due to the strain relaxation effect of trench defects. Moreover, the luminescence enhancement originates from the strong emission of the red MQWs inside trench defects. The mechanisms behind the superior luminescent properties of red MQWs within trench defects are explored in detail. Red InGaN LEDs with an internal quantum efficiency of 16.4% are achieved by modulating the trench defects. The method of achieving InGaN-based red emission by introducing trench defects is simple and reproducible, requiring no additional substrate designs. This research provides a novel pathway toward achieving high-efficiency red InGaN LEDs.
We report on a search for a new, short-range, spin-dependent interaction using a modified version of the experimental apparatus used to measure the permanent neutron electric dipole moment at the Paul Scherrer Institute. This interaction, which could be mediated by axion-like particles, concerned the unpolarized nucleons (protons and neutrons) near the material surfaces of the apparatus and polarized ultracold neutrons stored in vacuum. The dominant systematic uncertainty resulting from magnetic-field gradients was controlled to an unprecedented level of approximately 4 pT/cm using an array of optically-pumped cesium vapor magnetometers and magnetic-field maps independently recorded using a dedicated measurement device. No signature of a theoretically predicted new interaction was found, and we set a new limit on the product of the scalar and the pseudoscalar couplings $g_sg_p\lambda^2 < 8.3 \times 10^{-28}\,\text{m}^2$ (95% C.L.) in a range of $5\,\mu\text{m} < \lambda < 25\,\text{mm}$ for the monopole-dipole interaction. This new result confirms and improves our previous limit by a factor of 2.7 and provides the current tightest limit obtained with free neutrons.
We present the magnetically shielded room (MSR) for the n2EDM experiment at the Paul Scherrer Institute which features an interior cubic volume with each side of length 2.92m, thus providing an accessible space of 25m3. The MSR has 87 openings up to 220mm diameter to operate the experimental apparatus inside, and an intermediate space between the layers for sensitive signal processing electronics. The characterization measurements show a remanent magnetic field in the central 1m3 below 100pT, and a field below 600pT in the entire inner volume, up to 4\,cm to the walls. The quasi-static shielding factor at 0.01\,Hz measured with a sinusoidal 2muT peak-to-peak signal is about 100,000 in all three spatial directions and rises fast with frequency to reach 10^8 above 1Hz.
We report a study of isoelectronic chemical substitution in the recently discovered quantum critical ferromagnet CeRh$_6$Ge$_4$. Upon silicon-doping, the ferromagnetic ordering temperature of CeRh$_6$(Ge$_{1-x}$Si$_x$)$_4$ is continuously suppressed, and no transition is observed beyond $x_c$$\approx$0.125. Non-Fermi liquid behavior with $C/T \propto$log($T^*/T$) is observed close to $x_c$, indicating the existence of strong quantum fluctuations, while the $T$-linear behavior observed upon pressurizing the parent compound is absent in the resistivity, which appears to be a consequence of the disorder induced by silicon doping. Our findings provide evidence for the role played by disorder on the unusual ferromagnetic quantum criticality in CeRh$_6$Ge$_4$, and provides further evidence for understanding the origin of this behavior.
CeRh$_6$Ge$_4$ is an unusual example of a stoichiometric heavy fermion ferromagnet, which can be cleanly tuned by hydrostatic pressure to a quantum critical point. In order to understand the origin of this anomalous behavior, we have characterized the magnetic ordering and crystalline electric field (CEF) scheme of this system. While magnetic Bragg peaks are not resolved in neutron powder diffraction, coherent oscillations are observed in zero-field $μ$SR below $T_{\rm C}$, which are consistent with in-plane ferromagnetic ordering consisting of reduced Ce moments. From analyzing the magnetic susceptibility and inelastic neutron scattering, we propose a CEF-level scheme which accounts for the easy-plane magnetocrystalline anisotropy, where the low lying first excited CEF exhibits significantly stronger hybridization than the ground state. These results suggest that the orbital anisotropy of the ground state and low lying excited state doublets are important for realizing anisotropic electronic coupling between the $f$- and conduction electrons, which gives rise to the highly anisotropic hybridization observed in photoemission experiments.
The self-consistent nonlinear dynamics of a relativistic charged particle beam interacting with its complete self-fields is a fundamental problem underpinning many of the accelerator design issues in high brightness beam applications, as well as the development of advanced accelerators. Particularly, synchrotron radiation induced effects in a magnetic dispersive beamline element can lead to collective beam instabilities and emittance growth. A novel beam dynamic code is developed based on a Lagrangian method for the calculation of the particles' radiation near-fields using wavefront/wavelet meshes via the Green's function of the Maxwell equations. These fields are then interpolated onto a moving mesh for dynamic update of the beam. This method allows radiation co-propagation and self-consistent interaction with the beam in the simulation at greatly reduced numerical errors. Multiple levels of parallelisms are inherent in this method and implemented in our code CoSyR to enable at-scale simulations of nonlinear beam dynamics on modern computing platforms using MPI, multi-threading, and GPUs. CoSyR has been used to evaluate the transverse and longitudinal coherent radiation effects on the beam and to investigate beam optics designs proposed for mitigation of beam brightness degradation in a magnetic bunch compressor. In this paper, the design of CoSyR, as well as the benchmark with other coherent synchrotron radiation models, are described and discussed.
