Showing 1–30 of 30 results for author: Zhai, Z

Element-Specific Visualization of Layer-Parity and Twist-Dependent Magnetism in CrSBr

Authors: Aalok Tiwari, Shubhada Patil, Ravi Kumar Bandapelli, Alevtina Smekhova, Abhishek Kumar, Wenhao Liu, I-Hsuan Kao, Zhenhong Cui, Raghvendra Posti, Brandon Tran, Zixin Zhai, Priti Yadav, Sandy Adhitia Ekahana, Alexander X. Gray, Bing Lv, Vivekanand Shukla, Florian Kronast, Simranjeet Singh, Jyoti Katoch

Abstract: Van der Waals (vdW) based antiferromagnets (AFMs) are an ideal platform for probing and understanding thickness- and twist-angle-dependent emergent spin phenomena. However, element-specific nanoscale characterization of the spin structure in atomically thin vdW-based AFMs systems and layer-parity effects remain elusive, making them crucial for both fundamental insight into low-dimensional magnetis… ▽ More Van der Waals (vdW) based antiferromagnets (AFMs) are an ideal platform for probing and understanding thickness- and twist-angle-dependent emergent spin phenomena. However, element-specific nanoscale characterization of the spin structure in atomically thin vdW-based AFMs systems and layer-parity effects remain elusive, making them crucial for both fundamental insight into low-dimensional magnetism and the rational design of spintronic devices based on these materials. Here, we utilize X-ray magnetic circular and linear dichroisms paired with photoemission electron microscopy to resolve the magnetic order in atomically thin CrSBr. Our comprehensive measurements reveal CrSBr magnetic structure at the nanoscale and its dependence on the layer number, surface encapsulation, temperature, and applied field. Moreover, in the orthogonally twisted bilayer configuration, obtained by twisting two CrSBr ferromagnetic monolayers by 90$^\circ$, the magnetic easy axis fundamentally differs from the individual monolayers, unlocking a new pathway for moiré magnetism. △ Less

Submitted 28 July, 2026; originally announced July 2026.

Enabling Electrical Readout of Néel vector reversal in a van der Waals Antiferromagnet

Authors: Raghvendra Posti, Ravi Kumar Bandepalli, Wenhao Liu, Anshuman Sahoo, Pratyush Saud, Zixin Zhai, Aswin L. N. Kondusamy, Zhenhong Cui, I-Hsuan Kao, Aalok Tiwari, Thomas Poirier, James H. Edgar, Kenji Watanabe, Takashi Taniguchi, Bing Lv, Jyoti Katoch, Simranjeet Singh

Abstract: Owing to its robustness against external perturbations and intrinsically ultrafast dynamics, the Néel vector in antiferromagnets (AFMs) can enable the development of next-generation spintronic and magnonic devices for memory and computing applications. To realize AFM-based magnetic memory devices, one of the key requirements is to demonstrate electrical readout of 180-degree reversal of Néel vecto… ▽ More Owing to its robustness against external perturbations and intrinsically ultrafast dynamics, the Néel vector in antiferromagnets (AFMs) can enable the development of next-generation spintronic and magnonic devices for memory and computing applications. To realize AFM-based magnetic memory devices, one of the key requirements is to demonstrate electrical readout of 180-degree reversal of Néel vector in thin film AFMs, which remains critically missing. In this work, we report experimental demonstration of a novel transport methodology to detect Néel vector reversal in atomically thin films of a van der Waals (vdW) based A-type AFM. For this, we utilize spin-dependent electronic band properties of CrSBr by coupling it to a spin-polarized layer, separated by a tunnel barrier. In this configuration, the spin-dependent tunnelling magnetoresistance (MR) becomes sensitive to the relative orientation between the magnetization of the reference electrode and the interfacial sublattice magnetization of the AFM layer, in turn enabling electrical detection of the Néel vector orientation. Importantly, the observed MR can also reveal 180-degree reversal of Néel vector in even-layers of CrSBr, wherein adjacent sublattice magnetic layers are exactly compensated and the net magnetization vanishes and thus establishes a broadly applicable strategy for electrical detection of Néel vector in vdW-based AFMs. △ Less

Submitted 23 June, 2026; originally announced June 2026.

Electrically switchable ferron upconversion in a van der Waals ferroelectric

Authors: Sujan Subedi, Wuzhang Fang, Fan Fei, Zixin Zhai, Jack P. Rollins, Carter Fox, Alaina Drew, Bing Lv, Yuan Ping, Jun Xiao

Abstract: Nonlinear phononics provides a powerful ultrafast route to control lattice excitations, enabling access to hidden quantum orders, phononic computing, and quantum transduction. However, dynamic control of anharmonic phonon interactions remains limited, as these interactions are typically fixed by the equilibrium crystal lattice and lack external tunability. Emergent ferrons in ferroelectrics, which… ▽ More Nonlinear phononics provides a powerful ultrafast route to control lattice excitations, enabling access to hidden quantum orders, phononic computing, and quantum transduction. However, dynamic control of anharmonic phonon interactions remains limited, as these interactions are typically fixed by the equilibrium crystal lattice and lack external tunability. Emergent ferrons in ferroelectrics, which are collective oscillations of the spontaneous electric polarization, may offer a promising platform to overcome this limitation by combining intrinsic phononic nonlinearity with direct electrical control of the ferroelectric order parameter. Here we report electrically controllable nonlinear ferron upconversion in the van der Waals ferroelectric NbOI2. We show that resonant THz excitation of a 3.1 THz ferron drives coherent upconversion to a 7.0 THz optical phonon. Using two-dimensional THz spectroscopy, we directly resolve off-diagonal coupling features and establish the nonlinear upconversion pathway. Supported by first-principles calculations and analytical modeling, we identify the microscopic origin as a cubic anharmonic lattice coupling. Importantly, in situ electric-field switching enables nonvolatile control of both the ferron dynamics and the associated upconversion process. The phase reversal and hysteretic behavior across the coercive fields establish that the ferron-mediated nonlinear phononic interaction is strongly dependent on the underlying ferroelectric order parameter. These results introduce ferron upconversion as a new and universal regime of nonlinear phononics in ferroelectrics and establish an electrically programmable platform for coherent lattice control, paving the way for ferronic information processing and quantum phononic transduction. △ Less

Submitted 19 March, 2026; originally announced March 2026.

