arXiv subjects

K. Zheng

Publications and source records attributed to K. Zheng.

13 recordsLinked to original sources

arXiv2025

About carrier's self-trapping and dynamical Rashba splitting in the two-dimensional hybrid perovskite (BA)$_2$(MA)$_2$Pb$_3$I$_{10}$

Time- and Angle-Resolved Photoelectron Spectroscopy (tr-ARPES) is employed to monitor photoexcited electrons in the two-dimensional hybrid perovskite (BA)$_2$(MA)$_2$Pb$_3$I$_{10}$. Photoelectron intensity maps are in good agreement with ab-initio calculations of the band structure. The effective mass is $-0.18 \pm 0.02 m_e$ and $0.12 \pm 0.02 m_e$ for holes and electrons, respectively. In the photoexcited state, spin-orbit splitting of the conduction band cannot be resolved. This sets the upper bound of photoinduced Rashba coupling to $\alpha_C<2.5$ eV\AA. The correlated electron-hole plasma evolves in Wannier excitons with Bohr radius of 2.8 nm, while no sign of self-trapping in small polarons is found within the investigated time window of up to 120 ps following photoexcitation.

cond-mat.other↗

arXiv2025

Sub-micron Circuit Fabrication on Diamond Anvils for Mesoscopic High-Pressure Experiments

We present a novel fabrication procedure to produce high-quality lift-off structures on diamond anvils extending from the culet down to the slanted facets. Feature sizes down to 500 nm are achieved through the use of a bi-layer resist stack and electron beam lithography. Device structures with strong adhesion to the diamond surface and high abrasion resistance are realized by optimizing the surface treatment. To benchmark our process, we fabricate a multi-lead tungsten circuit to measure changes of the superconducting transition temperature of zirconium across the structural phase transition at $\sim$30 GPa; showing a 4-fold jump of the critical temperature. Our process is easily reproducible in most traditional academic and industrial cleanroom facilities. This work paves the way for complex and high-precision fabrication and measurements inside diamond anvil cells and on other faceted crystalline samples.

cond-mat.mes-hall↗

arXiv2022

Nitrogen Plasma Passivated Niobium Resonators for Superconducting Quantum Circuits

Microwave loss in niobium metallic structures used for superconducting quantum circuits is limited by a native surface oxide layer formed over a timescale of minutes when exposed to an ambient environment. In this work, we show that nitrogen plasma treatment forms a niobium nitride layer at the metal-air interface which prevents such oxidation. X-ray photoelectron spectroscopy confirms the doping of nitrogen more than 5 nm into the surface and a suppressed oxygen presence. This passivation remains stable after aging for 15 days in an ambient environment. Cryogenic microwave characterization shows an average filling factor adjusted two-level-system loss tangent $\rm{Fδ_{TLS}}$ of $(2.9\pm0.5)\cdot10^{-7}$ for resonators with 3 $\rmμ$m center strip and $(1.0\pm0.3)\cdot10^{-7}$ for 20 $\rmμ$m center strip, exceeding the performance of unpassivated samples by a factor of four.

physics.app-ph↗

arXiv2021

Fabrication and surface treatment of electron-beam evaporated niobium for low-loss coplanar waveguide resonators

We characterize low-loss electron-beam evaporated niobium thin films deposited under ultra-high vacuum conditions. Slow deposition yields films with a high superconducting transition temperature ($9.20 \pm 0.06 \rm ~K$) as well as a residual resistivity ratio of $4.8$. We fabricate the films into coplanar waveguide resonators to extract the intrinsic loss due to the presence of two-level-system fluctuators using microwave measurements. For a coplanar waveguide resonator gap of $2~μ\rm m$, the films exhibit filling-factor-adjusted two-level-system loss tangents as low as $1.5 \times 10^{-7}$ with single-photon regime internal quality factors in excess of one million after removing native surface oxides of the niobium.

cond-mat.supr-con↗

arXiv2021

Optical Direct Write of Dolan--Niemeyer-Bridge Junctions for Transmon Qubits

We characterize highly coherent transmon qubits fabricated with a direct-write photolithography system. Multi-layer evaporation and oxidation allows us to change the critical current density by reducing the effective tunneling area and increasing the barrier thickness. Surface treatments before resist application and again before evaporation result in high coherence devices. With optimized surface treatments we achieve energy relaxation $T_1$ times in excess of $80\ μ$s for three dimensional transmon qubits with Josephson junction lithographic areas of 2 $μ\mathrm{m}^2$.

