Showing 1–50 of 60 results for author: Ha, J

Electrically tunable interfacial thermal conduction via electronic structure engineering in ${Au}$/$Bi_{1-x}$$Sb_{x}$ topological insulators

Authors: Min Young Kim, Joon Sang Kang, Jangwoo Ha, Hyungyu Jin, Sandy A. Ekahana, Pratik Saud, Chris Jozwiak, Eli Rotenberg, Aaron Bostwick, Jyoti Katoch, Joseph P. Heremans

Abstract: This work provides direct experimental evidence for the role of topological interface states in thermal conduction across a metal/topological insulator junction. It also shows that this conduction can be reversibly modulated by electrical current injection, offering a new approach toward active control of heat flow at solid-state interfaces. Specifically, the interfacial thermal conductance of… ▽ More This work provides direct experimental evidence for the role of topological interface states in thermal conduction across a metal/topological insulator junction. It also shows that this conduction can be reversibly modulated by electrical current injection, offering a new approach toward active control of heat flow at solid-state interfaces. Specifically, the interfacial thermal conductance of ${Au}$/$Bi_{89}$$Sb_{11}$ and ${Au}$/$Bi_{87}$$Sb_{13}$ junctions demonstrates distinct temperature- and bias-dependent behavior. Both responses are attributed to carrier redistribution between topological interface and bulk band states, driven thermally by Fermi-Dirac broadening and electrically by quasi-Fermi-level shifts and WKB tunneling into nearby bulk bands. Control experiments using trivial semimetals and insulating interlayers further confirm the topological specificity of the effect. Such electrically tunable interfacial heat conduction positions interface electronic structure engineering as a promising route for active thermal management. In doing so, it lays the groundwork for a mechanically robust alternative to conventional structure-driven thermal control compatible with increasingly dense, high-power solid-state devices. △ Less

Submitted 2 July, 2026; originally announced July 2026.

Strain induced magnetic phase transition and anomalous transport phenomena in RuO$_2$ and MnF$_2$

Authors: Xiuxian Yang, Zhangqi Wu, Xiangju Wang, Shifeng Qian, Ping Yang, Xiaodong Zhou, Jian Hao, Wanxiang Feng, Yinwei Li

Abstract: Collinear antiferromagnets with broken time-reversal symmetry have emerged as a fertile platform for spintronics. Using a general tight-binding model and first-principles calculations, we show that strain engineering provides a simple route to control magnetic phase transition and activate transverse responses in representative altermagnets RuO$_2$ and MnF$_2$. For pristine RuO$_2$ and MnF$_2$ wit… ▽ More Collinear antiferromagnets with broken time-reversal symmetry have emerged as a fertile platform for spintronics. Using a general tight-binding model and first-principles calculations, we show that strain engineering provides a simple route to control magnetic phase transition and activate transverse responses in representative altermagnets RuO$_2$ and MnF$_2$. For pristine RuO$_2$ and MnF$_2$ with Néel vector $\mathbf{n}\parallel$ [001], symmetry constrains the off-diagonal elements of the Hall conductivity tensor to vanish, thereby forbidding anomalous transport and magneto-optical responses. Shear strain applied along the $ac$ direction preserves the spin symmetry relating the two spin-opposite magnetic sublattices and therefore maintains the altermagnetic phase. By contrast, shear strain applied along the $ab$ direction breaks this spin symmetry and drives a transition from an altermagnetic phase to a partially compensated ferrimagnetic phase in metallic RuO$_2$ and to a fully compensated ferrimagnetic phase in semiconducting MnF$_2$. In addition, the lowered symmetry enables finite anomalous Hall, anomalous Nernst, and anomalous thermal Hall conductivities, as well as magneto-optical rotation angles, which are prohibited in the pristine systems. These responses exhibit a clear strain dependence and become progressively stronger as the strain amplitude increases. Our results establish strain engineering as an effective route to manipulate magnetic phases and functional responses in unconventional antiferromagnets, thereby expanding opportunities for antiferromagnetic spintronics and magneto-optical applications. △ Less

Submitted 21 June, 2026; v1 submitted 16 June, 2026; originally announced June 2026.

Optically driven thermodynamic transition from free- to locked-epitaxy

Authors: Renhong Liang, Mao Ye, Yiran Ying, Longlong Shu, Renkui Zheng, Haitao Huang, Jianhua Hao, Shuk-Yin Tong, Shanming Ke

Abstract: Controlling crystallographic orientation in quasi-van der Waals (vdW) epitaxy remains a fundamental challenge, especially for material systems located near the boundary between weakly and strongly coupled growth regimes. In such marginal systems, epitaxial selection is governed by a delicate thermodynamic competition between surface-energy penalties and interfacial interaction gains, giving rise t… ▽ More Controlling crystallographic orientation in quasi-van der Waals (vdW) epitaxy remains a fundamental challenge, especially for material systems located near the boundary between weakly and strongly coupled growth regimes. In such marginal systems, epitaxial selection is governed by a delicate thermodynamic competition between surface-energy penalties and interfacial interaction gains, giving rise to two archetypal limits: vdW-dominated free-epitaxy and strong interfacial coupling dominated locked-epitaxy. However, dynamically driving transitions between these regimes has remained elusive. Here, we demonstrate that external light irradiation can deterministically induce such a transition. Using the thermodynamically frustrated Fe4N/mica interface as a model system, we show that photo-excited carriers act as a chemical potentiator, significantly enhancing the interfacial chemical affinity. Within a quantitative thermodynamic description, this optical modulation increases the locking criterion (I_lock)-defined as the ratio of interfacial energy gain to surface-energy cost-beyond its critical threshold. As a result, the system switches from vdW-dominated free-epitaxy with (001) orientation to chemically locked-epitaxy with (111) orientation. Our findings establish light as a non-invasive and switchable control knob to dynamically reconfigure the interfacial energy landscape in quasi-vdW epitaxy, enabling programmable access to distinct epitaxial states beyond intrinsic material limitations. △ Less

Submitted 10 March, 2026; originally announced March 2026.

Comments: 20 pages, 4 figures

Multi-target density matrix renormalization group for 3D CFTs on the fuzzy sphere

Authors: Jin-Xiang Hao, Zheng Zhu, Yang Qi

Abstract: The fuzzy sphere regularization provides a powerful framework for studying three-dimensional (3D) conformal field theories (CFTs) by mapping them onto numerically tractable lattice models on the spherical lowest Landau level. However, the system sizes accessible to this method have been limited by the exact diagonalization (ED). In this work, we transcend this limitation by combining the fuzzy sph… ▽ More The fuzzy sphere regularization provides a powerful framework for studying three-dimensional (3D) conformal field theories (CFTs) by mapping them onto numerically tractable lattice models on the spherical lowest Landau level. However, the system sizes accessible to this method have been limited by the exact diagonalization (ED). In this work, we transcend this limitation by combining the fuzzy sphere regularization with a sophisticated multi-target density matrix renormalization group (DMRG) algorithm. Focusing on the 3D Ising-type model on the spherical lowest Landau level, we calculate the 24 low-lying energies at a larger system size than previously feasible with ED. At criticality, we extract the scaling dimensions of six primary operators, and the results show significantly improved agreement with bootstrap benchmarks compared to previous ED results at smaller sizes. Our approach allows us to efficiently target multiple excited states in larger systems beyond the reach of exact diagonalization. This study establishes the fuzzy sphere regularization combined with advanced DMRG techniques as a powerful and general framework for precision physics in 3D CFTs. △ Less

Submitted 26 January, 2026; originally announced January 2026.

Universal thermodynamic framework for quasi-van der Waals epitaxy

Authors: Renhong Liang, Mao Ye, Renkui Zheng, Jianhua Hao, Haitao Huang, Longlong Shu, Shanming Ke

Abstract: van der Waals (vdW) epitaxy is conventionally regarded as a rotation-free and strain-free growth mode driven by weak, isotropic interactions, yet many interfaces paradoxically exhibit strictly locked orientations that defy standard surface-energy models. We resolve this inconsistency by establishing a unified quantitative framework for 2D-3D systems, in which strong electrostatic and chemical inte… ▽ More van der Waals (vdW) epitaxy is conventionally regarded as a rotation-free and strain-free growth mode driven by weak, isotropic interactions, yet many interfaces paradoxically exhibit strictly locked orientations that defy standard surface-energy models. We resolve this inconsistency by establishing a unified quantitative framework for 2D-3D systems, in which strong electrostatic and chemical interactions compete with entropic forces. We introduce a two-tier descriptor set-the predictive index (I_pre) and the thermodynamic locking criterion (I_lock)-to quantify the energetic sufficiency for locked epitaxy. Our theory accurately predicted the competitive interactions at the interface within the 2D-3D system, precisely characterized whether the epitaxial layer underwent free growth or was constrained in a locked growth mode, demonstrating robust consistency with diverse experimental observations. This framework unifies orientation selection in 3D-on-2D films and rotational locking in 2D-on-3D layers within a single-phase diagram. Our work provides a generalizable, predictive route to controlling epitaxial orientation across a broad spectrum of layered heterostructure △ Less

Submitted 25 December, 2025; originally announced December 2025.

Comments: 23 pages, 5 figures

Highly Anisotropic Charge Dynamics and Spectral Weight Redistribution in the Trilayer Nickelate La$_{4}$Ni$_{3}$O$_{10}$

Authors: Zhe Liu, Jie Li, Deyuan Hu, Bingke Ji, Haoran Zhang, Jiahao Hao, Yaomin Dai, Qing Li, Mengjun Ou, Bing Xu, Yi Lu, Meng Wang, Hai-Hu Wen

Abstract: We study the $ab$-plane and $c$-axis charge dynamics of La$_{4}$Ni$_{3}$O$_{10}$ using optical spectroscopy. While a pronounced Drude profile, i.e. metallic response, is observed in the $ab$-plane optical conductivity $σ_{1}^{ab}(ω)$, the $c$-axis optical spectra $σ_{1}^{c}(ω)$ exhibit semiconducting behavior. The zero-frequency extrapolation of the optical conductivity… ▽ More We study the $ab$-plane and $c$-axis charge dynamics of La$_{4}$Ni$_{3}$O$_{10}$ using optical spectroscopy. While a pronounced Drude profile, i.e. metallic response, is observed in the $ab$-plane optical conductivity $σ_{1}^{ab}(ω)$, the $c$-axis optical spectra $σ_{1}^{c}(ω)$ exhibit semiconducting behavior. The zero-frequency extrapolation of the optical conductivity $σ_{1}(ω\rightarrow 0) \equiv 1/ρ_{\text{dc}}$ gives a resistivity anisotropy of $ρ_{c}/ρ_{ab} \simeq 366$ at 300~K for La$_{4}$Ni$_{3}$O$_{10}$, which is much larger than the values in iron-based superconductors but comparable to those in high-$T_{c}$ cuprates. The interband response is also highly anisotropic, showing salient orbital selectivity for light polarized in the $ab$ plane and along the $c$ axis. The interband-transition peaks in both $σ_{1}^{ab}(ω)$ and $σ_{1}^{c}(ω)$ are located at lower energies compared to density-functional-theory predictions, signifying considerable electronic correlations. By investigating the spectral weight transfer, we find that in the pristine phase, Coulomb correlations have a marked impact on the charge dynamics of \LNO, whereas in the density-wave state, a gap opens with the Ni-$d_{z^{2}}$ orbital being involved. △ Less

Submitted 3 December, 2025; v1 submitted 3 December, 2025; originally announced December 2025.