Highly Efficient Second/Third Harmonic Generation in van der Waals Layered Material AgScP2S6 with Anisotropic Polarization and Temperature Dependence

Authors: Mohamed Yaseen Noor, Ryan Siebenaller, Wenhao Liu, Zixin Zhai, Conrad Kuz, Simin Zhang, Mousumi Upadhyay Kahaly, Gergely Nagy, Aamir Mushtaq, Rahul Rao, Emmanuel Rowe, Benjamin S. Conner, Bing Lv, Michael A. Susner, Enam Chowdhury

Abstract: Single-crystal X-ray diffraction and nonlinear optical measurements, especially second- and third-harmonic generation (SHG/THG) are comprehensively investigated for the van der Waals layered material AgScP2S6 with a non-centrosymmetric P31c (159) space group. Linear optical constants are extracted using spectroscopic ellipsometry and applied in fitting the harmonic generation behavior. Polarizatio… ▽ More Single-crystal X-ray diffraction and nonlinear optical measurements, especially second- and third-harmonic generation (SHG/THG) are comprehensively investigated for the van der Waals layered material AgScP2S6 with a non-centrosymmetric P31c (159) space group. Linear optical constants are extracted using spectroscopic ellipsometry and applied in fitting the harmonic generation behavior. Polarization-resolved SHG and THG measurements exhibit pronounced anisotropy, with emission patterns well-described by theoretical models derived from the khi(2) and khi(3) tensor elements. The material demonstrates exceptionally high nonlinear susceptibilities, with khi(2) ~ 10^(-8) m/V and khi(3) ~ 10^(-17) m^2/V^2 which is a few orders of magnitude greater than comparable 2D materials reported in the literature. Temperature-dependent SHG and THG measurements from 300 K to 25 K reveal exponential decay in harmonic signal intensities, attributed to reduced carrier mobility, with no evidence of structural phase transitions, consistent with results from single crystal diffraction and heat capacity measurements. Polarization-resolved SHG and THG measurements also reveal distinct orientation and ellipticity trends, highlighting the anisotropic nonlinear tensor contributions and contrasting polarization selection rules in the material. These results establish AgScP2S6 as a high-performance, thermally stable, and highly anisotropic nonlinear candidate material suitable for compact photonic applications such as ultrafast optical modulators, polarization-sensitive detectors, and wavelength-tunable light sources. △ Less

Submitted 26 February, 2026; originally announced February 2026.

Visualizing Nanoscopic Acoustic Mode Competition in van der Waals Ferroelectric

Authors: Zhaodong Chu, Carter Fox, Zixin Zhai, Haihua Liu, Priti Yadav, Bing Lv, Yue Li, Thomas E Gage, Jun Xiao, Haidan Wen

Abstract: Understanding how low-dimensional ferroelectrics respond to ultrafast excitation at nanoscales is essential for controlling energy flow and mechanical functionality in next-generation polar devices, yet the nanoscopic structural response to ultrafast depolarization remains unresolved, obscuring the microscopic pathways of acoustic decoherence and energy dissipation. Here, we spatiotemporally resol… ▽ More Understanding how low-dimensional ferroelectrics respond to ultrafast excitation at nanoscales is essential for controlling energy flow and mechanical functionality in next-generation polar devices, yet the nanoscopic structural response to ultrafast depolarization remains unresolved, obscuring the microscopic pathways of acoustic decoherence and energy dissipation. Here, we spatiotemporally resolve lattice motion in the van der Waals ferroelectric NbOI2 using combined ultrafast electron microscopy and diffraction, revealing three acoustic phonons: two transverse shear modes and one longitudinal breathing mode. The transverse mode that shears the layers perpendicular to the in-plane polar axis dominates over that along the polar axis, reflecting anisotropic polarization-strain coupling. Real-space mapping uncovers spatially correlated heterogeneity in mode amplitudes and lifetimes. Regions dominated by a single shear mode exhibit significantly longer acoustic lifetimes than multimode regions, suggesting acoustic phonon-phonon scattering as a major source of decoherence. Our results provide a microscopic understanding of ultrafast depolarization-driven acoustic dynamics and spatially heterogeneous energy dissipation in van der Waals ferroelectrics. △ Less

Submitted 27 February, 2026; v1 submitted 11 February, 2026; originally announced February 2026.

Comments: 3 figures in main text, and 5 figures in SI

Magneto-Moiré Excitons in Twisted Bilayer CrSBr

Authors: Qiuyang Li, Anton Shubnic, Nishkarsh Agarwal, Adam Alfrey, Wenhao Liu, Zixin Zhai, Igor Lobanov, Valery Uzdin, Senlei Li, Yujie Yang, Wyatt Alpers, Kai Sun, Liuyan Zhao, Chunhui Rita Du, Bing Lv, Robert Hovden, Ivan A. Shelykh, Hui Deng

Abstract: Moiré superlattices in van der Waals materials have revolutionized the study of electronic and excitonic systems by creating periodic electrostatic potentials. Extending this concept to magnetic materials promises new pathways in merging spintronics with photonics. While moiré magnetism has been revealed with near-field probes and nonlinear optical techniques, the coupling of these magnetic textur… ▽ More Moiré superlattices in van der Waals materials have revolutionized the study of electronic and excitonic systems by creating periodic electrostatic potentials. Extending this concept to magnetic materials promises new pathways in merging spintronics with photonics. While moiré magnetism has been revealed with near-field probes and nonlinear optical techniques, the coupling of these magnetic textures to optical excitations - magneto-moiré excitons - remains unexplored. Here, we report the observation of magneto-moiré excitons in twisted bilayer CrSBr, correlated with moiré spin textures that emerge below a critical twist angle of ~2°. The nanoscale moiré spin texture imprints distinct signatures onto the optical spectrum, shifting the exciton energy via a periodic magnetic exchange field. First-principles calculations corroborate that these signatures arise from one-dimensional spin textures governed by the balance of exchange interactions and domain wall energy. Our results demonstrate that moiré magnetism can be used to engineer nanoscale excitonic energy landscapes, providing a new platform for magneto-optical sensing, quantum transduction, and control of non-collinear magnetism and topology through light. △ Less

Submitted 23 December, 2025; originally announced December 2025.

