Showing 1–49 of 49 results for author: Fang, F

Inertial synchronization of networked oscillators in arbitrary dimensions

Authors: Kirill Kovalenko, Bruce X. Dai, Fanshu Fang, Zerong Guo, Haoran Liu, Federico Botta, Charo I. del Genio, Stefano Boccaletti, Simona Olmi

Abstract: The Kuramoto model provides a paradigmatic framework for studying synchronization of interacting oscillators, and has been generalized to arbitrary dimensions to describe swarms, flocks and multi-dimensional opinion dynamics. Yet, existing formulations neglect inertia, a key mechanism known to enhance information propagation and collective responsiveness. Here, we introduce and analyze an inertial… ▽ More The Kuramoto model provides a paradigmatic framework for studying synchronization of interacting oscillators, and has been generalized to arbitrary dimensions to describe swarms, flocks and multi-dimensional opinion dynamics. Yet, existing formulations neglect inertia, a key mechanism known to enhance information propagation and collective responsiveness. Here, we introduce and analyze an inertial Kuramoto model in arbitrary dimensions. We show that inertia fundamentally alters the nature of the synchronization transition, inducing a crossover from continuous to discontinuous behavior, with the onset of hysteresis depending explicitly on both inertia and dimensionality parity. Our analytical theory is supported by extensive numerical simulations. These results establish inertia as a crucial ingredient of high-dimensional collective dynamics and reveal a novel universal structure in synchronization phenomena. △ Less

Submitted 24 August, 2026; originally announced August 2026.

Comments: 7 pages, 4 figures, accepted in Phys. Rev. E

Microscopic evidence for Fulde-Ferrel-Larkin-Ovchinnikov state and multiband effects in KFe$_2$As$_2$

Authors: X. Y. Liu, Z. Kao, J. Luo, J. Yang, A. F. Fang, J. Zhao, R. Zhou, Guo-qing Zheng

Abstract: The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is a superconducting phase characterized by broken translational-symmetry, where Cooper pairs form with non-zero momentum between Zeeman-split Fermi surfaces. This state is highly sensitive to band structure and pairing symmetry. In multiband superconductors, the FFLO state can significantly deviate from its standard form, but experimental verifica… ▽ More The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is a superconducting phase characterized by broken translational-symmetry, where Cooper pairs form with non-zero momentum between Zeeman-split Fermi surfaces. This state is highly sensitive to band structure and pairing symmetry. In multiband superconductors, the FFLO state can significantly deviate from its standard form, but experimental verification has remained challenging. Here, we present $^{75}$As nuclear magnetic resonance (NMR) measurements on the multiband superconductor KFe$_2$As$_2$. In the low-temperature, high-magnetic-field region above the upper critical field $B_{c2}$, we observe a clear increase in the second moment of the NMR spectrum, along with a strong enhancement in the spin-lattice relaxation rate divided by temperature 1/$T_1$$T$. These results indicate an emergence of superconducting spin smecticity and Andreev bound states from the spatially modulation of the superconducting gap, providing microscopic evidence for the FFLO state. The obtained phase diagram reveals a distinct boundary line between the FFLO and homogenous superconducting (HSC) states with a low critical temperature of the FFLO state $T^\ast \approx 0.2 T_c$, which can be attributed to the multiband effects in KFe$_2$As$_2$. Our results show that the iron-based superconductors are a good material platform for studying the FFLO state and highlight the importance of the multiband effects on this exotic phase. △ Less

Submitted 1 July, 2025; originally announced July 2025.

Comments: 16 pages, 4 figures

Journal ref: Phys. Rev. B 112, L020505 (2025)

Fully-gapped superconductivity with rotational symmetry breaking in pressurized kagome metal CsV$_3$Sb$_5$

Authors: X. Y. Feng, Z. Zhao, J. Luo, Y. Z. Zhou, J. Yang, A. F. Fang, H. T. Yang, H. -J. Gao, R. Zhou, Guo-qing Zheng