cond-mat.mes-hall↗

arXiv2021

A Microwave SQUID Multiplexer Optimized for Bolometric Applications

A microwave SQUID multiplexer ($μ$MUX) has been optimized for coupling to large arrays of superconducting transition-edge sensor (TES) bolometers. We present the scalable cryogenic multiplexer chip design in a 1820-channel multiplexer configuration for the 4-8 GHz rf band. The key metrics of yield, sensitivity, and crosstalk are determined through measurements of 455 readout channels, which span 4-5 GHz. The median white-noise level is 45 pA/$\sqrt{\textrm{Hz}}$, evaluated at 2 Hz, with a 1/f knee $\leq$ 20 mHz after common-mode subtraction. The white-noise level decreases the sensitivity of a TES bolometer optimized for detection of the cosmic microwave background at 150 GHz by only 3%. The measured crosstalk between any channel pair is $\leq$ 0.3%.

astro-ph.IM↗

arXiv2018

Magnetic Sensitivity of AlMn TESes and Shielding Considerations for Next-Generation CMB Surveys

In the next decade, new ground-based Cosmic Microwave Background (CMB) experiments such as Simons Observatory (SO), CCAT-prime, and CMB-S4 will increase the number of detectors observing the CMB by an order of magnitude or more, dramatically improving our understanding of cosmology and astrophysics. These projects will deploy receivers with as many as hundreds of thousands of transition edge sensor (TES) bolometers coupled to Superconducting Quantum Interference Device (SQUID)-based readout systems. It is well known that superconducting devices such as TESes and SQUIDs are sensitive to magnetic fields. However, the effects of magnetic fields on TESes are not easily predicted due to the complex behavior of the superconducting transition, which motivates direct measurements of the magnetic sensitivity of these devices. We present comparative four-lead measurements of the critical temperature versus applied magnetic field of AlMn TESes varying in geometry, doping, and leg length, including Advanced ACT (AdvACT) and POLARBEAR-2/Simons Array bolometers. Molybdenum-copper bilayer ACTPol TESes are also tested and are found to be more sensitive to magnetic fields than the AlMn devices. We present an observation of weak-link-like behavior in AlMn TESes at low critical currents. We also compare measurements of magnetic sensitivity for time division multiplexing SQUIDs and frequency division multiplexing microwave rf-SQUIDs. We discuss the implications of our measurements on the magnetic shielding required for future experiments that aim to map the CMB to near-fundamental limits.

astro-ph.IM↗

arXiv2016

Brute-Force Mapmaking with Compact Interferometers: A MITEoR Northern Sky Map from 128 MHz to 175 MHz

We present a new method for interferometric imaging that is ideal for the large fields of view and compact arrays common in 21 cm cosmology. We first demonstrate the method with simulations for two very different low frequency interferometers, the Murchison Widefield Array (MWA) and the MIT Epoch of Reionization (MITEoR) Experiment. We then apply the method to the MITEoR data set collected in July 2013 to obtain the first northern sky map from 128 MHz to 175 MHz at about 2 degree resolution, and find an overall spectral index of -2.73+/-0.11. The success of this imaging method bodes well for upcoming compact redundant low-frequency arrays such as HERA. Both the MITEoR interferometric data and the 150 MHz sky map are publicly available at http://space.mit.edu/home/tegmark/omniscope.html.

astro-ph.IM↗

arXiv2013

Soft x-rays absorption and high-resolution powder x-ray diffraction study of superconducting CaxLa(1-x)Ba(1.75-x)La(0.25+x)Cu3Oy system

We have studied the electronic structure of unoccupied states measured by O K-edge and Cu L-edge x-ray absorption spectroscopy (XAS), combined with crystal structure studied by high resolution powder x-ray diffraction (HRPXRD), of charge-compensated layered superconducting CaxLa(1-x)Ba(1.75-x)La(0.25+x)Cu3Oy (0<x<0.4, 6.4<y<7.3) cuprate. A detailed analysis shows that, apart from hole doping, chemical pressure on the electronically active CuO2 plane due to the lattice mismatch with the spacer layers greatly influences the superconducting properties of this system. The results suggest chemical pressure to be the most plausible parameter to control the maximum critical temperatures (Tcmax) in different cuprate families at optimum hole density.