Comments: 19 pages, 4 figures

Tuning Coupled Toroidic and Polar Orders in a Bilayer Antiferromagnet

Authors: Chuangtang Wang, Xiaoyu Guo, Zixin Zhai, Meixin Cheng, Sang-Wook Cheong, Adam W. Tsen, Bing Lv, Liuyan Zhao

Abstract: Magnetic toroidal order features a loop-like arrangement of magnetic dipole moments, thus breaking both spatial inversion (P) and time-reversal (T) symmetries while preserving their combined PT sym-metry. This PT symmetry enables a linear magnetoelectric effect, allowing the coupling between magnetic toroidicity and electric polarity. However, the detection and control of two-dimensional (2D) magn… ▽ More Magnetic toroidal order features a loop-like arrangement of magnetic dipole moments, thus breaking both spatial inversion (P) and time-reversal (T) symmetries while preserving their combined PT sym-metry. This PT symmetry enables a linear magnetoelectric effect, allowing the coupling between magnetic toroidicity and electric polarity. However, the detection and control of two-dimensional (2D) magnetic toroidal order and the investigation of its linear magnetoelectric response remain largely unexplored. Here, using bilayer CrSBr as a platform, which hosts an in-plane layer-antiferromagnetic (AFM) order and simultaneously exhibits a magnetic toroidal order, we show compelling evidence for tuning this 2D magnetic toroidicity and its induced electric polarity through magnetic-field-depend-ent second harmonic generation (SHG). Under an out-of-plane magnetic field, we decompose the SHG signal into a time-reversal-odd component that scales with the magnetic toroidal moment and a time-reversal-even component that is proportional to the electric polarization. When sweeping the magnetic field from positive to negative values, we observe that the magnetic toroidicity retains its sign but diminishes in magnitude at higher fields while the electric polarity flips its sign and increases in strength at increasing fields below a critical threshold. When applying an in-plane electric field along the Néel vector direction, together with an out-of-plane field, we find that the magnetic toroidal and electric polar domains are moved in a locked fashion. These findings underscore the promise of 2D magnetic toroidal order in realizing giant linear magnetoelectric effects, opening exciting possi-bilities for next-generation electronic, magnetic, optical, and photonic devices enabled by 2D mag-netoelectrics. △ Less

Submitted 16 September, 2025; originally announced September 2025.

Comments: 12 pages, 4 figures

High-Sensitivity Photonic Crystal Biosensors using Topological Light Trapping

Authors: Zhengzheng Zhai, Sajeev John

Abstract: Photonic crystals (PCs) with localized optical cavity modes arising from topological domain-wall line defects are simulated for optical biosensing by numerical solution of Maxwell's equations. These consist of a square lattice of square silicon blocks with a significant photonic band gap (PBG). Optical transmission through the PBG at specific frequencies occurs by defect-mediated optical tunneling… ▽ More Photonic crystals (PCs) with localized optical cavity modes arising from topological domain-wall line defects are simulated for optical biosensing by numerical solution of Maxwell's equations. These consist of a square lattice of square silicon blocks with a significant photonic band gap (PBG). Optical transmission through the PBG at specific frequencies occurs by defect-mediated optical tunneling. Biofluid flows perpendicular to light propagation, through a channel containing the PC, defined by silica side-walls and an underlying silica substrate. Replacing the silicon blocks with thin silicon strips throughout the domain-wall region, analyte binding coincides with regions of maximal field intensity. As a result, the sensitivity is improved by almost 16 times higher than the previous designs. We analyze optical mode hybridization of two nearby domain walls and its close relation to the transmission-levels and correlations in frequency shifts of nearby optical resonances in response to analyte-bindings. We illustrate three high-sensitivity chips each with three domain-wall defects, all of which can distinguish three analyte-bindings and their combinations completely in a single spectroscopic measurement. In a photonic crystal, consisting of silicon squares embedded in a water background and a 5-micron lattice spacing, the biosensor sensitivity to a thin analyte binding layer is nearly 3000 nm/RIU, and to the overall background biofluid is over 8000 nm/RIU. △ Less

Submitted 2 January, 2026; v1 submitted 6 August, 2025; originally announced August 2025.

High Entropy Engineering of Magnetic Kagome Lattice (Gd,Tb,Dy,Ho,Er)Mn6Sn6

Authors: Wenhao Liu, Nikhil Uday Dhale, Youzhe Chen, Pramanand Joshi, Zixin Zhai, Xiqu Wang, Ping Liu, Robert J. Birgeneau, Boris Maiorov, Christopher A. Mizzi, Bing Lv

Abstract: The magnetic kagome lattice compound RMn6Sn6 (R=rare earth) is an emerging platform to exploit the interplay between magnetism and topological electronic states where a variety of exciting findings such as flat bands, Dirac points as well as the dramatic dependence of magnetic order on the rare-earth element have been reported. High entropy through rare earth alloying, on the other hand, provides… ▽ More The magnetic kagome lattice compound RMn6Sn6 (R=rare earth) is an emerging platform to exploit the interplay between magnetism and topological electronic states where a variety of exciting findings such as flat bands, Dirac points as well as the dramatic dependence of magnetic order on the rare-earth element have been reported. High entropy through rare earth alloying, on the other hand, provides another knob to control over the physical properties in this system. Here, by the marriage of high entropy and the magnetic kagome lattice, we obtain (Gd,Tb,Dy,Ho,Er)Mn6Sn6 single crystals and systematically investigate their magnetic and transport properties. Different from the parent phases, the high entropy 166 material displays multiple novel magnetic transitions induced by temperature and external magnetic fields. Furthermore, linear magnetoresistance persisting up to 20 T has been revealed at 4 K. The intrinsic nontrivial band topology also survives in the high entropy form, as evidenced by the intrinsic anomalous Hall effect. Our results highlight high entropy as a powerful approach for tuning the interplay of charge, spin and lattice degree of freedom in magnetic topological materials. △ Less

Submitted 30 July, 2025; originally announced July 2025.

Tunable symmetry breaking in a hexagonal-stacked moiré magnet

Authors: Zeliang Sun, Gaihua Ye, Xiaohan Wan, Ning Mao, Cynthia Nnokwe, Senlei Li, Nishkarsh Agarwal, Siddhartha Sarkar, Zixin Zhai, Bing Lv, Robert Hovden, Chunhui Rita Du, Yang Zhang, Kai Sun, Rui He, Liuyan Zhao

Abstract: Symmetry plays a central role in defining magnetic phases, making tunable symmetry breaking across magnetic transitions highly desirable for discovering non-trivial magnetism. Magnetic moiré superlattices, formed by twisting two-dimensional (2D) magnetic crystals, have been theoretically proposed and experimentally explored as platforms for unconventional magnetic states. However, despite recent a… ▽ More Symmetry plays a central role in defining magnetic phases, making tunable symmetry breaking across magnetic transitions highly desirable for discovering non-trivial magnetism. Magnetic moiré superlattices, formed by twisting two-dimensional (2D) magnetic crystals, have been theoretically proposed and experimentally explored as platforms for unconventional magnetic states. However, despite recent advances, tuning symmetry breaking in moiré magnetism remains limited, as twisted 2D magnets, such as rhombohedral (R)-stacked twisted CrI_3, largely inherit the magnetic properties and symmetries of their constituent layers. Here, in hexagonal-stacked twisted double bilayer (H-tDB) CrI_3, we demonstrate clear symmetry evolution as the twist angle increases from 180^{\circ} to 190^{\circ}. While the net magnetization remains zero across this twist angle range, the magnetic phase breaks only the three-fold rotational symmetry at 180^{\circ}, but it breaks all of the rotational, mirror, and time-reversal symmetries at intermediate twist angles between 181^{\circ} and 185^{\circ}, and all broken symmetries are recovered at 190^{\circ}. These pronounced symmetry breakings at intermediate twist angles are accompanied by metamagnetic behaviors, evidenced by symmetric double hysteresis loops around zero magnetic field. Together, these results reveal that H-tDB CrI_3 at intermediate twist angles host a distinct moiré magnetic phase, featuring periodic in-plane spin textures with broken rotational, mirror, and time-reversal symmetries, which is markedly different from the out-of-plane layered antiferromagnetism in bilayer CrI_3 and the predominantly out-of-plane moiré magnetism in R-tDB CrI_3. Our work establishes H-stacked CrI_3 moiré magnets as a versatile platform for engineering magnetic properties, including and likely beyond complex spin textures. △ Less