Comments: 8 pages, 3 figures. Comments are welcome and appreciated

Surface-Localized Magnetic Order in RuO2 Thin Films Revealed by Low-Energy Muon Probes

Authors: Akashdeep Akashdeep, Sachin Krishnia, Jae-Hyun Ha, Siyeon An, Maik Gaerner, Thomas Prokscha, Andreas Suter, Gianluca Janka, Günter Reiss, Timo Kuschel, Dong-Soo Han, Angelo Di Bernardo, Zaher Salman, Gerhard Jakob, Mathias Kläui

Abstract: Ruthenium dioxide (RuO2) has recently emerged as a candidate altermagnet, yet its intrinsic magnetic ground state, particularly in thin films, remains debated. This study aims to clarify the nature and spatial extent of the magnetic order in RuO2 thin films grown under different conditions. Thin films of RuO2 with thicknesses of 30 nm and 33 nm are fabricated by pulsed laser deposition and sputter… ▽ More Ruthenium dioxide (RuO2) has recently emerged as a candidate altermagnet, yet its intrinsic magnetic ground state, particularly in thin films, remains debated. This study aims to clarify the nature and spatial extent of the magnetic order in RuO2 thin films grown under different conditions. Thin films of RuO2 with thicknesses of 30 nm and 33 nm are fabricated by pulsed laser deposition and sputtering onto TiO2(110) and Al2O3(1-102) substrates, respectively. Low-energy muon spin rotation/relaxation (LE-muSR) with depth-resolved sensitivity measurements is performed in transverse magnetic fields (TF) from 4 K to 290 K. The muSR data collected with a muon implantation energy of 1 keV reveal that magnetic signals originate from the near-surface region of the film (<10 nm), and the affected volume fraction is at most about 8.5%. The localized magnetic response is consistent across different substrates, growth techniques, and parameter sets, suggesting a common origin related to surface defects and dimensionality effects. The combined use of TF-muSR and the study of depth-dependent implantation with low-energy muons provides direct evidence for surface-confined, inhomogeneous static magnetic order in RuO2 thin films, helping reconcile discrepancies. These findings underscore the importance of considering reduced-dimensional contributions and motivate further investigation into the role of defects, strain, and stoichiometry on the magnetic properties of RuO2, especially at the surface. △ Less

Submitted 6 February, 2026; v1 submitted 9 October, 2025; originally announced October 2025.

Journal ref: Appl. Phys. Lett. 128, 022406 (2026)

Fingerprinting Organic Molecules for the Inverse Design of Two-Dimensional Hybrid Perovskites with Target Energetics

Authors: Yongxin Lyu, Yifan Zhou, Yu Zhang, Yang Yang, Bosen Zou, Qiang Weng, Tong Xie, Claudio Cazorla, Jianhua Hao, Jun Yin, Tom Wu

Abstract: Artificial intelligence (AI)-assisted workflows have transformed materials discovery, enabling rapid exploration of chemical spaces of functional materials. Endowed with extraordinary optoelectronic properties, two-dimensional (2D) hybrid perovskites represent an exciting frontier, but current efforts to design 2D perovskites rely heavily on trial-and-error and expert intuition approaches, leaving… ▽ More Artificial intelligence (AI)-assisted workflows have transformed materials discovery, enabling rapid exploration of chemical spaces of functional materials. Endowed with extraordinary optoelectronic properties, two-dimensional (2D) hybrid perovskites represent an exciting frontier, but current efforts to design 2D perovskites rely heavily on trial-and-error and expert intuition approaches, leaving most of the chemical space unexplored and compromising the design of hybrid materials with desired properties. Here, we introduce an inverse design workflow for Dion-Jacobson perovskites that is built on an invertible fingerprint representation for millions of conjugated diammonium organic spacers. By incorporating high-throughput density functional theory (DFT) calculations, interpretable machine learning, and synthesis feasibility screening, we identified new organic spacer candidates with deterministic energy level alignment between the organic and the inorganic motifs in the 2D hybrid perovskites. These results highlight the power of integrating invertible, physically meaningful molecular representations into AI-assisted design, streamlining the property-targeted design of hybrid materials. △ Less

Submitted 29 September, 2025; originally announced September 2025.

On the ambient conditions crystal structure of AgSbTe2

Authors: Baihong Sun, Sergei Grazhdannikov, Muhamed Dawod, Lunhua He, Jiazheng Hao, Thomas Meier, Yansun Yao, Yaron Amouyal, Elissaios Stavrou

Abstract: We present a combined X-ray and neutron diffraction, Raman spectroscopy, and 121Sb NMR studies of AgSbTe2, supported by first-principles calculations aiming to elucidate its crystal structure. While diffraction methods cannot unambiguously resolve the structure, Raman and NMR data, together with electric field gradient calculations, strongly support the rhombohedral R-3m phase. Moreover, the agree… ▽ More We present a combined X-ray and neutron diffraction, Raman spectroscopy, and 121Sb NMR studies of AgSbTe2, supported by first-principles calculations aiming to elucidate its crystal structure. While diffraction methods cannot unambiguously resolve the structure, Raman and NMR data, together with electric field gradient calculations, strongly support the rhombohedral R-3m phase. Moreover, the agreement between experimental and calculated Raman spectra further corroborates this result, resolving the 60-year sold debate about the exact crystal structure of the AgSbTe2 compound. △ Less

Submitted 16 June, 2025; originally announced June 2025.

Scalable thru-hole epitaxy of GaN through self-adjusting $h$-BN masks via solution-processed 2D stacks

Authors: Jongwoo Ha, Minah Choi, Jieun Yang, Chinkyo Kim

Abstract: Selective epitaxy on 2D-material masks is a promising pathway for achieving localized, defect-suppressed GaN growth, but conventional 2D transfer processes limit scalability and interface control. Here, we demonstrate a thru-hole epitaxy (THE) method that enables vertically connected and laterally overgrown GaN domains through a spin-coated, solution-processed stack of hexagonal boron nitride (… ▽ More Selective epitaxy on 2D-material masks is a promising pathway for achieving localized, defect-suppressed GaN growth, but conventional 2D transfer processes limit scalability and interface control. Here, we demonstrate a thru-hole epitaxy (THE) method that enables vertically connected and laterally overgrown GaN domains through a spin-coated, solution-processed stack of hexagonal boron nitride ($h$-BN) flakes. The disordered $h$-BN mask exhibits a self-adjusting structure during growth, which locally reconfigures to allow percolative precursor transport and coherent GaN nucleation beneath otherwise blocking layers. Comprehensive structural analyses using scanning electron microscopy, Raman mapping, and high-resolution transmission electron microscopy confirm both the presence of epitaxial GaN beneath the h-BN and suppression of threading dislocations. This strategy eliminates the need for patterned 2D mask transfers and demonstrates a scalable route to selective-area GaN growth on arbitrary substrates, relevant to future micro-LED and photonic integration platforms. △ Less

Submitted 16 May, 2025; originally announced May 2025.

Pyrochlore NaYbO2: A potential Quantum Spin Liquid Candidate

Authors: Chuanyan Fan, Tieyan Chang, Longlong Fan, Simon J. Teat, Feiyu Li, Xiaoran Feng, Chao Liu, Shi-lei Wang, Huifen Ren, Jiazheng Hao, Zhaohui Dong, Lunhua He, Shanpeng Wang, Chengwang Niu, Yu-Sheng Chen, Xutang Tao, Junjie Zhang

Abstract: The search for quantum spin liquids (QSL) and chemical doping in such materials to explore superconductivity have continuously attracted intense interest. Here, we report the discovery of a potential QSL candidate, pyrochlore-lattice beta-NaYbO2. Colorless and transparent NaYbO2 single crystals, layered alpha-NaYbO2 (~250 um on edge) and octahedral beta-NaYbO2 (~50 um on edge), were grown for the… ▽ More The search for quantum spin liquids (QSL) and chemical doping in such materials to explore superconductivity have continuously attracted intense interest. Here, we report the discovery of a potential QSL candidate, pyrochlore-lattice beta-NaYbO2. Colorless and transparent NaYbO2 single crystals, layered alpha-NaYbO2 (~250 um on edge) and octahedral beta-NaYbO2 (~50 um on edge), were grown for the first time. Synchrotron X-ray single crystal diffraction unambiguously determined that the newfound beta-NaYbO2 belongs to the three-dimensional pyrochlore structure characterized by the R-3m space group, corroborated by synchrotron X-ray and neutron powder diffraction and pair distribution function. Magnetic measurements revealed no long-range magnetic order or spin glass behavior down to 0.4 K with a low boundary spin frustration factor of 17.5, suggesting a potential QSL ground state. Under high magnetic fields, the potential QSL state was broken and spins order. Our findings reveal that NaYbO2 is a fertile playground for studying novel quantum states. △ Less

Submitted 25 January, 2025; originally announced January 2025.

Comments: This document is the unedited author's version of a Submitted Work that was subsequently accepted for publication in Journal of the American Chemical Society, copyright American Chemical Society after peer review. To access the final edited and published work, a link will be provided soon

Journal ref: Journal of the American Chemical Society 147, 5693-5702 (2025)

Prediction of high-Tc superconductivity in ternary actinium beryllium hydrides at low pressure

Authors: Kun Gao, Wenwen Cui, Jingming Shi, Artur P. Durajski, Jian Hao, Silvana Botti, Miguel A. L. Marques, Yinwei Li

Abstract: Hydrogen-rich superconductors are promising candidates to achieve room-temperature superconductivity. However, the extreme pressures needed to stabilize these structures significantly limit their practical applications. An effective strategy to reduce the external pressure is to add a light element M that binds with H to form MHx units, acting as a chemical precompressor. We exemplify this idea by… ▽ More Hydrogen-rich superconductors are promising candidates to achieve room-temperature superconductivity. However, the extreme pressures needed to stabilize these structures significantly limit their practical applications. An effective strategy to reduce the external pressure is to add a light element M that binds with H to form MHx units, acting as a chemical precompressor. We exemplify this idea by performing ab initio calculations of the Ac-Be-H phase diagram, proving that the metallization pressure of Ac-H binaries, for which critical temperatures as high as 200 K were predicted at 200 GPa, can be significantly reduced via beryllium incorporation. We identify three thermodynamically stable (AcBe2H10, AcBeH8, and AcBe2H14) and four metastable compounds (fcc AcBeH8, AcBeH10, AcBeH12 and AcBe2H16). All of them are superconductors. In particular, fcc AcBeH8 remains dynamically stable down to 10 GPa, where it exhibits a superconducting transition temperature Tc of 181 K. The Be-H bonds are responsible for the exceptional properties of these ternary compounds and allow them to remain dynamically stable close to ambient pressure. Our results suggest that high-Tc superconductivity in hydrides is achievable at low pressure and may stimulate experimental synthesis of ternary hydrides. △ Less

Submitted 6 August, 2025; v1 submitted 28 November, 2024; originally announced November 2024.

Journal ref: Physical Review B 109,014501(2024)

Prediction of high-Tc superconductivity under submegabar pressure in ternary actinium borohydrides

Authors: Tingting Gu, Wenwen Cui, Jian Hao, Jingming Shi, Artur P. Durajski, Hanyu Liu, Yinwei Li

Abstract: Ternary hydrides are considered as the ideal candidates with high critical temperature (Tc) stabilized at submegabar pressure, evidenced by the recent discoveries in LaBeH8 (110 K at 80 GPa) and LaB2H8 (106 K at 90 GPa). Here, we investigate the crystal structures and superconductivity of an Ac-B-H system under pressures of 100 and 200 GPa by using an advanced structure method combined with first-… ▽ More Ternary hydrides are considered as the ideal candidates with high critical temperature (Tc) stabilized at submegabar pressure, evidenced by the recent discoveries in LaBeH8 (110 K at 80 GPa) and LaB2H8 (106 K at 90 GPa). Here, we investigate the crystal structures and superconductivity of an Ac-B-H system under pressures of 100 and 200 GPa by using an advanced structure method combined with first-principles calculations. As a result, nine stable compounds were identified, where B atoms are bonded with H atoms in the formation with diverse BHx motifs, e.g., methanelike (BH4), polythenelike, (BH2)n,andBH6 octahedron. Among them, seven Ac-B-H compounds were found to become superconductive. In particular, AcBH7 was estimated to have a Tc of 122 K at 70 GPa. Our in-depth analysis reveals that the B-H interactions in the BH6 units play a key role in its high superconductivity and stability at submegabar pressure. Our current results provide a guidance for future experiments to synthesize ternary hydride superconductors with high-Tc at moderate pressure. △ Less

Submitted 28 November, 2024; originally announced November 2024.