Next generation high performance (HP) tokamaks risk damage from unmitigated disruptions at high current and power. Achieving reliable disruption prediction for a device's HP operation based on its low performance (LP) data is key to success. In this letter, through explorative data analysis and dedicated numerical experiments on multiple existing tokamaks, we demonstrate how the operational regimes of tokamaks can affect the power of a trained disruption predictor. First, our results suggest data-driven disruption predictors trained on abundant LP discharges work poorly on the HP regime of the same tokamak, which is a consequence of the distinct distributions of the tightly correlated signals related to disruptions in these two regimes. Second, we find that matching operational parameters among tokamaks strongly improves cross-machine accuracy which implies our model learns from the underlying scalings of dimensionless physics parameters like q_{95}, β_{p} and confirms the importance of these parameters in disruption physics and cross machine domain matching from the data-driven perspective. Finally, our results show how in the absence of HP data from the target devices, the best predictivity of the HP regime for the target machine can be achieved by combining LP data from the target with HP data from other machines. These results provide a possible disruption predictor development strategy for next generation tokamaks, such as ITER and SPARC, and highlight the importance of developing on existing machines baseline scenario discharges of future tokamaks to collect more relevant disruptive data.
Magnetic Johnson-Nyquist noise (JNN) originating from metal electrodes, used to create a static electric field in neutron electric-dipole-moment (nEDM) experiments, may limit the sensitivity of measurements. We present here the first dedicated study on JNN applied to a large-scale long-measurement-time experiment with the implementation of a co-magnetometry. In this study, we derive surface- and volume-averaged root-mean-square normal noise amplitudes at a certain frequency bandwidth for a cylindrical geometry. In addition, we model the source of noise as a finite number of current dipoles and demonstrate a method to simulate temporal and three-dimensional spatial dependencies of JNN. The calculations are applied to estimate the impact of JNN on measurements with the new apparatus, n2EDM, at the Paul Scherrer Institute. We demonstrate that the performances of the optically pumped $^{133}$Cs magnetometers and $^{199}$Hg co-magnetometers, which will be used in the apparatus, are not limited by JNN. Further, we find that in measurements deploying a co-magnetometer system, the impact of JNN is negligible for nEDM searches down to a sensitivity of $4\,\times\,10^{-28}\,e\cdot{\rm cm}$ in a single measurement; therefore, the use of economically and mechanically favored solid aluminum electrodes is possible.
We report an infrared spectroscopy study of the axion topological insulator candidate EuIn$_2$As$_2$ for which the Eu moments exhibit an A-type antiferromagnetic (AFM) order below $T_N \simeq 18 \mathrm{K}$. The low energy response is composed of a weak Drude peak at the origin, a pronounced infrared-active phonon mode at 185 cm$^{-1}$ and a free carrier plasma edge around 600 cm$^{-1}$. The interband transitions start above 800 cm$^{-1}$ and give rise to a series of weak absorption bands at 5\,000 and 12\,000 cm$^{-1}$ and strong ones at 20\,000, 27\,500 and 32\,000 cm$^{-1}$. The AFM transition gives rise to pronounced anomalies of the charge response in terms of a cusp-like maximum of the free carrier scattering rate around $T_N$ and large magnetic splittings of the interband transitions at 5\,000 and 12\,000 cm$^{-1}$. The phonon mode at 185 cm$^{-1}$ has also an anomalous temperature dependence around $T_N$ which suggests that it couples to the fluctuations of the Eu spins. The combined data provide evidence for a strong interaction amongst the charge, spin and lattice degrees of freedom.
Direct evidence of micro-turbulence effect on the onset of neoclassical tearing mode (NTM) is reported for the first time in this letter. A puzzling positive correlation between critical width of seed island of NTM and normalized plasma pressure beta_p is first observed employing a novel method for clearly separating the processes of seed island and the onset of NTM in the EAST tokamak. Different from the methods developed before, the width of the seed island is well controlled by slowly ramping up the current in resonant magnetic perturbation coils. It is revealed that the positive correlation is mainly attributed to the enhancement of perpendicular transport by micro-turbulence, which overcomes the destabilizing effect of beta_p on the onset of NTM. Reduced magnetohydrodynamics (MHD) modeling well reproduced the two states of nonlinear bifurcations observed in this experiment by including the finite transport effect. This result provides a new route for understanding multi-scale interaction in plasma physics.