Submitted 20 June, 2025; originally announced June 2025.

Twist Engineering of Anisotropic Excitonic and Optical Properties of a Two-Dimensional Magnetic Semiconductor

Authors: Qiuyang Li, Xiaohan Wan, Senlei Li, Adam Alfrey, Wenhao Liu, Zixin Zhai, Wyatt Alpers, Yujie Yang, Irmina Wladyszewska, Christiano W. Beach, Liuyan Zhao, Bing Lv, Chunhui Rita Du, Kai Sun, Hui Deng

Abstract: Two dimensional (2D) van der Waals (vdW) magnetic semiconductors are a new class of quantum materials for studying the emergent physics of excitons and spins in the 2D limit. Twist engineering provides a powerful tool to manipulate the fundamental properties of 2D vdW materials. Here, we show that twist engineering of the anisotropic ferromagnetic monolayer semiconductor, CrSBr, leads to bilayer m… ▽ More Two dimensional (2D) van der Waals (vdW) magnetic semiconductors are a new class of quantum materials for studying the emergent physics of excitons and spins in the 2D limit. Twist engineering provides a powerful tool to manipulate the fundamental properties of 2D vdW materials. Here, we show that twist engineering of the anisotropic ferromagnetic monolayer semiconductor, CrSBr, leads to bilayer magnetic semiconductors with continuously tunable magnetic moment, dielectric anisotropy, exciton energy and linear dichroism. We furthermore provide a model for exciton energy in the media with tunable anisotropy. These results advance fundamental studies on 2D vdW materials and open doors to applications to nano-optics, twistronics, and spintronics. △ Less

Submitted 30 April, 2025; originally announced May 2025.

Comments: 10 pages, 4 figures

Journal ref: Phys. Rev. Lett. 135, 156901 (2025)

Colossal terahertz emission with ultrafast tunability based on van der Waals ferroelectric NbOI$_2$

Authors: Sujan Subedi, Wenhao Liu, Wuzhang Fang, Carter Fox, Zixin Zhai, Fan Fei, Yuan Ping, Bing Lv, Jun Xiao

Abstract: Terahertz (THz) technology is critical for quantum material physics, biomedical imaging, ultrafast electronics, and next-generation wireless communications. However, standing in the way of widespread applications is the scarcity of efficient ultrafast THz sources with on-demand fast modulation and easy on-chip integration capability. Here we report the discovery of colossal THz emission from a van… ▽ More Terahertz (THz) technology is critical for quantum material physics, biomedical imaging, ultrafast electronics, and next-generation wireless communications. However, standing in the way of widespread applications is the scarcity of efficient ultrafast THz sources with on-demand fast modulation and easy on-chip integration capability. Here we report the discovery of colossal THz emission from a van der Waals (vdW) ferroelectric semiconductor NbOI$_2$. Using THz emission spectroscopy, we observe a THz generation efficiency an order of magnitude higher than that of ZnTe, a standard nonlinear crystal for ultrafast THz generation. We further uncover the underlying generation mechanisms associated with its large ferroelectric polarization by studying the THz emission dependence on excitation wavelength, incident polarization and fluence. Moreover, we demonstrate the ultrafast coherent amplification and annihilation of the THz emission and associated coherent phonon oscillations by employing a double-pump scheme. These findings combined with first-principles calculations, inform new understanding of the THz light-matter interaction in emergent vdW ferroelectrics and pave the way to develop high-performance THz devices on them for quantum materials sensing and ultrafast electronics. △ Less

Submitted 10 December, 2024; originally announced December 2024.

Journal ref: Advanced Optical Materials (2025)

Revealing unusual bandgap shifts with temperature and bandgap renormalization effect in phase-stabilized metal halide perovskite thin films

Authors: Haochen Zhang, Zhixuan Bi, Zehua Zhai, Han Gao, Yuwei Liu, Meiling Jin, Meng Ye, Xuanzhang Li, Haowen Liu, Yuegang Zhang, Xiang Li, Hairen Tan, Yong Xu, Luyi Yang

Abstract: Hybrid organic-inorganic metal halide perovskites are emerging materials in photovoltaics, whose bandgap is one of the most crucial parameters governing their light harvesting performance. Here we present the temperature and photocarrier density dependence of the bandgap in two phase-stabilized perovskite thin films (MA0.3FA0.7PbI3 and MA0.3FA0.7Pb0.5Sn0.5I3) using photoluminescence and absorption… ▽ More Hybrid organic-inorganic metal halide perovskites are emerging materials in photovoltaics, whose bandgap is one of the most crucial parameters governing their light harvesting performance. Here we present the temperature and photocarrier density dependence of the bandgap in two phase-stabilized perovskite thin films (MA0.3FA0.7PbI3 and MA0.3FA0.7Pb0.5Sn0.5I3) using photoluminescence and absorption spectroscopy. Contrasting bandgap shifts with temperature are observed between the two perovskites. Using X-ray diffraction and in situ high-pressure photoluminescence spectroscopy, we show that thermal expansion plays only a minor role in the large bandgap blueshift, which is attributed to the enhanced structural stability of our samples. Our first-principles calculations further demonstrate the significant impact of thermally induced lattice distortions on the bandgap widening. We propose that the anomalous trends are caused by the competition between static and dynamic distortions. Additionally, both the bandgap renormalization and band filling effects are directly observed for the first time in fluence-dependent photoluminescence measurements and are employed to estimate the exciton effective mass. Our results provide new insights into the basic understanding of thermal and charge-accumulation effects on the band structure of hybrid perovskite thin films. △ Less

Submitted 28 November, 2023; v1 submitted 21 August, 2023; originally announced August 2023.