Journal ref: Physical Review B 110,064105(2024)

Control of ferromagnetism of Vanadium Oxide thin films by oxidation states

Authors: Kwonjin Park, Jaeyong Cho, Soobeom Lee, Jaehun Cho, Jae-Hyun Ha, Jinyong Jung, Dongryul Kim, Won-Chang Choi, Jung-Il Hong, Chun-Yeol You

Abstract: Vanadium oxide (VOx) is a material of significant interest due to its metal-insulator transition (MIT) properties as well as its diverse stable antiferromagnetism depending on the valence states of V and O with distinct MIT transitions and Néel temperatures. Although several studies reported the ferromagnetism in the VOx, it was mostly associated with impurities or defects, and pure VOx has rarely… ▽ More Vanadium oxide (VOx) is a material of significant interest due to its metal-insulator transition (MIT) properties as well as its diverse stable antiferromagnetism depending on the valence states of V and O with distinct MIT transitions and Néel temperatures. Although several studies reported the ferromagnetism in the VOx, it was mostly associated with impurities or defects, and pure VOx has rarely been reported as ferromagnetic. Our research presents clear evidence of ferromagnetism in the VOx thin films, exhibiting a saturation magnetization of approximately 14 kA/m at 300 K. We fabricated 20-nm thick VOx thin films via reactive sputtering from a metallic vanadium target in various oxygen atmosphere. The oxidation states of ferromagnetic VOx films show an ill-defined stoichiometry of V2O3+p, where p = 0.05, 0.23, 0.49, with predominantly disordered microstructures. Ferromagnetic nature of these VOx films is confirmed through a strong antiferromagnetic exchange coupling with the neighboring ferromagnetic layer in the VOx/Co bilayers, in which the spin configurations of Co layer is influenced strongly due to the additional anisotropy introduced by VOx layer. The present study highlights the potential of VOx as an emerging functional magnetic material with tunability by oxidation states for modern spintronic applications. △ Less

Submitted 25 November, 2024; originally announced November 2024.

Comments: 6 figures, and supporting information with 3 figures

Evolution of Electronic Correlations in the Ruddlesden-Popper Nickelates

Authors: Zhe Liu, Jie Li, Mengwu Huo, Bingke Ji, Jiahao Hao, Yaomin Dai, Mengjun Ou, Qing Li, Hualei Sun, Bing Xu, Yi Lu, Meng Wang, Hai-Hu Wen

Abstract: We report on optical studies of the Ruddlesden-Popper nickelates La$_{n+1}$Ni$_{n}$O$_{3n+1}$ with $n = 2$ (La$_{3}$Ni$_{2}$O$_{7}$), $n = 3$ (La$_{4}$Ni$_{3}$O$_{10}$) and $n = \infty$ (LaNiO$_{3}$). As the number of the NiO$_{6}$ octahedra layers $n$ grows, the ratio of the kinetic energy determined from the experimental optical conductivity and that from band theory… ▽ More We report on optical studies of the Ruddlesden-Popper nickelates La$_{n+1}$Ni$_{n}$O$_{3n+1}$ with $n = 2$ (La$_{3}$Ni$_{2}$O$_{7}$), $n = 3$ (La$_{4}$Ni$_{3}$O$_{10}$) and $n = \infty$ (LaNiO$_{3}$). As the number of the NiO$_{6}$ octahedra layers $n$ grows, the ratio of the kinetic energy determined from the experimental optical conductivity and that from band theory $K_{\text{exp}}/K_{\text{band}}$ increases, suggesting a reduction of electronic correlations. While the strong electronic correlations in the bilayer La$_{3}$Ni$_{2}$O$_{7}$ place it on the verge of the Mott insulating phase, the trilayer La$_{4}$Ni$_{3}$O$_{10}$ and infinite-layer LaNiO$_{3}$ exhibit moderate electronic correlations, falling into the regime of correlated metals. The evolution of the electronic correlations in La$_{n+1}$Ni$_{n}$O$_{3n+1}$ is likely to be dominated by the Ni-$d_{z^2}$ orbital. Our results provide important information for understanding the superconductivity in Ruddlesden-Popper nickelates. △ Less

Submitted 13 November, 2024; originally announced November 2024.

Comments: 7 pages, 4 figures. Comments are welcome and appreciated

Surface Exciton Polariton

Authors: Jason Hao, Jeffrey Owrutsky, Daniel Ratchford, Blake Simpkins, Alexander L. Efros

Abstract: In this paper, we have developed a theory describing surface exciton polariton (SEPs) that accounts for the spatial dispersion of the dielectric constant connected with exciton momentum. Due to strong coupling between light and bulk excitons in the frequency separation, $\hbarω_{LT}$, between the longitudinal and transverse exciton, the SEP is formed and behaves at partially light and partially ma… ▽ More In this paper, we have developed a theory describing surface exciton polariton (SEPs) that accounts for the spatial dispersion of the dielectric constant connected with exciton momentum. Due to strong coupling between light and bulk excitons in the frequency separation, $\hbarω_{LT}$, between the longitudinal and transverse exciton, the SEP is formed and behaves at partially light and partially matter. The dispersion of the SEP was found through a combined solution of Maxwell's and Thomas-Hopfield's equations. The analytical theory describes SEPs at any bulk exciton/vacuum interface and provides its complete dispersion if one knows $\hbarω_{LT}$, the exciton effective mass, $M$, and the high frequency dielectric constant, $κ_\infty$. The presented theory is in excellent agreement with the only numerical modeling of this problem, which was conducted for SEPs at a ZnO/vacuum interface. Calculations show the spatial dispersion of the dielectric constant leads to rather small broadening of the photon-like quasi-particle and suggests using SEPs for long-range coherence transfer. △ Less

Submitted 30 October, 2024; originally announced November 2024.

Emergence of steady quantum transport in a superconducting processor

Authors: Pengfei Zhang, Yu Gao, Xiansong Xu, Ning Wang, Hang Dong, Chu Guo, Jinfeng Deng, Xu Zhang, Jiachen Chen, Shibo Xu, Ke Wang, Yaozu Wu, Chuanyu Zhang, Feitong Jin, Xuhao Zhu, Aosai Zhang, Yiren Zou, Ziqi Tan, Zhengyi Cui, Zitian Zhu, Fanhao Shen, Tingting Li, Jiarun Zhong, Zehang Bao, Liangtian Zhao , et al. (7 additional authors not shown)

Abstract: Non-equilibrium quantum transport is crucial to technological advances ranging from nanoelectronics to thermal management. In essence, it deals with the coherent transfer of energy and (quasi-)particles through quantum channels between thermodynamic baths. A complete understanding of quantum transport thus requires the ability to simulate and probe macroscopic and microscopic physics on equal foot… ▽ More Non-equilibrium quantum transport is crucial to technological advances ranging from nanoelectronics to thermal management. In essence, it deals with the coherent transfer of energy and (quasi-)particles through quantum channels between thermodynamic baths. A complete understanding of quantum transport thus requires the ability to simulate and probe macroscopic and microscopic physics on equal footing. Using a superconducting quantum processor, we demonstrate the emergence of non-equilibrium steady quantum transport by emulating the baths with qubit ladders and realising steady particle currents between the baths. We experimentally show that the currents are independent of the microscopic details of bath initialisation, and their temporal fluctuations decrease rapidly with the size of the baths, emulating those predicted by thermodynamic baths. The above characteristics are experimental evidence of pure-state statistical mechanics and prethermalisation in non-equilibrium many-body quantum systems. Furthermore, by utilising precise controls and measurements with single-site resolution, we demonstrate the capability to tune steady currents by manipulating the macroscopic properties of the baths, including filling and spectral properties. Our investigation paves the way for a new generation of experimental exploration of non-equilibrium quantum transport in strongly correlated quantum matter. △ Less

Submitted 11 November, 2024; originally announced November 2024.

Comments: 7 pages, 4 figures

Journal ref: Nat. Commun. 15, 10115 (2024)

Enhanced transverse electron transport via disordered composite formation

Authors: Sang J. Park, Hojun Lee, Jongjun M. Lee, Jangwoo Ha, Hyun-Woo Lee, Hyungyu Jin

Abstract: Transverse electron transport in magnetic materials - manifested in effects such as the anomalous Hall and Nernst effects - holds promise for spintronic and thermoelectric applications. While recent advances have focused on enhancing such transport through topological single crystals via intrinsic mechanisms linked to Berry curvature, practical limitations remain due to their mechanical fragility… ▽ More Transverse electron transport in magnetic materials - manifested in effects such as the anomalous Hall and Nernst effects - holds promise for spintronic and thermoelectric applications. While recent advances have focused on enhancing such transport through topological single crystals via intrinsic mechanisms linked to Berry curvature, practical limitations remain due to their mechanical fragility and narrow material scope. Here, we demonstrate a distinct approach for transverse transport enhancement based on composite formation. Using both theoretical modeling and experiments, we show that disordered mixtures of two ferromagnetic materials can exhibit significantly stronger transverse electron deflection than either constituent alone. This enhancement originates from meandering electron pathways created by the disordered mixture of two materials and does not rely on long-range crystalline order. The identified requirements for this mechanism can be broadly satisfied across different material systems, offering a universal and tunable strategy to engineer large transverse responses in structurally robust platforms. △ Less

Submitted 20 March, 2026; v1 submitted 6 November, 2024; originally announced November 2024.

Comments: 9+18 pages, 5+14 figures, Accepted version in Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 136, 056301 (2026)

A family of air-stable chalcogenide solid electrolytes in Li$_2$BMQ$_4$ (B = Ca, Sr and Ba; M = Si, Ge and Sn; Q = O, S and Se) systems

Authors: Huican Mao, Xiang Zhu, Guangmao Li, Jie Pang, Junfeng Hao, Liqi Wang, Hailong Yu, Youguo Shi, Fan Wu, Shilie Pan, Ruijuan Xiao, Hong Li, Liquan Chen

Abstract: Combining high-throughput first-principles calculations and experimental measurements, we have identified a novel family of fast lithium-ion chalcogenide conductors in Li$_2$BMQ$_4$ (2114, B = Ca, Sr and Ba; M = Si, Ge and Sn; Q = O, S and Se) systems. Our calculations demonstrate that most of the thermodynamically and kinetically stable sulfides and selenides in this new system exhibit ultralow L… ▽ More Combining high-throughput first-principles calculations and experimental measurements, we have identified a novel family of fast lithium-ion chalcogenide conductors in Li$_2$BMQ$_4$ (2114, B = Ca, Sr and Ba; M = Si, Ge and Sn; Q = O, S and Se) systems. Our calculations demonstrate that most of the thermodynamically and kinetically stable sulfides and selenides in this new system exhibit ultralow Li$^+$ ion migration activation energy (0.16 eV ~ 0.56 eV) and considerable bandgaps varying between ~ 2 eV and 3.5 eV. We have successfully synthesized Li$_2$BaSnS$_4$ and Li$_2$SrSiS$_4$, and they exhibit excellent moisture stability through H$_2$S gas measurements. Electrochemical impedance measurements indicate 2114 systems show the typical features of solid ionic conductors, with a room-temperature Li$^+$ conductivity close to 5$\times$10$^{-4}$ mS/cm aligning with our molecular dynamics simulations. Furthermore, we have theoretically investigated the substitution of Cl$^-$ at S$^{2-}$ site. The doped compounds display significantly higher conductivity, with an increase of about three orders of magnitude (up to a maximum of 0.72 mS/cm) compared to the undoped compounds. These findings offer valuable insights for the further exploration of potential chalcogenide solid electrolyte materials with robust air stability and enhanced ionic conductivity for practical applications in lithium-ion batteries. △ Less

Submitted 6 May, 2024; originally announced May 2024.