We have studied the superconducting gap structure of LaPt$_2$Si$_2$ by measuring the temperature dependence of the London penetration depth shift $Δλ(T)$ and point contact spectroscopy of single crystals. $Δλ(T)$ shows an exponential temperature dependence at low temperatures, and the derived normalized superfluid density $ρ_{s}(T)$ can be well described by a single-gap s-wave model. The point-contact conductance spectra can also be well fitted by an s-wave Blonder-Tinkham-Klapwijk model, where the gap value shows a typical BCS temperature and magnetic field dependence consistent with type-II superconductivity. These results suggest fully gapped superconductivity in LaPt$_2$Si$_2$, with moderately strong electron-phonon coupling.
We present the design of a next-generation experiment, n2EDM, currently under construction at the ultracold neutron source at the Paul Scherrer Institute (PSI) with the aim of carrying out a high-precision search for an electric dipole moment of the neutron. The project builds on experience gained with the previous apparatus operated at PSI until 2017, and is expected to deliver an order of magnitude better sensitivity with provision for further substantial improvements. An overview is given of the experimental method and setup, the sensitivity requirements for the apparatus are derived, and its technical design is described.
With infrared spectroscopy we studied the bulk electronic properties of the topological antiferromagnet MnBi$_2$Te$_4$ with $T_N \simeq 25~\mathrm{K}$. With the support of band structure calculations, we assign the intra- and interband excitations and determine the band gap of $E_g \approx$ 0.17 eV. We also obtain evidence for two types of conduction bands with light and very heavy carriers. The multiband free carrier response gives rise to an unusually strong increase of the combined plasma frequency, $ω_{\mathrm{pl}}$, below 300 K. The band reconstruction below $T_N$, yields an additional increase of $ω_{\mathrm{pl}}$ and a splitting of the transition between the two conduction bands by about 54 meV. Our study thus reveals a complex and strongly temperature dependent multi-band low-energy response that has important implications for the study of the surface states and device applications.
Symmetry plays a central role in conventional and topological phases of matter, making the ability to optically drive symmetry change a critical step in developing future technologies that rely on such control. Topological materials, like the newly discovered topological semimetals, are particularly sensitive to a breaking or restoring of time-reversal and crystalline symmetries, which affect both bulk and surface electronic states. While previous studies have focused on controlling symmetry via coupling to the crystal lattice, we demonstrate here an all-electronic mechanism based on photocurrent generation. Using second-harmonic generation spectroscopy as a sensitive probe of symmetry change, we observe an ultrafast breaking of time-reversal and spatial symmetries following femtosecond optical excitation in the prototypical type-I Weyl semimetal TaAs. Our results show that optically driven photocurrents can be tailored to explicitly break electronic symmetry in a generic fashion, opening up the possibility of driving phase transitions between symmetry-protected states on ultrafast time scales.
We report measurements of the electrical resistivity and ac magnetic susceptibility of single crystalline LaPt$_2$Si$_2$ under pressure, in order to investigate the interplay of superconductivity and CDW order. LaPt$_2$Si$_2$ exhibits a first order phase transition from a tetragonal to orthorhombic structure, accompanied by the onset of CDW order below $T_{\rm{CDW}}$ = 76 K, while superconductivity occurs at a lower temperature of $T_{\rm{c}}$ = 1.87 K. We find that the application of pressure initially suppresses the CDW transition, but enhances $T_{\rm{c}}$. At pressures above 2.4 GPa, CDW order vanishes, while both $T_{\rm{c}}$ and the resistivity $A$-coefficient reach a maximum value around this pressure. Our results suggest that the occurrence of a superconducting dome can be accounted for within the framework of BCS theory, where there is a maximum in the density of states upon the closure of the CDW gap.
Many theories beyond the Standard Model postulate short-range modifications to gravity which produce deviations of Newton's gravitational potential from a strict $1/r$ dependence. It is common to analyze experiments searching for these modifications using a potential of the form $V^{\prime}(r)=-\frac{GMm}{r} [1+α\exp{(-r/λ)}]$. The best present constraints on $α$ for $λ<100$\,nm come from neutron scattering and often employ comparisons of different measurements of the coherent neutron scattering amplitudes $b$. We analyze the internal consistency of existing data from two different types of measurements of low energy neutron scattering amplitudes: neutron interferometry, which involves squared momentum transfers $q^{2}=0$, and neutron gravity reflectometry, which involves squared momentum transfers $q^{2}=8mV_{opt}$ where $m$ is the neutron mass and $V_{opt}$ is the neutron optical potential of the medium. We show that the fractional difference $\frac{Δb}{|b|}$ averaged over the 7 elements where high precision data exists on the same material from both measurement methods is $[2.2 \pm 1.4] \times 10^{-4}$. We also show that $\frac{Δb}{|b|}$ for this data is insensitive both to exotic Yukawa interactions and also to the electromagnetic neutron-atom interactions proportional to the neutron-electron scattering length $b_{ne}$ and the neutron polarizability scattering amplitude $b_{pol}$. This result will be useful in any future global analyses of neutron scattering data to determine $b_{ne}$ and bound $α$ and $λ$. We also discuss how various neutron interferometric and scattering techniques with cold and ultracold neutrons can be used to improve the precision of $b$ measurements and make some specific proposals.