Journal ref: Advanced Functional Materials 34, 2302214 (2024)

Spin Coherence and Spin Relaxation in Hybrid Organic-Inorganic Lead and Mixed Lead-Tin Perovskites

Authors: Haochen Zhang, Zehua Zhai, Zhixuan Bi, Han Gao, Meng Ye, Yong Xu, Hairen Tan, Luyi Yang

Abstract: Metal halide perovskites make up a promising class of materials for semiconductor spintronics. Here we report a systematic investigation of coherent spin precession, spin dephasing and spin relaxation of electrons and holes in two hybrid organic-inorganic perovskites MA0.3FA0.7PbI3 and MA0.3FA0.7Pb0.5Sn0.5I3 using time-resolved Faraday rotation spectroscopy. With applied in-plane magnetic fields,… ▽ More Metal halide perovskites make up a promising class of materials for semiconductor spintronics. Here we report a systematic investigation of coherent spin precession, spin dephasing and spin relaxation of electrons and holes in two hybrid organic-inorganic perovskites MA0.3FA0.7PbI3 and MA0.3FA0.7Pb0.5Sn0.5I3 using time-resolved Faraday rotation spectroscopy. With applied in-plane magnetic fields, we observe robust Larmor spin precession of electrons and holes that persists for hundreds of picoseconds. The spin dephasing and relaxation processes are likely to be sensitive to the defect levels. Temperature-dependent measurements give further insights into the spin relaxation channels. The extracted electron Landé g-factors (3.75 and 4.36) are the biggest among the reported values in inorganic or hybrid perovskites. Both the electron and hole g-factors shift dramatically with temperature, which we propose to originate from thermal lattice vibration effects on the band structure. These results lay the foundation for further design and use of lead- and tin-based perovskites for spintronic applications. △ Less

Submitted 1 September, 2023; v1 submitted 6 August, 2023; originally announced August 2023.

Journal ref: Nano Letters 23, 7917-7920 (2023)

Surface engineering for cellulose as a boosted Layer-by-Layer assembly: excellent flame retardancy and improved durability with introduction of bio-based "molecular glue"

Authors: Can Fu, Xiaoli Xu, Guang-Zhong Yin, Baoyun Xu, Pingyang Li, Bo Ai, Zhongjie Zhai, Fei GaO, Jinguo Zhai, De-Yi Wang

Abstract: Layer-by-Layer (LbL) assembly was attractive as a versatile tool to address the flammability of cotton, while the washing fastness of LbL coating stayed an issue. Aiming to tackle this issue, LbL layers consisted of phenylphosphonic acid (PHA) and 3-aminopropyltriethoxysilane (APTES) was deposited on polydopamine (PDA)-coated cotton. The prepared cotton reached 31.4% of limiting oxygen index (LOI)… ▽ More Layer-by-Layer (LbL) assembly was attractive as a versatile tool to address the flammability of cotton, while the washing fastness of LbL coating stayed an issue. Aiming to tackle this issue, LbL layers consisted of phenylphosphonic acid (PHA) and 3-aminopropyltriethoxysilane (APTES) was deposited on polydopamine (PDA)-coated cotton. The prepared cotton reached 31.4% of limiting oxygen index (LOI), and extinguished immediately after removing the ignitor. Peak of heat release rate (pHRR) attenuated around 36 % compared with pure cotton. A combined barrier and quenching mechanisms were proposed. Moreover, enhanced washing durability (24.1% of LOI) was achieved even after 50 detergent laundering cycles. A facile, boosted LbL approach with proposed π-π stacking interactions between PDA abundant aromatic structures and benzene ring in PHA from LbL layers, is first to put forward to construct durable efficient flame retardant (FR) cotton. This work attempted to enlighten more thoughts and design for durable FR cotton fabrics. △ Less

Submitted 22 January, 2022; originally announced February 2022.

Curving the space by non-Hermiticity

Authors: Chenwei Lv, Ren Zhang, Zhengzheng Zhai, Qi Zhou

Abstract: Quantum systems are often classified into Hermitian and non-Hermitian ones. Extraordinary non-Hermitian phenomena, ranging from the non-Hermitian skin effect to the supersensitivity to boundary conditions, have been widely explored. Whereas these intriguing phenomena have been considered peculiar to non-Hermitian systems, we show that they can be naturally explained by a duality between non-Hermit… ▽ More Quantum systems are often classified into Hermitian and non-Hermitian ones. Extraordinary non-Hermitian phenomena, ranging from the non-Hermitian skin effect to the supersensitivity to boundary conditions, have been widely explored. Whereas these intriguing phenomena have been considered peculiar to non-Hermitian systems, we show that they can be naturally explained by a duality between non-Hermitian models in flat spaces and their counterparts, which could be Hermitian, in curved spaces. For instance, prototypical one-dimensional (1D) chains with uniform chiral tunnelings are equivalent to their duals in two-dimensional (2D) hyperbolic spaces with or without magnetic fields, and non-uniform tunnelings could further tailor local curvatures. Such a duality unfolds deep geometric roots of non-Hermitian phenomena, delivers an unprecedented routine connecting Hermitian and non-Hermitian physics, and gives rise to a theoretical perspective reformulating our understandings of curvatures and distance. In practice, it provides experimentalists with a powerful two-fold application, using non-Hermiticity as a new protocol to engineer curvatures or implementing synthetic curved spaces to explore non-Hermitian quantum physics. △ Less

Submitted 29 December, 2021; v1 submitted 4 June, 2021; originally announced June 2021.

Comments: 7 pages (3 figures) + 6 pages (3 figures). Added non-Hermitian realizations of quantum Hall states in curved spaces

Fractonic gauge theory of smectics

Authors: Zhengzheng Zhai, Leo Radzihovsky

Abstract: Motivated by striped correlated quantum matter, and the recently developed duality between elasticity of a two-dimensional (2D) crystal and a gauge theory, we derive a dual coupled U(1) vector gauge theory for a two-dimensional (2D) quantum smectic, where the disclination is mapped onto the fractonic charge, that we demonstrate can only move transversely to smectic layers. This smectic gauge theor… ▽ More Motivated by striped correlated quantum matter, and the recently developed duality between elasticity of a two-dimensional (2D) crystal and a gauge theory, we derive a dual coupled U(1) vector gauge theory for a two-dimensional (2D) quantum smectic, where the disclination is mapped onto the fractonic charge, that we demonstrate can only move transversely to smectic layers. This smectic gauge theory dual also emerges from a gauge dual of a quantum crystal after a Higgs transition corresponding to a single flavor of its dipole condensation, an anisotropic quantum melting via dislocation proliferation. A condensation of the second flavor of dislocations is described by another Higgs transition describing the smectic-to-nematic melting. We also utilize the electrostatic limit of this duality to formulate a melting of a 2D classical smectic in terms of a higher derivative sine- Gordon model, demonstrating its instability to a nematic at any nonzero temperature. Generalizing this classical duality to a 3D smectic, gives formulation of a 3D nematic-to-smectic transition in terms of an anisotropic Abelian-Higgs model. △ Less

Submitted 1 November, 2022; v1 submitted 3 December, 2020; originally announced December 2020.