On-liquid-gallium surface synthesis of ultra-smooth conductive metal-organic framework thin films

Authors: Jinxin Liu, Yunxu Chen, Xing Huang, Yanhan Ren, Mike Hambsch, David Bodesheim, Darius Pohl, Xiaodong Li, Marielle Deconinck, Bowen Zhang, Markus Löffler, Zhongquan Liao, Fengxiang Zhao, Arezoo Dianat, Gianaurelio Cuniberti, Yana Vaynzof, Junfeng Gao, Jingcheng Hao, Stefan C. B. Mannsfeld, Xinliang Feng, Renhao Dong

Abstract: Conductive metal-organic frameworks (MOFs) are emerging electroactive materials for (opto-)electronics. However, it remains a great challenge to achieve reliable MOF-based devices via the existing synthesis methods that are compatible with the complementary metal-oxide-semiconductor technology, as the surface roughness of thus-far synthetic MOF films or pellets is rather high for efficient electro… ▽ More Conductive metal-organic frameworks (MOFs) are emerging electroactive materials for (opto-)electronics. However, it remains a great challenge to achieve reliable MOF-based devices via the existing synthesis methods that are compatible with the complementary metal-oxide-semiconductor technology, as the surface roughness of thus-far synthetic MOF films or pellets is rather high for efficient electrode contact. Here, we develop an on-liquid-gallium surface synthesis (OLGSS) strategy under chemical vapor deposition (CVD) conditions for the controlled growth of two-dimensional conjugated MOF (2D c-MOF) thin films with ten-fold improvement of surface flatness (surface roughness can reach as low as ~2 Å) compared with MOF films grown by the traditional methods. Supported by theoretical modeling, we unveil a layer-by-layer CVD growth mode for constructing flattening surfaces, that is triggered by the high adhesion energy between gallium (Ga) and planar aromatic ligands. We further demonstrate the generality of the as-proposed OLGSS strategy by reproducing such a flat surface over nine different 2D c-MOF films with variable thicknesses (~2 to 208 nm) and large lateral sizes (over 1 cm2). The resultant ultra-smooth 2D c-MOF films enable the formation of high-quality electrical contacts with gold (Au) electrodes, leading to a reduction of contact resistance by over ten orders of magnitude compared to the traditional uneven MOF films. Furthermore, due to the efficient interfacial interaction benifited from the high-quality contacts, the prepared van der Waals heterostructure (vdWH) of OLGSS c-MOF and MoS2 exhibits intriguing photoluminescence (PL) enhancement, PL peak shift and large work function modulation. The establishment of the reliable OLGSS method provides the chances to push the development of MOF electronics and the construction of multicomponent MOF-based heterostructure materials. △ Less

Submitted 17 April, 2024; originally announced April 2024.

Creating and controlling global Greenberger-Horne-Zeilinger entanglement on quantum processors

Authors: Zehang Bao, Shibo Xu, Zixuan Song, Ke Wang, Liang Xiang, Zitian Zhu, Jiachen Chen, Feitong Jin, Xuhao Zhu, Yu Gao, Yaozu Wu, Chuanyu Zhang, Ning Wang, Yiren Zou, Ziqi Tan, Aosai Zhang, Zhengyi Cui, Fanhao Shen, Jiarun Zhong, Tingting Li, Jinfeng Deng, Xu Zhang, Hang Dong, Pengfei Zhang, Yang-Ren Liu , et al. (8 additional authors not shown)

Abstract: Greenberger-Horne-Zeilinger (GHZ) states, also known as two-component Schrödinger cats, play vital roles in the foundation of quantum physics and, more attractively, in future quantum technologies such as fault-tolerant quantum computation. Enlargement in size and coherent control of GHZ states are both crucial for harnessing entanglement in advanced computational tasks with practical advantages,… ▽ More Greenberger-Horne-Zeilinger (GHZ) states, also known as two-component Schrödinger cats, play vital roles in the foundation of quantum physics and, more attractively, in future quantum technologies such as fault-tolerant quantum computation. Enlargement in size and coherent control of GHZ states are both crucial for harnessing entanglement in advanced computational tasks with practical advantages, which unfortunately pose tremendous challenges as GHZ states are vulnerable to noise. Here we propose a general strategy for creating, preserving, and manipulating large-scale GHZ entanglement, and demonstrate a series of experiments underlined by high-fidelity digital quantum circuits. For initialization, we employ a scalable protocol to create genuinely entangled GHZ states with up to 60 qubits, almost doubling the previous size record. For protection, we take a new perspective on discrete time crystals (DTCs), originally for exploring exotic nonequilibrium quantum matters, and embed a GHZ state into the eigenstates of a tailor-made cat scar DTC to extend its lifetime. For manipulation, we switch the DTC eigenstates with in-situ quantum gates to modify the effectiveness of the GHZ protection. Our findings establish a viable path towards coherent operations on large-scale entanglement, and further highlight superconducting processors as a promising platform to explore nonequilibrium quantum matters and emerging applications. △ Less

Submitted 13 October, 2024; v1 submitted 16 January, 2024; originally announced January 2024.

Comments: 7 pages, 4 figures + supplementary information

Journal ref: Nat. Commun. 15, 8823 (2024)

Observation of a 1/3 Magnetisation Plateau Phase as Evidence for the Kitaev Interaction in a Honeycomb-Lattice Antiferromagnet

Authors: Yanyan Shangguan, Song Bao, Zhao-Yang Dong, Ning Xi, Yi-Peng Gao, Zhen Ma, Wei Wang, Zhongyuan Qi, Shuai Zhang, Zhentao Huang, Junbo Liao, Xiaoxue Zhao, Bo Zhang, Shufan Cheng, Hao Xu, Dehong Yu, Richard A. Mole, Naoki Murai, Seiko Ohira-Kawamura, Lunhua He, Jiazheng Hao, Qing-Bo Yan, Fengqi Song, Wei Li, Shun-Li Yu , et al. (2 additional authors not shown)

Abstract: Fractional magnetisation plateaus, in which the magnetisation is pinned at a fraction of its saturated value within a range of external magnetic field, are spectacular macroscopic manifestations of the collective quantum behaviours. One prominent example of the plateau phase is found in spin-1/2 triangular-lattice antiferromagnets featuring strong geometrical frustration, and is often interpreted… ▽ More Fractional magnetisation plateaus, in which the magnetisation is pinned at a fraction of its saturated value within a range of external magnetic field, are spectacular macroscopic manifestations of the collective quantum behaviours. One prominent example of the plateau phase is found in spin-1/2 triangular-lattice antiferromagnets featuring strong geometrical frustration, and is often interpreted as quantum-fluctuation-stabilised state in magnetic field via the "order-by-disorder" mechanism. Here, we observe an unprecedented 1/3 magnetisation plateau between 5.2 and 7.4 T at 2 K in a spin-1 antiferromagnet Na$_3$Ni$_2$BiO$_6$ with a honeycomb lattice, where conventionally no geometrical frustration is anticipated. By carrying out elastic neutron scattering measurements, we propose the spin structure of the plateau phase to be an unusual partial spin-flop ferrimagnetic order, transitioning from the zigzag antiferromagnetic order in zero field. Our theoretical calculations show that the plateau phase is stabilised by the bond-anisotropic Kitaev interaction. These results provide a new paradigm for the exploration of rich quantum phases in frustrated magnets and exotic Kitaev physics in high-spin systems. △ Less

Submitted 26 December, 2023; originally announced December 2023.

Comments: Submitted version, 10 pages, 5 figures. Final version has been published in Nature Physics

Journal ref: Nature Physics 19, 1883-1889 (2023)

Orientation disparity in GaN/graphene/$m$-sapphire: control-based re-examination of thru-hole epitaxy

Authors: Su Young An, Hyunkyu Lee, Gunhoon Beak, Hyeonoh Jo, Jae Hun Kim, Jongwoo Ha, Jieun Yang, Changwook Dong, Jaewu Choi, Joonwon Lim, Chinkyo Kim

Abstract: The crystallographic orientation of films grown on 2D-masked substrates is often used to infer the pathway among remote, van der Waals, and thru-hole (pinhole-seeded) epitaxy. However, attribution of a specific growth mechanism based on orientation can be ambiguous unless mask continuity and substrate pre-treatment are evaluated within a single process window. We compare GaN grown under identical… ▽ More The crystallographic orientation of films grown on 2D-masked substrates is often used to infer the pathway among remote, van der Waals, and thru-hole (pinhole-seeded) epitaxy. However, attribution of a specific growth mechanism based on orientation can be ambiguous unless mask continuity and substrate pre-treatment are evaluated within a single process window. We compare GaN grown under identical conditions on four m-plane sapphire templates: (i) bare, (ii) "graphene-grown" (high-temperature Ar/H2 with CH4 on), (iii) "anneal-only" (high-temperature Ar/H2 with CH4 off), and (iv) graphene oxide spin-coated and reduced on pristine sapphire. GaN selects (103) on graphene-grown and anneal-only m-plane sapphire, selects (100) on bare m-plane sapphire, and is predominantly (100) with a minority (103) on graphene oxide spin-coated and reduced/pristine m-plane sapphire. High-resolution TEM shows that, on partly graphene-covered samples, nucleation occurs on exposed sapphire (thru-hole), not on graphene, providing mechanism evidence independent of orientation. Within this window, the substrate surface state set by high-temperature Ar/H2 pre-treatment (rather than mask continuity) primarily governs orientation, while open-area effects can play a secondary role. Thus, preferred orientation alone may not determine the growth mechanism; mask continuity and substrate pre-treatment must be explicitly controlled when using orientation as evidence for mechanism assignment. △ Less

Submitted 20 December, 2025; v1 submitted 30 August, 2023; originally announced August 2023.

Comments: 15 pages, 5 figures

Electronic correlations and partial gap in the bilayer nickelate La$_{3}$Ni$_{2}$O$_{7}$

Authors: Zhe Liu, Mengwu Huo, Jie Li, Qing Li, Yuecong Liu, Yaomin Dai, Xiaoxiang Zhou, Jiahao Hao, Yi Lu, Meng Wang, Hai-Hu Wen

Abstract: The discovery of superconductivity with a critical temperature of about 80~K in La$_{3}$Ni$_{2}$O$_{7}$ single crystals under pressure has received enormous attention. La$_{3}$Ni$_{2}$O$_{7}$ is not superconducting under ambient pressure but exhibits a transition at $T^{\ast} \simeq 115$~K. Understanding the electronic correlations and charge dynamics is an important step towards the origin of sup… ▽ More The discovery of superconductivity with a critical temperature of about 80~K in La$_{3}$Ni$_{2}$O$_{7}$ single crystals under pressure has received enormous attention. La$_{3}$Ni$_{2}$O$_{7}$ is not superconducting under ambient pressure but exhibits a transition at $T^{\ast} \simeq 115$~K. Understanding the electronic correlations and charge dynamics is an important step towards the origin of superconductivity and other instabilities. Here, our optical study shows that La$_{3}$Ni$_{2}$O$_{7}$ features strong electronic correlations which significantly reduce the electron's kinetic energy and place this system in the proximity of the Mott phase. The low-frequency optical conductivity reveals two Drude components arising from multiple bands at the Fermi level. The transition at $T^{\ast}$ removes the Drude component exhibiting non-Fermi liquid behavior, whereas the one with Fermi-liquid behavior is barely affected. These observations in combination with theoretical results suggest that the Fermi surface dominated by the Ni-$d_{3z^{2}-r^{2}}$ orbital is removed due to the transition at $T^{\ast}$. Our experimental results provide pivotal information for understanding the transition at $T^{\ast}$ and superconductivity in La$_{3}$Ni$_{2}$O$_{7}$. △ Less

Submitted 2 April, 2024; v1 submitted 6 July, 2023; originally announced July 2023.