cond-mat.supr-con↗

arXiv2013

Band filling effect on polaron localization in La$_{1-x}($Ca$_y$Sr$_{1-y}$)$_{x}$MnO$_3$ manganites

We report on an a $μ$SR and $^{55}$Mn NMR investigation of the magnetic order parameter as a function of temperature in the optimally doped La$_{5/8}$(Ca$_y$Sr$_{1-y}$)$_{3/8}$MnO$_3$ and in the underdoped La$_{1-x}$Sr$_{x}$MnO$_3$ and La$_{1-x}$Ca$_{x}$MnO$_3$ metallic manganite families. The study is aimed at unraveling the effect of lattice distortions, implicitly controlled by the Ca-Sr isoelectronic substitution, from that of hole doping $x$ on the Curie temperature $T_c$ and the order of the magnetic transition. At optimal doping, the transitions are second order at all $y$ values, including the $y=1$ (La$_{5/8}$Ca$_{3/8}$MnO$_3$) end member. In contrast, they are first order in the underdoped samples, which show a finite (truncated) order parameter at the Curie point, including La$_{0.75}$Sr$_{0.25}$MnO$_3$ whose $T_c$ is much higher than that of La$_{5/8}$Ca$_{3/8}$MnO$_3$. The order parameter curves, on the other hand, exhibit a very minor dependence on $x$, if truncation is excepted. This suggests that the effective exchange interaction between Mn ions is essentially governed by local distortions, in agreement with the original double-exchange model, while truncation is primarily, if not entirely, an effect of under- or overdoping. A phase diagram, separating in the $x-y$ plane polaron-driven first order transitions from regular second order transitions governed by critical fluctuations, is proposed for the La$_{1-x}($Ca$_y$Sr$_{1-y}$)$_{x}$MnO$_3$ system.

cond-mat.str-el↗

arXiv2010

The magnetic states of lightly hole-doped cuprates in the clean limit

We have performed extensive zero field muSR experiments on pure YBa2Cu3O6+y and diluted Y-rare-earth substituted Y0.92Eu0.08Ba2Cu3O6+y and Y0.925Nd0.075Ba2Cu3O6+y at light hole-doping. A common magnetic behavior is detected for all the three families, demonstrating negligible effects of the isovalent Y-substituent disorder. Two distinct regimes are identified, separated by a crossover, whose origin is attributed to the concurrent thermal activation of spin and charge degrees of freedom: a thermally activated and a re-entrant quenched-hole antiferromagnetic regime. The peculiar temperature and hole density dependence of the magnetic moment m(h,T) fit a model with a (spin) activation energy for the crossover between the two regimes throughout the entire investigated range. The magnetic moment is suppressed by a simple dilution mechanism both in the reentrant regime (0<h<0.056) and in the so-called Cluster Spin Glass state coexisting with superconductivity (0.056<h<0.08). We argue a common magnetic ground state for these two doping regions and dub it quenched-hole antiferromagnet. Conversely frustration prevails in the thermally activated regime, that vanishes at the same concentration where superconductivity emerges, suggesting the presence of a quantum critical point at h_c=0.056(2).

cond-mat.supr-con↗

arXiv2009

Experimental evidence of chemical-pressure-controlled superconductivity in cuprates

X-ray absorption spectroscopy (XAS) and high resolution X-ray diffraction are combined to study the interplay between electronic and lattice structures in controlling the superconductivity in cuprates with a model charge-compensated CaxLa1-xBa1.75-xLa0.25+xCu3Oy (0<x<0.5, y=7.13) system. In spite of a large change in Tc, the doped holes, determined by the Cu L and O K XAS, hardly show any variation with the x. On the other hand, the CuO2 plaquette size shows a systematic change due to different size of substituted cations. The results provide a direct evidence for the chemical pressure being a key parameter for controlling the superconducting ground state of the cuprates.

cond-mat.supr-con↗

arXiv2008

Joint Transmitter-Receiver Design for the Downlink Multiuser Spatial Multiplexing MIMO System

This paper proposes a joint transmitter-receiver design to minimize the weighted sum power under the post-processing signal-to-interference-and-noise ratio (post-SINR) constraints for all subchannels. Simulation results demonstrate that the algorithm can not only satisfy the post-SINR constraints but also easily adjust the power distribution among the users by changing the weights accordingly. Hence the algorithm can be used to alleviates the adjacent cell interference by reducing the transmitting power to the edge users without performance penalty.

cs.IT↗