Comments: 16+5 pages, 11 figures; minor typos corrected, AOP published version

Journal ref: Annals of Physics 435 (2021) 168509

Crystal-to-Fracton Tensor Gauge Theory Dualities

Authors: Michael Pretko, Zhengzheng Zhai, Leo Radzihovsky

Abstract: We demonstrate several explicit duality mappings between elasticity of two-dimensional crystals and fracton tensor gauge theories, expanding on recent works by two of the present authors. We begin by dualizing the quantum elasticity theory of an ordinary commensurate crystal, which maps directly onto a fracton tensor gauge theory, in a natural tensor analogue of the conventional particle-vortex du… ▽ More We demonstrate several explicit duality mappings between elasticity of two-dimensional crystals and fracton tensor gauge theories, expanding on recent works by two of the present authors. We begin by dualizing the quantum elasticity theory of an ordinary commensurate crystal, which maps directly onto a fracton tensor gauge theory, in a natural tensor analogue of the conventional particle-vortex duality transformation of a superfluid. The transverse and longitudinal phonons of a crystal map onto the two gapless gauge modes of the tensor gauge theory, while the topological lattice defects map onto the gauge charges, with disclinations corresponding to isolated fractons and dislocations corresponding to dipoles of fractons. We use the classical limit of this duality to make new predictions for the finite-temperature phase diagram of fracton models, and provide a simpler derivation of the Halperin-Nelson-Young theory of thermal melting of two-dimensional solids. We extend this duality to incorporate bosonic statistics, which is necessary for a description of the quantum melting transitions. We thereby derive a hybrid vector-tensor gauge theory which describes a supersolid phase, hosting both crystalline and superfluid orders. The structure of this gauge theory puts constraints on the quantum phase diagram of bosons, and also leads to the concept of symmetry enriched fracton order. We formulate the extension of these dualities to systems breaking time-reversal symmetry. We also discuss the broader implications of these dualities, such as a possible connection between fracton phases and the study of interacting topological crystalline insulators. △ Less

Submitted 31 October, 2019; v1 submitted 29 July, 2019; originally announced July 2019.

Comments: 32+3 pages

Journal ref: Phys. Rev. B 100, 134113 (2019)

Two-dimensional melting via sine-Gordon duality

Authors: Zhengzheng Zhai, Leo Radzihovsky

Abstract: Motivated by the recently developed duality between elasticity of a crystal and a symmetric tensor gauge theory by Pretko and Radzihovsky, we explore its classical analog, that is a dual theory of the dislocation-mediated melting of a two-dimensional crystal, formulated in terms of a higher derivative vector sine-Gordon model. It provides a transparent description of the continuous two-stage melti… ▽ More Motivated by the recently developed duality between elasticity of a crystal and a symmetric tensor gauge theory by Pretko and Radzihovsky, we explore its classical analog, that is a dual theory of the dislocation-mediated melting of a two-dimensional crystal, formulated in terms of a higher derivative vector sine-Gordon model. It provides a transparent description of the continuous two-stage melting in terms of the renormalization-group relevance of two cosine operators that control the sequential unbinding of dislocations and disclinations, respectively corresponding to the crystal-to-hexatic and hexatic-to-isotropic fluid transitions. This renormalization-group analysis compactly reproduces seminal results of the Coulomb gas description, such as the flows of the elastic couplings and of the dislocation and disclination fugacities, as well the temperature dependence of the associated correlation lengths. △ Less

Submitted 7 October, 2019; v1 submitted 2 May, 2019; originally announced May 2019.

Comments: 8+6 pages, 3 figures published version

Journal ref: Phys. Rev. B 100, 094105 (2019)

Highly-efficient spintronic terahertz emitter enabled by metal-dielectric photonic crystal

Authors: Zheng Feng, Rui Yu, Yu Zhou, Hai Lu, Wei Tan, Hu Deng, Quancheng Liu, Zhaohui Zhai, Liguo Zhu, Jianwang Cai, Bingfeng Miao, Haifeng Ding

Abstract: Spintronic terahertz (THz) emitter provides the advantages such as apparently broader spectrum, significantly lower cost, and more flexibility in compared with the commercial THz emitters, and thus attracts great interests recently. In past few years, efforts have been made in optimizing the material composition and structure geometry, and the conversion efficiency has been improved close to that… ▽ More Spintronic terahertz (THz) emitter provides the advantages such as apparently broader spectrum, significantly lower cost, and more flexibility in compared with the commercial THz emitters, and thus attracts great interests recently. In past few years, efforts have been made in optimizing the material composition and structure geometry, and the conversion efficiency has been improved close to that of ZnTe crystal. One of the drawbacks of the current designs is the rather limited laser absorption - more than 50% energy is wasted and the conversion efficiency is thus limited. Here, we theoretically propose and experimentally demonstrate a novel device that fully utilizes the laser intensity and significantly improves the conversion efficiency. The device, which consists of a metal-dielectric photonic crystal structure, utilizes the interference between the multiple scattering waves to simultaneously suppress the reflection and transmission of the laser, and to reshape the laser field distributions. The experimentally detected laser absorption and THz generations show one-to-one correspondence with the theoretical calculations. We achieve the strongest THz pulse emission that presents a 1.7 times improvement compared to the currently designed spintronic emitter. This work opens a new pathway to improve the performance of spintronic THz emitter from the perspective of optics. △ Less

Submitted 9 July, 2018; originally announced July 2018.

Comments: 22 pages,4 figures

Journal ref: Advanced Optical Materials, 1800965 (2018)

Anomalous scattering rate and microwave absorption in $Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}$ and $YBa_{2}Cu_{3}O_{7-δ}$

Authors: C. Kusko, Z. Zhai, R. S. Markiewicz, S. Sridhar

Abstract: We determine the scattering rate from microwave measurements for an optimally doped Bi-2212 single crystal, using a simple two fluid model with a d-wave symmetry order parameter. In the superconducting state, the calculated scattering rate is three orders of magnitude smaller than that determined from ARPES experiments. A similar anomalously large decrease in the scattering rate is also required… ▽ More We determine the scattering rate from microwave measurements for an optimally doped Bi-2212 single crystal, using a simple two fluid model with a d-wave symmetry order parameter. In the superconducting state, the calculated scattering rate is three orders of magnitude smaller than that determined from ARPES experiments. A similar anomalously large decrease in the scattering rate is also required to explain the data within a gap-quasiparticle scenario for other HTS, such as $YBa_{2}Cu_{3}O_{7-δ}$. The results suggest that the assumption of normal excitations vanishing at low $T$ is invalid and an additional charge mode is responsible for the microwave absorption. △ Less

Submitted 15 February, 2001; originally announced February 2001.