Abstract: The discovery of the kagome metal CsV$_3$Sb$_5$ has generated significant interest in its complex physical properties, particularly its superconducting behavior under different pressures, though its nature remains debated. Here, we performed low-temperature, high-pressure $^{121/123}$Sb nuclear quadrupole resonance (NQR) measurements to explore the superconducting pairing symmetry in CsV$_3$Sb… ▽ More The discovery of the kagome metal CsV$_3$Sb$_5$ has generated significant interest in its complex physical properties, particularly its superconducting behavior under different pressures, though its nature remains debated. Here, we performed low-temperature, high-pressure $^{121/123}$Sb nuclear quadrupole resonance (NQR) measurements to explore the superconducting pairing symmetry in CsV$_3$Sb$_5$. At ambient pressure, we found that the spin-lattice relaxation rate 1/$T_1$ exhibits a kink at $T \sim$ 0.4 $T_\textrm{c}$ within the superconducting state and follows a $T^3$ variation as temperature further decreases. This suggests the presence of two superconducting gaps with line nodes in the smaller one. As pressure increases beyond $P_{\rm c} \sim 1.85$ GPa, where the charge-density wave phase is completely suppressed, 1/$T_1$ shows no Hebel-Slichter peak just below $T_\textrm{c}$, and decreases rapidly, even faster than $T^5$, indicating that the gap is fully opened for pressures above $P_{\rm c}$. In this high pressure region, the angular dependence of the in-plane upper critical magnetic field $H_{\rm c2}$ breaks the $C_6$ rotational symmetry. We propose the $s+id$ pairing at $P > P_{\rm c}$ which explains both the 1/$T_1$ and $H_{\rm c2}$ behaviors. Our findings indicate that CsV$_3$Sb$_5$ is an unconventional superconductor and its superconducting state is even more exotic at high pressures. △ Less

Submitted 2 April, 2025; originally announced April 2025.

Comments: 25 pages, 4 figures, to appear in Nature Communications

Journal ref: Nat. Commun. 16, 3643 (2025)

Antiferromagnetic Spin Fluctuations and Structural Transition in Cluster Mott Insulator Candidate Nb3Cl8 Revealed by 93Nb- and 35Cl-NMR

Authors: Y. Z. Zhou, X. Han, J. Luo, D. T. Wu, A. F. Fang, B. Shen, B. J. Feng, Y. G. Shi, J. Yang, R. Zhou

Abstract: Motivated by recent studies of the cluster Mott insulator candidate compound Nb3Cl8, this study performs 93Nb and 35Cl nuclear magnetic resonance (NMR) measurements to investigate the electron correlations. Below the structural transition temperature Ts ~ 97 K, all satellites of the 93Nb NMR spectra split into three distinct peaks, which suggests symmetry lowering due to the structural transition… ▽ More Motivated by recent studies of the cluster Mott insulator candidate compound Nb3Cl8, this study performs 93Nb and 35Cl nuclear magnetic resonance (NMR) measurements to investigate the electron correlations. Below the structural transition temperature Ts ~ 97 K, all satellites of the 93Nb NMR spectra split into three distinct peaks, which suggests symmetry lowering due to the structural transition and could be attributed to the change in the Nb-Nb bond-lengths of the Nb3 clusters. The spin-lattice relaxation rate 1/T1 divided by the temperature T, 1/T1T, increases upon cooling to Ts for all Cl sites, whereas only the Knight shift K of Cl located at the center of the Nb3 clusters exhibits a temperature dependence similar to that observed in magnetic susceptibility. These findings collectively demonstrate the existence of strong spin correlations between the Nb atoms in Nb3Cl8, which are closely associated with Mottness. △ Less

Submitted 17 March, 2025; originally announced March 2025.

Comments: 13 pages, 4 figures, 1 table. Supplementary materials: 9 pages, 7 figures

Journal ref: Chinese Physics Letters 42, 037303 (2025)

Nanoindentation responses of Fe-Cr alloys from room temperature to 600 °C

Authors: L. Kurpaska, M. Clozel, J. H. OConnell, I. Jozwik, E. Wyszkowska, W. Y. Huo, W. Chrominski, D. Kalita, S. T. Nori, F. Fang, J. Jagielski, J. H. Neethling

Abstract: In this work, the evolution of nanomechanical properties was studied systematically as a function of temperature, chemical, and microstructural complexity of different Fe-based alloys. Experiments were performed at different temperatures (room temperature, 200 C, 400 C, 600 C) using the nanoindentation technique on low activation Fe9Cr-1WVTa (Eurofer97), model Fe-9Cr-NiSiP, Fe-9Cr alloys, and pure… ▽ More In this work, the evolution of nanomechanical properties was studied systematically as a function of temperature, chemical, and microstructural complexity of different Fe-based alloys. Experiments were performed at different temperatures (room temperature, 200 C, 400 C, 600 C) using the nanoindentation technique on low activation Fe9Cr-1WVTa (Eurofer97), model Fe-9Cr-NiSiP, Fe-9Cr alloys, and pure iron samples, followed by microstructural observations. The results show varying softening and hardening effects depending on the experimental temperature, demonstrating Portevin-Le-Chatelier effect, i.e., dynamic strain aging phenomenon in model alloys. Sources of the dynamic strain aging instabilities were traced back to the interaction between dislocations and alloying elements such as interstitial carbon and substitutional chromium. The materials undergo dynamic recovery and recrystallization below the regions of high-temperature indentation depending on the pre-indentation dislocation density and the alloy composition. Our findings help in the understanding of the structure and mechanical property relationship in complex Eurofer97 alloy at high-temperatures for potential nuclear applications as structural materials. △ Less

Submitted 21 February, 2025; originally announced February 2025.