Comments: 26 pages, 4 figures, Comments are welcome and appreciated

Journal ref: Nature Communications 15, 7570 (2024)

Effects of Nb Doping on the Charge-Density Wave and Electronic Correlations in the Kagome Metal Cs(V$_{1-x}$Nb$_{x}$)$_{3}$Sb$_{5}$

Authors: Xiaoxiang Zhou, Yongkai Li, Zhe Liu, Jiahao Hao, Yaomin Dai, Zhiwei Wang, Yugui Yao, Hai-Hu Wen

Abstract: The transport and optical properties of the Nb-doped Cs(V$_{1-x}$Nb$_{x}$)$_{3}$Sb$_{5}$ with x = 0.03 and 0.07 have been investigated and compared with those of the undoped CsV$_{3}$Sb$_{5}$. Upon Nb doping, the charge-density wave (CDW) transition temperature $T_{\text{CDW}}$ is suppressed, and the superconducting temperature $T_{c}$ rises. The residual resistivity ratio decreases with Nb doping… ▽ More The transport and optical properties of the Nb-doped Cs(V$_{1-x}$Nb$_{x}$)$_{3}$Sb$_{5}$ with x = 0.03 and 0.07 have been investigated and compared with those of the undoped CsV$_{3}$Sb$_{5}$. Upon Nb doping, the charge-density wave (CDW) transition temperature $T_{\text{CDW}}$ is suppressed, and the superconducting temperature $T_{c}$ rises. The residual resistivity ratio decreases with Nb doping, suggesting an increase of disorder. For all compounds, the optical conductivity in the pristine phase reveals two Drude components (D1 and D2). The substitution of Nb causes an increase of D1 alongside a reduction of D2 in weight, which implies a change of the Fermi surface. The total Drude weight is reduced with increasing Nb content, signifying an enhancement of electronic correlations. Below $T_{\text{CDW}}$, while the optical conductivity clearly manifests the CDW gap in all materials, the gapped portion of the Fermi surface shrinks as the Nb content grows. A comprehensive analysis indicates that the change of the Fermi surface, the enhancement of electronic correlations, the shrinkage of the removed Fermi surface by the CDW gap, and the increase of disorder may all have a considerable impact on the interplay between the CDW and superconductivity in Cs(V$_{1-x}$Nb$_{x}$)$_{3}$Sb$_{5}$. △ Less

Submitted 13 March, 2023; v1 submitted 13 March, 2023; originally announced March 2023.

Comments: 7 pages, 3 figures

Journal ref: Physical Review B 107,125124 (2023)

Phase diagram of a one-dimensional Ising model with an anomalous Z_2 symmetry

Authors: Jin-Xiang Hao, Wei Li, Yang Qi

Abstract: Anomalous global symmetries, which can be realized on the boundary of symmetry-protected topological phases, bring\wlx{s} new phases and phase transitions to condensed matter physics. In this work, we study a one-dimensional model with an anomalous $\mathbb Z_2$ symmetry, using the density-matrix renormalization group method. Besides a symmetry-breaking ferromagnetic phase, we find a gapless phase… ▽ More Anomalous global symmetries, which can be realized on the boundary of symmetry-protected topological phases, bring\wlx{s} new phases and phase transitions to condensed matter physics. In this work, we study a one-dimensional model with an anomalous $\mathbb Z_2$ symmetry, using the density-matrix renormalization group method. Besides a symmetry-breaking ferromagnetic phase, we find a gapless phase described by the Tomonaga-Luttinger Liquid theory with the central charge $c=1$. Our numerical finding is compatible of theoretical constraints on possible phases resulting from the symmetry anomaly. △ Less

Submitted 5 July, 2026; v1 submitted 10 November, 2022; originally announced November 2022.

Electronic Correlations and Evolution of the Charge-Density Wave in the Kagome Metals $A$V$_{3}$Sb$_{5}$ ($A$ = K, Rb, Cs)

Authors: Xiaoxiang Zhou, Yongkai Li, Xinwei Fan, Jiahao Hao, Ying Xiang, Zhe Liu, Yaomin Dai, Zhiwei Wang, Yugui Yao, Hai-Hu Wen

Abstract: The kagome metals $A$V$_{3}$Sb$_{5}$ ($A$ = K, Rb, Cs) have attracted enormous interest as they exhibit intertwined charge-density wave (CDW) and superconductivity. The alkali-metal dependence of these characteristics contains pivotal information about the CDW and its interplay with superconductivity. Here, we report optical studies of $A$V$_{3}$Sb$_{5}$ across the whole family. With increasing al… ▽ More The kagome metals $A$V$_{3}$Sb$_{5}$ ($A$ = K, Rb, Cs) have attracted enormous interest as they exhibit intertwined charge-density wave (CDW) and superconductivity. The alkali-metal dependence of these characteristics contains pivotal information about the CDW and its interplay with superconductivity. Here, we report optical studies of $A$V$_{3}$Sb$_{5}$ across the whole family. With increasing alkali-metal atom radius from K to Cs, the CDW gap increases monotonically, whereas $T_{\text{CDW}}$ first rises and then drops, at variance with conventional CDW. While the Fermi surface gapped by the CDW grows, $T_{c}$ is elevated in CsV$_{3}$Sb$_{5}$, indicating that the interplay between the CDW and superconductivity is not simply a competition for the density of states near \EF. More importantly, we observe an enhancement of electronic correlations in CsV$_{3}$Sb$_{5}$, which suppresses the CDW but enhances superconductivity, thus accounting for the above peculiar observations. Our results suggest electronic correlations as an important factor in manipulating the CDW and its entanglement with superconductivity in $A$V$_{3}$Sb$_{5}$. △ Less

Submitted 21 August, 2022; originally announced August 2022.

Comments: 6 pages, 4 figures. Comments are welcome and appreciated

Doping-induced structural transformation in the spin-1/2 triangular-lattice antiferromagnet Na$_{2}$Ba$_{1-x}$Sr$_{x}$Co(PO$_{4}$)$_{2}$

Authors: Chuandi Zhang, Qianhui Xu, Xu-Tao Zeng, Chao Lyu, Zhengwang Lin, Jiazheng Hao, Sihao Deng, Lunhua He, Yinguo Xiao, Yu Ye, Ziyu Chen, Xian-Lei Sheng, Wentao Jin

Abstract: The effects of Sr doping on the structural properties of Na$_{2}$BaCo(PO$_{4}$)$_{2}$, a spin-1/2 triangular-lattice antiferromagnet as a quantum spin liquid candidate, are investigated by complementary x-ray and neutron powder diffraction measurements. It is found that in Na$_{2}$Ba$_{1-x}$Sr$_{x}$Co(PO$_{4}$)$_{2}$ (NBSCPO), the trigonal phase (space group $\mathit{P}$$\bar{3}$$\mathit{m}$1) wit… ▽ More The effects of Sr doping on the structural properties of Na$_{2}$BaCo(PO$_{4}$)$_{2}$, a spin-1/2 triangular-lattice antiferromagnet as a quantum spin liquid candidate, are investigated by complementary x-ray and neutron powder diffraction measurements. It is found that in Na$_{2}$Ba$_{1-x}$Sr$_{x}$Co(PO$_{4}$)$_{2}$ (NBSCPO), the trigonal phase (space group $\mathit{P}$$\bar{3}$$\mathit{m}$1) with a perfect triangular lattice of Co$^{2+}$ ions is structurally stable when the doping level of Sr is below 30% ($\mathit{x}$ $\le$ 0.3), while a pure monoclinic phase (space group $\mathit{P}$2$_{1}$/$\mathit{a}$) with slight rotations of CoO$_{6}$ octahedra and displacements of Ba$^{2+}$/Sr$^{2+}$ ions will be established when the Sr doping level is above 60% ($\mathit{x}$ $\ge$ 0.6). Such a doping-induced structural transformation in NBSCPO is supported by first-principles calculations and Raman spectroscopy. Na$_{2}$SrCo(PO$_{4}$)$_{2}$, a novel spin-1/2 triangular-lattice antiferromagnet with glaserite-type structure, although monoclinically distorted, exhibits no long-range magnetic order down to 2 K and a similar negative Curie-Weiss temperature as Na$_{2}$BaCo(PO$_{4}$)$_{2}$ with a perfect triangular lattice, suggesting the robustness of magnetic exchange interaction against the Ba/Sr substitutions. △ Less

Submitted 17 May, 2022; originally announced May 2022.

Comments: 26 pages, 7 figures

Journal ref: Journal of Alloys and Compounds 905, 164147 (2022)

Topological $p_z$-wave nodal-line superconductivity with flat surface bands in the AH$_{x}$Cr${_3}$As${_3}$ (A=Na, K, Rb, Cs) superconductors

Authors: Juan-Juan Hao, Ming Zhang, Xian-Xin Wu, Fan Yang

Abstract: We study the pairing symmetry and the topological properties of the hydrogen-doped ACr$_3$As$_3$ superconductors. Based on our first-principle band structure with spin-orbit-coupling (SOC), we construct tight binding model including the on-site SOC terms, equipped with the multi-orbital Hubbard interactions. Then using the random-phase-approximation (RPA) approach, we calculate the pairing phase d… ▽ More We study the pairing symmetry and the topological properties of the hydrogen-doped ACr$_3$As$_3$ superconductors. Based on our first-principle band structure with spin-orbit-coupling (SOC), we construct tight binding model including the on-site SOC terms, equipped with the multi-orbital Hubbard interactions. Then using the random-phase-approximation (RPA) approach, we calculate the pairing phase diagram of this model. Our RPA results yield the triplet pz-wave pairing in the ($\left(\uparrow\downarrow+\downarrow\uparrow\right)$) spin channel to be the leading pairing symmetry all over the experiment relevant hydrogen-doping regimes. This pairing state belongs to the spin-U(1)-symmetry protected time-reversal-invariant topological nodal-line superconductivity. Determined by the momentum-dependent topological invariant Z (kx; ky), the whole (001) surface Brillouin zone is covered with topological flat bands with different regimes covered with different numbers of flat surface bands, which can be detected by the scanning tunneling microscope experiments. △ Less

Submitted 24 January, 2022; originally announced January 2022.

Comments: 7 pages, 6 figures

Report number: EPL 138 66001

Swelling, Softening and Elastocapillary Adhesion of Cooked Pasta

Authors: Jonghyun Hwang, Jonghyun Ha, Ryan Siu, Yun Seong Kim, Sameh Tawfick

Abstract: The diverse chemical and physical reactions encountered during cooking connect us to science every day. Here, we theoretically and experimentally investigate the swelling and softening of pasta due to liquid imbibition, as well as the elastic deformation and adhesion of pasta due to capillary force. As water diffuses into the pasta during cooking, it softens gradually from the outside inward as st… ▽ More The diverse chemical and physical reactions encountered during cooking connect us to science every day. Here, we theoretically and experimentally investigate the swelling and softening of pasta due to liquid imbibition, as well as the elastic deformation and adhesion of pasta due to capillary force. As water diffuses into the pasta during cooking, it softens gradually from the outside inward as starch swells. The softening follows three sequential regimes: Regime I shows a slow decrease of modulus with cooking time; Regime II, the glassy to rubbery transition region, is characterized by very fast, several orders of magnitude drop in modulus; and regime III, the rubbery region, has an asymptotic modulus about four orders of magnitude lower than the raw pasta. We present experiments and theory to capture these regimes and relate them to the heterogeneous microstructure changes associated with swelling. Interestingly, we observe a modulus drop of two orders of magnitude within the range of 'al dente' cooking duration, and we find the modulus to be extremely sensitive to the amount of salt added to the boiling water. While most chefs can gauge the pasta by tasting its texture, our proposed experiment, which only requires a measurement with a ruler, can precisely provide an optimal cooking time finely tuned for various kinds of pasta shapes. △ Less

Submitted 24 January, 2022; originally announced January 2022.

Many-body Hilbert space scarring on a superconducting processor

Authors: Pengfei Zhang, Hang Dong, Yu Gao, Liangtian Zhao, Jie Hao, Qiujiang Guo, Jiachen Chen, Jinfeng Deng, Bobo Liu, Wenhui Ren, Yunyan Yao, Xu Zhang, Shibo Xu, Ke Wang, Feitong Jin, Xuhao Zhu, Hekang Li, Chao Song, Zhen Wang, Fangli Liu, Zlatko Papić, Lei Ying, H. Wang, Ying-Cheng Lai

Abstract: Quantum many-body scarring (QMBS) -- a recently discovered form of weak ergodicity breaking in strongly-interacting quantum systems -- presents opportunities for mitigating thermalization-induced decoherence in quantum information processsing. However, the existing experimental realizations of QMBS are based on kinetically-constrained systems where an emergent dynamical symmetry "shields" such sta… ▽ More Quantum many-body scarring (QMBS) -- a recently discovered form of weak ergodicity breaking in strongly-interacting quantum systems -- presents opportunities for mitigating thermalization-induced decoherence in quantum information processsing. However, the existing experimental realizations of QMBS are based on kinetically-constrained systems where an emergent dynamical symmetry "shields" such states from the thermalizing bulk of the spectrum. Here, we experimentally realize a distinct kind of QMBS phenomena by approximately decoupling a part of the many-body Hilbert space in the computational basis. Utilizing a programmable superconducting processor with 30 qubits and tunable couplings, we realize Hilbert space scarring in a non-constrained model in different geometries, including a linear chain as well as a quasi-one-dimensional comb geometry. By performing full quantum state tomography on 4-qubit subsystems, we provide strong evidence for QMBS states by measuring qubit population dynamics, quantum fidelity and entanglement entropy following a quench from initial product states. Our experimental findings broaden the realm of QMBS mechanisms and pave the way to exploiting correlations in QMBS states for applications in quantum information technology. △ Less

Submitted 18 March, 2022; v1 submitted 10 January, 2022; originally announced January 2022.