Comments: 7 pages, 2 figures, Presented at International Workshop on HTS in High Frequency Fields, Capri, May 2000

Onset of dielectric modes at 110K and 60K due to local lattice distortions in non-superconducting YBa_{2}Cu_{3}O_{6.0} crystals

Authors: Z. Zhai, P. V. Parimi, J. B. Sokoloff, A. Erb, S. Sridhar

Abstract: We report the observation of two dielectric transitions at 110K and 60K in the microwave response of non-superconducting YBa_{2}Cu_{3}O_{6.0} crystals. The transitions are characterized by a change in polarizability and presence of loss peaks, associated with overdamped dielectric modes. An explanation is presented in terms of changes in polarizability of the apical O atoms in the Ba-O layer, af… ▽ More We report the observation of two dielectric transitions at 110K and 60K in the microwave response of non-superconducting YBa_{2}Cu_{3}O_{6.0} crystals. The transitions are characterized by a change in polarizability and presence of loss peaks, associated with overdamped dielectric modes. An explanation is presented in terms of changes in polarizability of the apical O atoms in the Ba-O layer, affected by lattice softening at 110K, due to change in buckling of the Cu-O layer. The onset of another mode at 60K strongly suggests an additional local lattice change at this temperature. Thus microwave dielectric measurements are sensitive indicators of lattice softening which may be relevant to superconductivity. △ Less

Submitted 31 May, 2000; originally announced May 2000.

Comments: 5 pages, 3 ps format figures

Non-quasiparticle microwave absorption in $Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}$

Authors: S. Sridhar, Z. Zhai

Abstract: We show that a non-quasiparticle charge collective mode, in parallel and coincident with the d-wave pair conductivity, leads to a quantitative understanding of microwave surface impedance measurements on superconducting $Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}$. The analysis suggests an inhomogeneous charge ground state in $Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}$ and other HTS. We show that a non-quasiparticle charge collective mode, in parallel and coincident with the d-wave pair conductivity, leads to a quantitative understanding of microwave surface impedance measurements on superconducting $Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}$. The analysis suggests an inhomogeneous charge ground state in $Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}$ and other HTS. △ Less

Submitted 7 April, 2000; originally announced April 2000.

Comments: 6 pages, 3 figures

Precision microwave dielectric and magnetic susceptibility measurements of correlated electronic materials using superconducting cavities

Authors: Z. Zhai, C. Kusko, N. Hakim, S. Sridhar

Abstract: We analyze microwave cavity perturbation methods, and show that the technique is an excellent, precision method to study the dynamic magnetic and dielectric response in the $GHz$ frequency range. Using superconducting cavities, we obtain exceptionally high precision and sensitivity for measurements of relative changes. A dynamic electromagnetic susceptibility… ▽ More We analyze microwave cavity perturbation methods, and show that the technique is an excellent, precision method to study the dynamic magnetic and dielectric response in the $GHz$ frequency range. Using superconducting cavities, we obtain exceptionally high precision and sensitivity for measurements of relative changes. A dynamic electromagnetic susceptibility $\tildeζ(T)=ζ^{\prime}+iζ^{\prime \prime}$ is introduced, which is obtained from the measured parameters: the shift of cavity resonant frequency $δf$ and quality factor $Q$. We focus on the case of a spherical sample placed at the center of a cylindrical cavity resonant in the $TE_{011}$ mode. Depending on the sample characteristics, the magnetic permeability $\tildeμ$, the dielectric permittivity $\tildeε$ and the complex conductivity $\tildeσ$ can be extracted from $\tildeζ_{H}$. A full spherical wave analysis of the cavity perturbation is given. This analysis has led to the observation of new phenomena in novel low dimensional materials. △ Less

Submitted 19 January, 2000; originally announced January 2000.

Comments: 16 pages, 5 figures

Microwave Absorption Peaks : Signatures of Spin Dynamics in Cuprates

Authors: S. Sridhar, Z. Zhai, A. Erb

Abstract: We show that a common feature of temperature-dependent microwave absorption is the presence of absorption peaks. $ac$ loss peaks can arise when the internal $T$-dependent magnetic relaxation time crosses the measurement frequency. These features are observed in the \QTR{em}{insulating} ($Sr_{x}Ca_{14-x}Cu_{24}O_{41}$, $La_{5/3}Sr_{1/3}NiO_{4}$ and $YBa_{2}Cu_{3}O_{6.0}$), \QTR{em}{pseudo-gap} (… ▽ More We show that a common feature of temperature-dependent microwave absorption is the presence of absorption peaks. $ac$ loss peaks can arise when the internal $T$-dependent magnetic relaxation time crosses the measurement frequency. These features are observed in the \QTR{em}{insulating} ($Sr_{x}Ca_{14-x}Cu_{24}O_{41}$, $La_{5/3}Sr_{1/3}NiO_{4}$ and $YBa_{2}Cu_{3}O_{6.0}$), \QTR{em}{pseudo-gap} ($T>T_{c}$ in underdoped $YBa_{2}Cu_{3}O_{7-δ}$, $Hg:1223$ and $Hg:1201$) and \QTR{em}{superconducting} ($T<T_{c}$) states of the oxides. The commonality of these features suggests a magnetic (spin) mechanism, rather than a quasiparticle origin, for the so-called ``conductivity'' peaks observed in the cuprate superconductors. △ Less

Submitted 17 June, 1999; v1 submitted 3 June, 1999; originally announced June 1999.

Comments: 7 pages, 2 ps format figures

Observation of spin freezing and relaxation at microwave frequencies in the spin ladder compound $Sr_{14-x}Ca_{x}Cu_{24}O_{41}$

Authors: Z. Zhai, P. V. Patanjali, N. Hakim, J. B. Sokoloff, S. Sridhar, U. Ammerahl, A. Vietkine, A. Revcolevschi

Abstract: We report the observation of a frequency ($ω$) and temperature (T) -dependent loss peak in $χ''$ and accompanying dispersion in $χ'$ from microwave (2-18GHz) measurements of the complex susceptibility $\tildeχ(ω, T)=χ'+iχ''$ of $Sr_{14-x}Ca_{x}Cu_{24}O_{41}$. We associate this phenomenon with a rapid decrease of spin disorder and corresponding spin relaxation rate, representing a ``spin freezing… ▽ More We report the observation of a frequency ($ω$) and temperature (T) -dependent loss peak in $χ''$ and accompanying dispersion in $χ'$ from microwave (2-18GHz) measurements of the complex susceptibility $\tildeχ(ω, T)=χ'+iχ''$ of $Sr_{14-x}Ca_{x}Cu_{24}O_{41}$. We associate this phenomenon with a rapid decrease of spin disorder and corresponding spin relaxation rate, representing a ``spin freezing transition'' accompanying charge ordering which occurs at temperatures $\sim 250K$. Our results enable direct quantitative measurements of the spin relaxation rate, and yield information on spin dynamics in these materials and the related compounds $SrCuO_{2}$ and $Sr_{2}CuO_{3}$. △ Less

Submitted 11 March, 1999; originally announced March 1999.