Microscopic evidence of charge- and spin-density waves in La$_3$Ni$_2$O$_{7-δ}$ revealed by $^{139}$La-NQR

Authors: J. Luo, J. Feng, G. Wang, N. N. Wang, J. Dou, A. F. Fang, J. Yang, J. G. Cheng, Guo-qing Zheng, R. Zhou

Abstract: The recent discovery of superconductivity in La$_3$Ni$_2$O$_{7-δ}$ with a transition temperature $T_c$ close to 80 K at high pressures has attracted significant attention, due particularly to a possible density wave (DW) transition occurring near the superconducting dome. Identifying the type of DW order is crucial for understanding the origin of superconductivity in this system. However, owing to… ▽ More The recent discovery of superconductivity in La$_3$Ni$_2$O$_{7-δ}$ with a transition temperature $T_c$ close to 80 K at high pressures has attracted significant attention, due particularly to a possible density wave (DW) transition occurring near the superconducting dome. Identifying the type of DW order is crucial for understanding the origin of superconductivity in this system. However, owing to the presence of La$_4$Ni$_3$O$_{10}$ and other intergrowth phases in La$_3$Ni$_2$O$_{7-δ}$ samples, extracting the intrinsic information from the La$_3$Ni$_2$O$_7$ phase is challenging. In this study, we employed $^{139}$La nuclear quadrupole resonance (NQR) measurements to eliminate the influence of other structural phases in the sample and obtain microscopic insights into the DW transition in La$_3$Ni$_2$O$_{7-δ}$. Below the DW transition temperature $T_{\rm DW} \sim$ 153K, we observe a distinct splitting in the $\pm$ 5/2 $\leftrightarrow$ $\pm$ 7/2 transition of the NQR resonance peak at the La(2) site, while only a line broadening is seen in the $\pm$ 3/2 $\leftrightarrow$ $\pm$ 5/2 transition peak. Through further analysis of the spectra, we show that the line splitting is due to a unidirectional charge modulation. A magnetic line broadening is also observed below $T_{\rm DW}$, accompanied by a large enhancement of the spin-lattice relaxation rate, indicating the formation of magnetically ordered moments in the DW state. Our results suggest a simultaneous formation of charge- and spin-density wave order in La$_3$Ni$_2$O$_{7-δ}$ , thereby offering critical insights into the electronic correlations in Ni-based superconductors. △ Less

Submitted 17 June, 2025; v1 submitted 19 January, 2025; originally announced January 2025.

Comments: 16 pages, 5 figures

Journal ref: Chinese Physics Letters 42, 067402 (2025)

Emergent disorder and sub-ballistic dynamics in quantum simulations of the Ising model using Rydberg atom arrays

Authors: Ceren B. Dag, Hanzhen Ma, P. Myles Eugenio, Fang Fang, Susanne F. Yelin

Abstract: Rydberg atom arrays with Van der Waals interactions provide a controllable path to simulate the locally connected transverse-field Ising model (TFIM), a prototypical model in statistical mechanics. Remotely operating the publicly accessible Aquila Rydberg atom array, we experimentally investigate the physics of TFIM far from equilibrium and uncover significant deviations from the theoretical predi… ▽ More Rydberg atom arrays with Van der Waals interactions provide a controllable path to simulate the locally connected transverse-field Ising model (TFIM), a prototypical model in statistical mechanics. Remotely operating the publicly accessible Aquila Rydberg atom array, we experimentally investigate the physics of TFIM far from equilibrium and uncover significant deviations from the theoretical predictions. Rather than the expected ballistic spread of correlations, the Rydberg simulator exhibits a subballistic spread, along with a logarithmic scaling of entanglement entropy in time - all while the system mostly retains its initial magnetization. By modeling the atom motion, we trace these effects to an emergent disorder in Rydberg atom arrays, which we characterize with a minimal random spin model. We further experimentally explore the different dynamical regimes hosted in the system by varying the lattice spacing and the Rabi frequency. Our findings highlight the crucial role of atom motion in the many-body dynamics of Rydberg atom arrays at the TFIM limit, and propose simple benchmark measurements to test for its presence in future experiments. △ Less

Submitted 3 September, 2025; v1 submitted 20 November, 2024; originally announced November 2024.