Report number: s41567-022-01784-9

Journal ref: Nature Physics 19, 120 (2022)

Oscillating paramagnetic Meissner effect and Berezinskii-Kosterlitz-Thouless transition in $Bi_2Sr_2CaCu_2O_{8+δ}$ monolayer

Authors: S. Y. Wang, Y. Yu, J. X. Hao, Y. Feng, J. J. Zhu, Y. S. Lin, B. K. Xiang, H. Ru, Y. P. Pan, G. D. Gu, K. Watanabe, T. Taniguchi, Y. Qi, Y. Zhang, Y. H. Wang

Abstract: Monolayers of a prototypical cuprate high transition-temperature ($T_C$) superconductor $Bi_2Sr_2CaCu_2O_{8+δ}$ (Bi2212) was recently found to show $T_C$ and other electronic properties similar to those of the bulk. The robustness of superconductivity in an ideal two-dimensional (2D) system was an intriguing fact that defied the Mermin-Wagner theorem. Here, we took advantage of the high sensitivit… ▽ More Monolayers of a prototypical cuprate high transition-temperature ($T_C$) superconductor $Bi_2Sr_2CaCu_2O_{8+δ}$ (Bi2212) was recently found to show $T_C$ and other electronic properties similar to those of the bulk. The robustness of superconductivity in an ideal two-dimensional (2D) system was an intriguing fact that defied the Mermin-Wagner theorem. Here, we took advantage of the high sensitivity of scanning SQUID susceptometry to image the phase stiffness throughout the phase transition of Bi2212 in the 2D limit. We found susceptibility oscillated with flux between diamagnetism and paramagnetism in a Fraunhofer-like pattern up till $T_C$. The temperature and sample size-dependence of the modulation period agreed well with our Coulomb gas analogy of a finite 2D system based on Berezinskii-Kosterlitz-Thouless (BKT) transition. In the multilayers, the susceptibility oscillation differed in a small temperature regime below $T_C$ in consistent with a dimensional-crossover led by interlayer coupling. Serving as strong evidence of BKT transition in the bulk, there appeared a sharp superfluid density jump at zero-field and paramagnetism at small fields just below $T_C$. These results unified the phase transitions from the monolayer Bi2212 to the bulk as BKT transition with finite interlayer coupling. This elucidating picture favored the pre-formed pairs scenario for the underdoped cuprates regardless of lattice dimensionality. △ Less

Submitted 9 December, 2021; originally announced December 2021.

Lifshitz transition enhanced triplet $p_z$-wave superconductivity in hydrogen doped KCr$_3$As$_3$

Authors: Ming Zhang, Juan-Juan Hao, Xianxin Wu, Fan Yang

Abstract: The recently synthesized air-insensitive hydrogen doped KCr$_3$As$_3$ superconductor has aroused great research interests. This material has, for the first time in the research area of the quasi-one-dimensional Cr-based superconductivity (SC), realized a tunability through charge doping, which will potentially significantly push the development of this area. Here based on the band structure from f… ▽ More The recently synthesized air-insensitive hydrogen doped KCr$_3$As$_3$ superconductor has aroused great research interests. This material has, for the first time in the research area of the quasi-one-dimensional Cr-based superconductivity (SC), realized a tunability through charge doping, which will potentially significantly push the development of this area. Here based on the band structure from first-principle calculations, we construct a six-band tight-binding (TB) model equipped with multi-orbital Hubbard interactions, and adopt the random-phase-approximation approach to study the hydrogen-doping dependence of the pairing symmetry and superconducting $T_c$. Under the rigid-band approximation, our pairing phase diagram is occupied by the triplet $p_z$-wave pairing through out the hydrogen-doping regime $x\in (0.4,1)$ in which SC has been experimentally detected. Remarkably, the $x$-dependence of $T_c$ shows a peak at the 3D-quasi-1D Lifshitz transition point, although the total density of state exhibit a dip there. A thorough investigation of the band structure reveals type-II van-Hove singularities (VHSs) in the $γ$ band, which favor the formation of the triplet SC. It turns out that the $γ$- Fermi surface (FS) comprises two flat quasi-1D FS sheets almost parallel to the $k_z=0$ plane and six almost perpendicular tube-like FS sheets, and the type-II VHS just lies in the boundary between these two FS parts. Furthermore, the $\left|k_z\right|$ of the VH planes reaches the maximum near the Lifshitz-transition point, which pushes the $T_c$ of the $p_z$-wave SC to the maximum. Our results appeal more experimental access into this intriguing superconductor. △ Less

Submitted 31 March, 2022; v1 submitted 29 November, 2021; originally announced November 2021.

Comments: 11 pages, 6 figures, accepted version by Phys. Rev. B

Pseudogap and Strong Pairing Induced by Incipient and Shallow Bands in the Quasi-Two-Dimensional KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$

Authors: Jiahao Hao, Wenshan Hong, Xiaoxiang Zhou, Ying Xiang, Yaomin Dai, Huan Yang, Shiliang Li, Huiqian Luo, Hai-Hu Wen

Abstract: The optical properties of KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ (K12442, $T_c = 33.5$~K) and KCa$_{2}$(Fe$_{0.95}$Ni$_{0.05}$)$_{4}$As$_{4}$F$_{2}$ (Ni-K12442, $T_c = 29$~K) have been examined at a large number of temperatures. For both samples, a nodeless superconducting gap is clearly observed in the optical conductivity at 5~K. The superconducting gap $Δ\simeq 8.7$~meV (… ▽ More The optical properties of KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ (K12442, $T_c = 33.5$~K) and KCa$_{2}$(Fe$_{0.95}$Ni$_{0.05}$)$_{4}$As$_{4}$F$_{2}$ (Ni-K12442, $T_c = 29$~K) have been examined at a large number of temperatures. For both samples, a nodeless superconducting gap is clearly observed in the optical conductivity at 5~K. The superconducting gap $Δ\simeq 8.7$~meV ($2Δ/k_{\text{B}}T_{c} \simeq 6.03$) in K12442, pointing towards strong-coupling Cooper pairs, but in sharp contrast, $Δ\simeq 4.6$~meV ($2Δ/k_{\text{B}}T_{c} \simeq 3.68$) in Ni-K12442, which agrees with the BCS weak-coupling pairing state. More intriguingly, below $T^{\ast} \simeq 75$~K, the optical conductivity of K12442 reveals a pseudogap that smoothly evolves into the superconducting gap below $T_{c}$, while no such behavior is detected in the electron-doped Ni-K12442. The comparison between the two samples hints that the pseudogap and strong-coupling Cooper pairs in K12442 may be intimately related to the shallow and incipient bands. We provide arguments supporting a preformed pairing mechanism of the pseudogap, but at the moment a magnetic scenario can not yet be excluded. △ Less

Submitted 21 August, 2022; v1 submitted 8 November, 2021; originally announced November 2021.

Comments: 8 pages, 5 figures, Comments are welcome and appreciated

Journal ref: PhysRevB.106.014523 (2022)

Superior Photo-carrier Diffusion Dynamics in Organic-inorganic Hybrid Perovskites Revealed by Spatiotemporal Conductivity Imaging

Authors: Xuejian Ma, Fei Zhang, Zhaodong Chu, Ji Hao, Xihan Chen, Jiamin Quan, Zhiyuan Huang, Xiaoming Wang, Xiaoqin Li, Yanfa Yan, Kai Zhu, Keji Lai

Abstract: The outstanding performance of organic-inorganic metal trihalide solar cells benefits from the exceptional photo-physical properties of both electrons and holes in the material. Here, we directly probe the free-carrier dynamics in Cs-doped FAPbI3 thin films by spatiotemporal photoconductivity imaging. Using charge transport layers to selectively quench one type of carriers, we show that the two re… ▽ More The outstanding performance of organic-inorganic metal trihalide solar cells benefits from the exceptional photo-physical properties of both electrons and holes in the material. Here, we directly probe the free-carrier dynamics in Cs-doped FAPbI3 thin films by spatiotemporal photoconductivity imaging. Using charge transport layers to selectively quench one type of carriers, we show that the two relaxation times on the order of 1 microsecond and 10 microseconds correspond to the lifetimes of electrons and holes in FACsPbI3, respectively. Strikingly, the diffusion mapping indicates that the difference in electron/hole lifetimes is largely compensated by their disparate mobility. Consequently, the long diffusion lengths (3 ~ 5 micrometers) of both carriers are comparable to each other, a feature closely related to the unique charge trapping and de-trapping processes in hybrid trihalide perovskites. Our results unveil the origin of superior diffusion dynamics in this material, crucially important for solar-cell applications. △ Less

Submitted 4 August, 2021; originally announced August 2021.

Seven-coordinated Silicon in a SiO2He Compound Formed under the Extreme Conditions of Planetary Interiors

Authors: Shicong Ding, Pan Zhang, Kang Yang, Cailong Liu, Jian Hao, Wenwen Cui, Jingming Shi, Yinwei Li

Abstract: Changes in atomic coordination numbers at high pressures are fundamental to condensed-matter physics because they initiate the emergence of unexpected structures and phenomena. Silicon is capable of forming eight-, nine-, and ten-coordinated structures under compression,in addition to the usual six-coordinated structures. The missing seven-coordinated silicon remains an open question, but here our… ▽ More Changes in atomic coordination numbers at high pressures are fundamental to condensed-matter physics because they initiate the emergence of unexpected structures and phenomena. Silicon is capable of forming eight-, nine-, and ten-coordinated structures under compression,in addition to the usual six-coordinated structures. The missing seven-coordinated silicon remains an open question, but here our theoretical study provides evidence for its existence at high pressures. A combination of a crystal-structure prediction method and first-principles calculations allowed prediction of a stable SiO2He compound containing unique SiO7 polyhedrons, which is a configuration unknown in any proposed silica phase. Consequently, seven-coordinated SiO7 is a possible form of silica at high pressures. Further calculations indicate that the SiO2He phase remains energetically stable with a solid character over a wide range of pressures exceeding 607 GPa and temperatures of 0-9000 K, covering the extreme conditions of the core-mantle boundary in super-Earth exoplanets, or even the Solar System's ice giant planets. Our results may provide theoretical guidance for the discovery of other silicides at high pressures, promote the exploration of materials at planetary core-mantle boundaries, and enable planetary models to be refined. △ Less

Submitted 7 July, 2021; originally announced July 2021.