Comments: 4 pages, 5 figures included in ps format

Non-linear microwave impedance of short and long Josephson Junctions

Authors: Z. Zhai, Patanjali V. Parimi, S. Sridhar

Abstract: The non-linear dependence on applied $ac$ field ($b_ω$) or current ($% i_ω$) of the microwave (ac) impedance $R_ω+iX_ω$ of both short and long Josephson junctions is calculated under a variety of excitation conditions. The dependence on the junction width is studied, for both field symmetric (current anti-symmetric) and field anti-symmetric (current symmetric) excitation configurations.The resis… ▽ More The non-linear dependence on applied $ac$ field ($b_ω$) or current ($% i_ω$) of the microwave (ac) impedance $R_ω+iX_ω$ of both short and long Josephson junctions is calculated under a variety of excitation conditions. The dependence on the junction width is studied, for both field symmetric (current anti-symmetric) and field anti-symmetric (current symmetric) excitation configurations.The resistance shows step-like features every time a fluxon (soliton) enters the junction, with a corresponding phase slip seen in the reactance. For finite widths the interference of fluxons leads to some interesting effects which are described. Many of these calculated results are observed in microwave impedance measurements on intrinsic and fabricated Josephson junctions in the high temperature superconductors, and new effects are suggested. When a $% dc$ field ($b_{dc}$) or current ($i_{dc}$) is applied, interesting phase locking effects are observed in the ac impedance $Z_ω$. In particular an almost periodic dependence on the dc bias is seen similar to that observed in microwave experiments at very low dc field bias. These results are generic to all systems with a $\cos (φ)$ potential in the overdamped limit and subjected to an ac drive. △ Less

Submitted 26 October, 1998; originally announced October 1998.

Comments: 7 pages, 11 figures

High frequency magneto-impedance of double perovskite $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$: secondary transitions at high temperatures

Authors: P. V. Patanjali, P. Theule, Z. Zhai, N. Hakim, S. Sridhar, R. Suryanarayanan, M. Apostu, G. Dhalenne, A. Revcolevschi

Abstract: Radio frequency magneto-impedance measurements clearly reveal a pronounced anomaly at 260K besides the main MI transition at 125K in the double perovskite material $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$. This feature is not seen clearly in static resistivity and magnetization measurements. We suggest that this anomaly represents short range magnetic correlations enhanced at radio frequencies, with the ea… ▽ More Radio frequency magneto-impedance measurements clearly reveal a pronounced anomaly at 260K besides the main MI transition at 125K in the double perovskite material $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$. This feature is not seen clearly in static resistivity and magnetization measurements. We suggest that this anomaly represents short range magnetic correlations enhanced at radio frequencies, with the easy axis along the c-axis . △ Less

Submitted 10 March, 1999; v1 submitted 26 October, 1998; originally announced October 1998.

Comments: four pages, three figures

Systematics of two-component superconductivity in $YBa_{2}Cu_{3}O_{6.95}$ from microwave measurements of high quality single crystals

Authors: H. Srikanth, Z. Zhai, S. Sridhar, A. Erb, E. Walker

Abstract: Systematic microwave surface impedance measurements of YBCO single crystals grown in $BaZrO_3$ crucibles reveal new properties that are not directly seen in similar measurements of other YBCO samples. Two key observations obtained from complex conductivity are: a new normal conductivity peak at around 80K and additional pairing below 65K. High pressure oxygenation of one of the crystals still yi… ▽ More Systematic microwave surface impedance measurements of YBCO single crystals grown in $BaZrO_3$ crucibles reveal new properties that are not directly seen in similar measurements of other YBCO samples. Two key observations obtained from complex conductivity are: a new normal conductivity peak at around 80K and additional pairing below 65K. High pressure oxygenation of one of the crystals still yields the same results ruling out any effect of macroscopic segregation of O-deficient regions. A single complex order parameter cannot describe these data, and the results suggest at least two superconducting components. Comparisons with model calculations done for various decoupled two-component scenarios (i.e. s+d, d+d) are presented. Systematics of three single crystals show that the 80K quasiparticle peak is correlated with the normal state inelastic scattering rate. Close to Tc, the data follow a mean-field behavior. Overall, our results strongly suggest the presence of multiple pairing temperature and energy scales in $YBa_{2}Cu_{3}O_{6.95}$. △ Less

Submitted 6 September, 1997; originally announced September 1997.

Comments: 14 pages, 2-column, Revtex, 5 embedded postscript figures, uses graphicx. Postscript version also available at http://sagar.physics.neu.edu/preprints.html

Journal ref: Phys. Rev B, Vol. 57, 7986 (1 April 1998)

Observation of coherent Josephson response in the non-linear ab-plane microwave impedance of $YBa_{2}Cu_{3}O_{6.95}$ single crystals

Authors: Z. Zhai, H. Srikanth, S. Sridhar, A. Erb, E. Walker, R. Flukiger

Abstract: We report novel non-linear phenomena in the $ab$-plane microwave impedance of $YBaCu_{2}O_{7-δ}$ single crystals. The $R_s$ vs. $H_{rf}$ data are well described by the non-linear RSJ model : $\dotφ+\sin φ=i_{rf}\cos ωt$. The entire crystal behaves like a single Josephson junction. The extraordinary coherence of the data suggests an intrinsic mechanism. We report novel non-linear phenomena in the $ab$-plane microwave impedance of $YBaCu_{2}O_{7-δ}$ single crystals. The $R_s$ vs. $H_{rf}$ data are well described by the non-linear RSJ model : $\dotφ+\sin φ=i_{rf}\cos ωt$. The entire crystal behaves like a single Josephson junction. The extraordinary coherence of the data suggests an intrinsic mechanism. △ Less

Submitted 4 August, 1997; originally announced August 1997.

Comments: 2 pages,1 figure, Submitted to Proc. of M^2SHTSC-V (Beijing), also available at http://sagar.physics.neu.edu/preprints