Origin of the Charge Density Wave in the Kagome Metal CsV$_{3}$Sb$_{5}$ as Revealed by Optical Spectroscopy

Authors: Xiaoxiang Zhou, Yongkai Li, Xinwei Fan, Jiahao Hao, Yaomin Dai, Zhiwei Wang, Yugui Yao, Hai-Hu Wen

Abstract: We report on a detailed study of the optical properties of CsV$_{3}$Sb$_{5}$ at a large number of temperatures above and below the charge-density-wave (CDW) transition. Above the CDW transition, the low-frequency optical conductivity reveals two Drude components with distinct widths. An examination of the band structure allows us to ascribe the narrow Drude to multiple light and Dirac bands, and t… ▽ More We report on a detailed study of the optical properties of CsV$_{3}$Sb$_{5}$ at a large number of temperatures above and below the charge-density-wave (CDW) transition. Above the CDW transition, the low-frequency optical conductivity reveals two Drude components with distinct widths. An examination of the band structure allows us to ascribe the narrow Drude to multiple light and Dirac bands, and the broad Drude to the heavy bands near the $M$ points which form saddle points near the Fermi level. Upon entering the CDW state, the opening of the CDW gap is clearly observed. A large portion of the broad Drude is removed by the gap, whereas the narrow Drude is not affected. Meanwhile, an absorption peak associated with interband transitions near the saddle points shifts to higher energy and grows in weight. These observations are consistent with the scenario that the CDW in CsV$_{3}$Sb$_{5}$ is driven by nesting of Fermi surfaces near the saddle points at $M$. △ Less

Submitted 8 July, 2021; v1 submitted 2 April, 2021; originally announced April 2021.

Comments: 6 pages, 2 figures. Comments are welcome and appreciated

Journal ref: Phys. Rev. B 104, 041101 (2021)

Charge-stripe Fluctuations in Nd$_{4}$Ni$_{3}$O$_{8}$ as Evidenced by Optical Spectroscopy

Authors: Jiahao Hao, Xinwei Fan, Qing Li, Xiaoxiang Zhou, Chengping He, Yaomin Dai, Bing Xu, Xiyu Zhu, Hai-Hu Wen

Abstract: We present an investigation into the optical properties of Nd$_{4}$Ni$_{3}$O$_{8}$ at different temperatures from 300 down to 5~K over a broad frequency range. The optical conductivity at 5~K is decomposed into IR-active phonons, a far-infrared band $α$, a mid-infrared band $β$, and a high-energy absorption edge. By comparing the measured optical conductivity to first-principles calculations and t… ▽ More We present an investigation into the optical properties of Nd$_{4}$Ni$_{3}$O$_{8}$ at different temperatures from 300 down to 5~K over a broad frequency range. The optical conductivity at 5~K is decomposed into IR-active phonons, a far-infrared band $α$, a mid-infrared band $β$, and a high-energy absorption edge. By comparing the measured optical conductivity to first-principles calculations and the optical response of other nickelates, we find that Nd$_{4}$Ni$_{3}$O$_{8}$ features evident charge-stripe fluctuations. The $β$ band is attributed to electronic transitions between the gapped Ni-$d_{x^2-y^2}$ bands due to fluctuating charge stripes, while the high-frequency absorption edge corresponds to the onset of transitions involving other high-energy bands. Furthermore, an analysis of the temperature-dependent optical spectral weight reveals a $T^{2}$ law, which is likely to originate from strong correlation effects. △ Less

Submitted 14 May, 2021; v1 submitted 8 January, 2021; originally announced January 2021.

Comments: 8 pages, 6 figures

Journal ref: Phys. Rev. B 103, 205120 (2021)

Helium Incorporation Stabilized Direct-gap Silicides

Authors: Shicong Ding, Jingming Shi, Jiahao Xie, Wenwen Cui, Pan Zhang, Kang Yang, Jian Hao, Meiling Xu, Qingxin Zeng, Lijun Zhang, Yinwei Li

Abstract: The search of direct-gap Si-based semiconductors is of great interest due to the potential application in many technologically relevant fields. This work examines the incorporation of He as a possible route to form a direct band gap in Si. Structure predictions and first-principles calculations have shown that He reacts with Si at high pressure, to form the stable compounds Si2He and Si3He. Both c… ▽ More The search of direct-gap Si-based semiconductors is of great interest due to the potential application in many technologically relevant fields. This work examines the incorporation of He as a possible route to form a direct band gap in Si. Structure predictions and first-principles calculations have shown that He reacts with Si at high pressure, to form the stable compounds Si2He and Si3He. Both compounds have host-guest structures consisting of a channel-like Si host framework filled with He guest atoms. The Si frameworks in two compounds could be persisted to ambient pressure after removal of He, forming two pure Si allotropes. Both Si-He compounds and both Si allotropes exhibit direct or quasi-direct band gaps of 0.84-1.34 eV, close to the optimal value (~1.3 eV) for solar cell applications. Analysis shows that Si2He with an electric-dipole-transition allowed band gap possesses higher absorption capacity than diamond cubic Si, which makes it to be a promising candidate material for thin-film solar cell. △ Less

Submitted 11 June, 2020; originally announced June 2020.

Cooper instability and superconductivity on the Penrose lattice

Authors: Yu-Bo Liu, Juan-Juan Hao, Yongyou Zhang, Ye Cao, Wei-Qiang Chen, Fan Yang

Abstract: Bulk superconductivity (SC) has recently been observed in the Al-Zn-Mg quasicrystal (QC). To settle several fundamental issues of the SC on the QC, we use an attractive Hubbard model to perform a systematic study on the Penrose lattice. The first issue is the Cooper instability of the QC, i.e., no Fermi surface under an infinitesimal attractive interaction. Starting from the two-electron problem o… ▽ More Bulk superconductivity (SC) has recently been observed in the Al-Zn-Mg quasicrystal (QC). To settle several fundamental issues of the SC on the QC, we use an attractive Hubbard model to perform a systematic study on the Penrose lattice. The first issue is the Cooper instability of the QC, i.e., no Fermi surface under an infinitesimal attractive interaction. Starting from the two-electron problem outside the filled Fermi-sea, we analytically prove that an infinitesimal Hubbard attraction can lead to the Cooper instability as long as the density of state is nonzero at the Fermi level, which provides the basis of the SC on the QC. Our numerical results yield that the Cooper pairing always takes place between the two time-reversal states, satisfying the Anderson's theorem. On this theorem, we perform a mean-field (MF) study at both zero and finite temperatures. The MF study also shows that an arbitrarily weak attraction can lead to the pairing order, with the resulted pairing state being well described by the BCS theory and the thermal dynamic behaviors being well consistent with experimental results. The second issue is about the superfluid density on the QC without translational symmetry. It's clarified that although the normal state of the system locates at the critical point of the metal-insulator transition, the pairing state exhibits real SC, carrying finite superfluid density that can be verified by the Meissner effect, consistent with experiment also. These revealed properties of the SC on the Penrose lattice are universal for all QCs. △ Less

Submitted 28 July, 2022; v1 submitted 15 February, 2020; originally announced February 2020.

Comments: 11 pages, 6 figures

Journal ref: Sci. China Phys. Mech. Astron. 65, 287411 (2022)

Device Chemistry of Graphene Transistors

Authors: B. C. Worley, S. Kim, T. J. Ha, S. Park, R. Haws, P. Rossky, D. Akinwande, A. Dodabalapur

Abstract: Graphene is an attractive material for microelectronics applications, given such favourable electrical characteristics as high mobility, high operating frequency, and good stability. If graphene is to be implemented in electronic devices on a mass scale, then it must be compatible with existing semiconductor industry fabrication processes. Unfortunately, such processing introduces defects and impu… ▽ More Graphene is an attractive material for microelectronics applications, given such favourable electrical characteristics as high mobility, high operating frequency, and good stability. If graphene is to be implemented in electronic devices on a mass scale, then it must be compatible with existing semiconductor industry fabrication processes. Unfortunately, such processing introduces defects and impurities to the graphene, which cause scattering of the charge carriers and changes in doping level. Scattering results in degradation of electrical performance, including lower mobility and Dirac point shifts. In this paper, we review methods by which to mitigate the effects of charged impurities and defects in graphene devices. Using capping layers such as fluoropolymers, statistically significant improvement of mobility, on/off ratio, and Dirac point voltage for graphene FETs have been demonstrated. These effects are also reversible and can be attributed to the presence of highly polar groups in these capping layers such as carbon-fluoride bonds in the fluoropolymer acting to electrostatically screen charged impurities and defects in or near the graphene. In other experiments, graphene FETs were exposed to vapour-phase, polar, organic molecules in an ambient environment. This resulted in significant improvement to electrical characteristics, and the magnitude of improvement to the Dirac point scaled with the dipole moment of the delivered molecule type. The potential profile produced in the plane of the graphene sheet by the impurities was calculated to be significantly reduced by the presence of polar molecules. We present strong evidence that the polar nature of capping layers or polar vapour molecules introduced to the surface of a graphene FET act to mitigate detrimental effects of charged impurities/defects. △ Less

Submitted 13 June, 2019; originally announced June 2019.

Singlet $s^\pm$-wave pairing in quasi-one-dimensional ACr$_3$As$_3$ (A=K, Rb, Cs) superconductors

Authors: Li-Da Zhang, Xiaoming Zhang, Juan-Juan Hao, Wen Huang, Fan Yang

Abstract: The recent discovery of quasi-one-dimensional Chromium-based superconductivity has generated much excitement. We study in this work the superconducting instabilities of a representative compound, the newly synthesized KCr$_3$As$_3$ superconductor. Based on inputs from density functional theory calculations, we first construct an effective multi-orbital tight-binding Hamiltonian to model its low-en… ▽ More The recent discovery of quasi-one-dimensional Chromium-based superconductivity has generated much excitement. We study in this work the superconducting instabilities of a representative compound, the newly synthesized KCr$_3$As$_3$ superconductor. Based on inputs from density functional theory calculations, we first construct an effective multi-orbital tight-binding Hamiltonian to model its low-energy band structure. We then employ standard random-phase approximation calculations to investigate the superconducting instabilities of the resultant multi-orbital Hubbard model. We find various pairing symmetries in the phase diagram in different interaction parameter regimes, including the triplet $f$-wave, $p_z$-wave and singlet $s^\pm$-wave pairings. We argue that the singlet $s^\pm$-wave pairing, which emerges at intermediate interaction strength, is realized in this material. This singlet pairing is driven by spin-density wave fluctuations enhanced by Fermi-surface nesting. We point out that phase-sensitive measurement can distinguish the $s$-wave pairing in KCr$_3$As$_3$ from the $p_z$-wave previously proposed for a related compound K$_2$Cr$_3$As$_3$. The $s^{\pm}$-wave pairing in KCr$_3$As$_3$ shall also exhibit a subgap spin resonance mode near the nesting vector, which can be tested by inelastic neutron scattering measurements. Another intriguing property of the $s^\pm$-pairing is that it can induce time-reversal invariant topological superconductivity in a semiconductor wire with large Rashba spin-orbit coupling via proximity effect. Our study shall be of general relevance to all superconductors in the family of ACr$_3$As$_3$ (A=K, Rb, Cs). △ Less

Submitted 12 March, 2019; v1 submitted 19 September, 2018; originally announced September 2018.

Journal ref: Phys. Rev. B 99, 094511 (2019)

Strain Engineering a $4a\times\sqrt{3}a$ Charge Density Wave Phase in Transition Metal Dichalcogenide 1T-VSe$_2$

Authors: Duming Zhang, Jeonghoon Ha, Hongwoo Baek, Yang-Hao Chan, Fabian D. Natterer, Alline F. Myers, Joshua D. Schumacher, William G. Cullen, Albert V. Davydov, Young Kuk, M. Y. Chou, Nikolai B. Zhitenev, Joseph A. Stroscio

Abstract: We report a rectangular charge density wave (CDW) phase in strained 1T-VSe$_2$ thin films synthesized by molecular beam epitaxy on c-sapphire substrates. The observed CDW structure exhibits an unconventional rectangular 4a{\times}{\sqrt{3a}} periodicity, as opposed to the previously reported hexagonal $4a\times4a$ structure in bulk crystals and exfoliated thin layered samples. Tunneling spectrosco… ▽ More We report a rectangular charge density wave (CDW) phase in strained 1T-VSe$_2$ thin films synthesized by molecular beam epitaxy on c-sapphire substrates. The observed CDW structure exhibits an unconventional rectangular 4a{\times}{\sqrt{3a}} periodicity, as opposed to the previously reported hexagonal $4a\times4a$ structure in bulk crystals and exfoliated thin layered samples. Tunneling spectroscopy shows a strong modulation of the local density of states of the same $4a\times\sqrt{3}a$ CDW periodicity and an energy gap of $2Δ_{CDW}=(9.1\pm0.1)$ meV. The CDW energy gap evolves into a full gap at temperatures below 500 mK, indicating a transition to an insulating phase at ultra-low temperatures. First-principles calculations confirm the stability of both $4a\times4a$ and $4a\times\sqrt{3}a$ structures arising from soft modes in the phonon dispersion. The unconventional structure becomes preferred in the presence of strain, in agreement with experimental findings. △ Less

Submitted 20 July, 2017; originally announced July 2017.

Journal ref: Physical Review Materials, 1, 024005 (2017)

Scanning Tunneling Spectroscopy of Proximity Superconductivity in Epitaxial Multilayer Graphene

Authors: Fabian D. Natterer, Jeonghoon Ha, Hongwoo Baek, Duming Zhang, William G. Cullen, Nikolai B. Zhitenev, Young Kuk, Joseph A. Stroscio

Abstract: We report on spatial measurements of the superconducting proximity effect in epitaxial graphene induced by a graphene-superconductor interface. Superconducting aluminum films were grown on epitaxial multilayer graphene on SiC. The aluminum films were discontinuous with networks of trenches in the film morphology reaching down to exposed graphene terraces. Scanning tunneling spectra measured on the… ▽ More We report on spatial measurements of the superconducting proximity effect in epitaxial graphene induced by a graphene-superconductor interface. Superconducting aluminum films were grown on epitaxial multilayer graphene on SiC. The aluminum films were discontinuous with networks of trenches in the film morphology reaching down to exposed graphene terraces. Scanning tunneling spectra measured on the graphene terraces show a clear decay of the superconducting energy gap with increasing separation from the graphene-aluminum edges. The spectra were well described by Bardeen-Cooper-Schrieffer (BCS) theory. The decay length for the superconducting energy gap in graphene was determined to be greater than 400 nm. Deviations in the exponentially decaying energy gap were also observed on a much smaller length scale of tens of nanometers. △ Less

Submitted 22 February, 2016; originally announced February 2016.

Journal ref: Physical Review B 93, 045406 (2016)

Heterodimer nanostructures induced energy focusing on metal film

Authors: Ting Liu, Jingjing Hao, Yingzhou Huang, Xun Su, Li Hu, Yurui Fang

Abstract: As an interesting surface plasmon phenomenon discovered several years ago, electromagnetic field redistribution in nanoparticle dimer on film system provides a novel thought to enhance the light power on a plain film which could been widely used in surface enhanced Raman scattering (SERS), solar cells, photo-catalysis, etc. Homodimers on film are mainly investigated in past years, while the proper… ▽ More As an interesting surface plasmon phenomenon discovered several years ago, electromagnetic field redistribution in nanoparticle dimer on film system provides a novel thought to enhance the light power on a plain film which could been widely used in surface enhanced Raman scattering (SERS), solar cells, photo-catalysis, etc. Homodimers on film are mainly investigated in past years, while the properties of heterodimers on film are still unclear. In this work, size difference induced electromagnetic field redistribution in Ag nanoparticle dimer on Au film system is investigated first. The results obtained from finite element method indicate that the smaller nanoparticle has much greater ability to focus light energy on Au film, which even reached more than 5 time compared to the larger one. Further researches indicate that this energy focusing ability has a strong relationship to the wavelength and diameter ration in dimer. Similar focusing phenomenon is found in the system of thick wire-smaller particle on film. Later, the SERS spectra collected in the small nanoparticle-large nanowire system provide an experimental evidence for this theoretic predication. Our results strengthen the understanding of surface plasmon on plane film and have potential application prospects in the surface plasmon related fields. △ Less

Submitted 22 November, 2015; originally announced November 2015.

Comments: 11 pages, 5 figures, including supporting information

Creating Nanostructured Superconductors On Demand by Local Current Annealing

Authors: Hongwoo Baek, Jeonghoon Ha, Duming Zhang, Bharath Natarajan, Jonathan P. Winterstein, Renu Sharma, Rongwei Hu, Kefeng Wang, Steven Ziemak, Johnpierre Paglione, Young Kuk, Nikolai B. Zhitenev, Joseph A. Stroscio

Abstract: Superconductivity results from a Bose condensate of Cooper-paired electrons with a macroscopic quantum wavefunction. Dramatic effects can occur when the region of the condensate is shaped and confined to the nanometer scale. Recent progress in nanostructured superconductors has revealed a route to topological superconductivity, with possible applications in quantum computing. However, challenges r… ▽ More Superconductivity results from a Bose condensate of Cooper-paired electrons with a macroscopic quantum wavefunction. Dramatic effects can occur when the region of the condensate is shaped and confined to the nanometer scale. Recent progress in nanostructured superconductors has revealed a route to topological superconductivity, with possible applications in quantum computing. However, challenges remain in controlling the shape and size of specific superconducting materials. Here, we report a new method to create nanostructured superconductors by partial crystallization of the half-Heusler material, YPtBi. Superconducting islands, with diameters in the range of 100 nm, were reproducibly created by local current annealing of disordered YPtBi in the tunneling junction of a scanning tunneling microscope (STM). We characterize the superconducting island properties by scanning tunneling spectroscopic measurements to determine the gap energy, critical temperature and field, coherence length, and vortex formations. These results show unique properties of a confined superconductor and demonstrate that this new method holds promise to create tailored superconductors for a wide variety of nanometer scale applications. △ Less

Submitted 1 October, 2015; originally announced October 2015.

Comments: 12 figures

Journal ref: Physical Review B 92, 094510 (2015)

Dissociation products and structures of solid H2S at strong compression

Authors: Yinwei Li, Lin Wang, Hanyu Liu, Yunwei Zhang, Jian Hao, Chris J. Pickard, Joseph R. Nelson, Richard J. Needs, Wentao Li, Yanwei Huang, Ion Errea, Matteo Calandra, Francesco Mauri, Yanming Ma

Abstract: Hydrogen sulfides have recently received a great deal of interest due to the record high superconducting temperatures of up to 203 K observed on strong compression of dihydrogen sulfide (H2S). A joint theoretical and experimental study is presented in which decomposition products and structures of compressed H2S are characterized, and their superconducting properties are calculated. In addition to… ▽ More Hydrogen sulfides have recently received a great deal of interest due to the record high superconducting temperatures of up to 203 K observed on strong compression of dihydrogen sulfide (H2S). A joint theoretical and experimental study is presented in which decomposition products and structures of compressed H2S are characterized, and their superconducting properties are calculated. In addition to the experimentally known H2S and H3S phases, our first-principles structure searches have identified several energetically competitive stoichiometries that have not been reported previously; H2S3, H3S2, and H4S3. In particular, H4S3 is predicted to be thermodynamically stable within a large pressure range of 25-113 GPa. High-pressure room-temperature X-ray diffraction measurements confirm the presence of H3S and H4S3 through decomposition of H2S that emerge at 27 GPa and coexist with residual H2S, at least up to the highest pressure studied in our experiments of 140 GPa. Electron-phonon coupling calculations show that H4S3 has a small Tc of below 2 K, and that H2S is mainly responsible for the observed superconductivity of samples prepared at low temperature (<100K). △ Less

Submitted 31 August, 2015; v1 submitted 16 August, 2015; originally announced August 2015.

Comments: 5 pages, 5 figures

Journal ref: Phys. Rev. B 93, 020103(R) (2016)

High-energy-density and superhard nitrogen-rich B-N compounds

Authors: Yinwei Li, Jian Hao, Hanyu Liu, Siyu Lu, John S. Tse

Abstract: The pressure-induced transformation of diatomic nitrogen into non-molecular polymeric phases may produce potentially useful high-energy-density materials. We combine first-principles calculations with structure searching to predict a new class of nitrogen-rich boron nitrides with a stoichiometry of B3N5 that are stable or metastable relative to solid N2 and h-BN at ambient pressure. The most stabl… ▽ More The pressure-induced transformation of diatomic nitrogen into non-molecular polymeric phases may produce potentially useful high-energy-density materials. We combine first-principles calculations with structure searching to predict a new class of nitrogen-rich boron nitrides with a stoichiometry of B3N5 that are stable or metastable relative to solid N2 and h-BN at ambient pressure. The most stable phase at ambient pressure has a layered structure (h-B3N5) containing hexagonal B3N3 layers sandwiched with intercalated freely rotating N2 molecules. At 15 GPa, a three-dimensional C2221 structure with single N-N bonds becomes the most stable. This pressure is much lower than that required for triple-to-single bond transformation in pure solid nitrogen (110 GPa). More importantly, C2221-B3N5 is metastable, and can be recovered under ambient conditions. Its energy density of 3.44 kJ/g makes it a potential high-energy-density material. In addition, stress-strain calculations estimate a Vickers hardness of 44 GPa. Structure searching reveals a new clathrate sodalite-like BN structure that is metastable under ambient conditions. △ Less

Submitted 14 August, 2015; originally announced August 2015.

Comments: 16 pages, 5 figures, accepted by PRL

Quasiparticle scattering from topological crystalline insulator SnTe (001) surface states

Authors: Duming Zhang, Hongwoo Baek, Jeonghoon Ha, Tong Zhang, Jonathan E. Wyrick, Albert V. Davydov, Young Kuk, Joseph A. Stroscio

Abstract: Recently, the topological classification of electronic states has been extended to a new class of matter known as topological crystalline insulators. Similar to topological insulators, topological crystalline insulators also have spin-momentum locked surface states; but they only exist on specific crystal planes that are protected by crystal reflection symmetry. Here, we report an ultra-low temper… ▽ More Recently, the topological classification of electronic states has been extended to a new class of matter known as topological crystalline insulators. Similar to topological insulators, topological crystalline insulators also have spin-momentum locked surface states; but they only exist on specific crystal planes that are protected by crystal reflection symmetry. Here, we report an ultra-low temperature scanning tunneling microscopy and spectroscopy study on topological crystalline insulator SnTe nanoplates grown by molecular beam epitaxy. We observed quasiparticle interference patterns on the SnTe (001) surface that can be interpreted in terms of electron scattering from the four Fermi pockets of the topological crystalline insulator surface states in the first surface Brillouin zone. A quantitative analysis of the energy dispersion of the quasiparticle interference intensity shows two high energy features related to the crossing point beyond the Lifshitz transition when the two neighboring low energy surface bands near the point merge. A comparison between the experimental and computed quasiparticle interference patterns reveals possible spin texture of the surface states. △ Less

Submitted 5 May, 2014; originally announced May 2014.

Journal ref: Physical Review B 89, 245445 (2014)

The metallization and superconductivity of dense hydrogen sulfide

Authors: Yinwei Li, Jian Hao, Yanling Li, Yanming Ma

Abstract: Hydrogen sulfide (H2S) is a prototype molecular system and a sister molecule of water. The phase diagram of solid H2S at high pressures remains largely unexplored arising from the challenges in dealing with the looser S-H bond and larger atomic core difference between H and S. Metallization is yet achieved for water ice, but it was established for H2S above 96 GPa. However, the metallic structure… ▽ More Hydrogen sulfide (H2S) is a prototype molecular system and a sister molecule of water. The phase diagram of solid H2S at high pressures remains largely unexplored arising from the challenges in dealing with the looser S-H bond and larger atomic core difference between H and S. Metallization is yet achieved for water ice, but it was established for H2S above 96 GPa. However, the metallic structure of H2S remains elusive, greatly impeding the understanding of its metallicity and the potential superconductivity. We have performed an extensive structural study on solid H2S under high pressures through unbiased first-principles structure predictions based on swarm intelligence. Besides the findings of best-known candidate structures for nonmetallic phases IV and V, we are able to establish stable metallic structures violating an earlier proposal of elemental decomposition into sulfur and hydrogen [PRL 85, 1254 (2000)]. Our study unraveled a superconductive potential of metallic H2S with an estimated maximal transition temperature of ~ 80 K at 160 GPa, higher than those predicted for most archetypal hydrogen-containing compounds (e.g., SiH4 and GeH4, etc). △ Less

Submitted 11 February, 2014; originally announced February 2014.

Comments: 15 pages, 4 figures

Journal ref: J. Chem. Phys. 140, 174712 (2014)