Comments: Revised with the addition of decoherence calculations, one End Matter figure and additional SM figures

Journal ref: Phys. Rev. Lett. 135, 250403 (2025)

Probing critical phenomena in open quantum systems using atom arrays

Authors: Fang Fang, Kenneth Wang, Vincent S. Liu, Yu Wang, Ryan Cimmino, Julia Wei, Marcus Bintz, Avery Parr, Jack Kemp, Kang-Kuen Ni, Norman Y. Yao

Abstract: At continuous phase transitions, quantum many-body systems exhibit scale-invariance and complex, emergent universal behavior. Most strikingly, at a quantum critical point, correlations decay as a power law, with exponents determined by a set of universal scaling dimensions. Experimentally probing such power-law correlations is extremely challenging, owing to the complex interplay between decoheren… ▽ More At continuous phase transitions, quantum many-body systems exhibit scale-invariance and complex, emergent universal behavior. Most strikingly, at a quantum critical point, correlations decay as a power law, with exponents determined by a set of universal scaling dimensions. Experimentally probing such power-law correlations is extremely challenging, owing to the complex interplay between decoherence, the vanishing energy gap, and boundary effects. Here, we employ a Rydberg quantum simulator to adiabatically prepare critical ground states of both a one-dimensional ring and a two-dimensional square lattice. By accounting for and tuning the openness of our quantum system, which is well-captured by the introduction of a single phenomenological length scale, we are able to directly observe power-law correlations and extract the corresponding scaling dimensions. Moreover, in two dimensions, we observe a decoupling between phase transitions in the bulk and on the boundary, allowing us to identify two distinct boundary universality classes. Our work demonstrates that direct adiabatic preparation of critical states in quantum simulators can complement recent approaches to studying quantum criticality using the Kibble-Zurek mechanism or digital quantum circuits. △ Less

Submitted 15 November, 2025; v1 submitted 23 February, 2024; originally announced February 2024.

Microstructure and mechanical properties of mechanically-alloyed CoCrFeNi high-entropy alloys using low ball-to-powder ratio

Authors: A. Olejarz, W. Y. Huo, M. Zielinski, R. Diduszko, E. Wyszkowska, A. Kosinska, D. Kalita, I. Jozwik, M. Chmielewski, F. Fang, L. Kurpaska

Abstract: High-entropy alloys are extensively studied due to their very promising properties. However manufacturing methods currently used to prepare HEAs are complicated, costly, and likely non-industrially scalable processes. This limits their evolution and poses questions regarding the material's applicability in the future. Considering the abovementioned point, we developed a novel methodology for effic… ▽ More High-entropy alloys are extensively studied due to their very promising properties. However manufacturing methods currently used to prepare HEAs are complicated, costly, and likely non-industrially scalable processes. This limits their evolution and poses questions regarding the material's applicability in the future. Considering the abovementioned point, we developed a novel methodology for efficient HEA production using a low ball-to-powder ratio (BPR). Using different milling times, we manufactured four HEA powder precursors using a BPR of 5:1, which were later sintered via the Spark Plasma Sintering technique and heat treated. Microstructural characterization was performed by optical microscopy, Scanning Electron Microscopy equipped with EDS and EBSD detectors, and X-ray diffraction. Mechanical properties were measured using nano and microhardness techniques. In this work, we follow the structural evolution of the material and connect it with the strengthening effect as a function of milling time. Furthermore, we discuss the impact of different sintering and annealing conditions, proving that HEAs characterized by high mechanical properties may be manufactured using low BPR. △ Less

Submitted 6 April, 2023; originally announced April 2023.

Journal ref: Volume 938, 25 March 2023, 168196

Commensurate-to-incommensurate transition of charge-density-wave order and a possible quantum critical point in pressurized kagome metal CsV$_3$Sb$_5$

Authors: X. Y. Feng, Z. Zhao, J. Luo, J. Yang, A. F. Fang, H. T. Yang, H. J. Gao, R. Zhou, Guo-qing Zheng

Abstract: Clarifying the interplay between charge density waves (CDWs) and superconductivity is important in the kagome metal CsV$_3$Sb$_5$, and pressure ($P$) can play a crucial role. Here, we present $^{121/123}$Sb nuclear quadrupole resonance (NQR) measurements under hydrostatic pressures up to 2.43 GPa in CsV$_3$Sb$_5$ single crystals. We demonstrate that the CDW gradually changes from a commensurate mo… ▽ More Clarifying the interplay between charge density waves (CDWs) and superconductivity is important in the kagome metal CsV$_3$Sb$_5$, and pressure ($P$) can play a crucial role. Here, we present $^{121/123}$Sb nuclear quadrupole resonance (NQR) measurements under hydrostatic pressures up to 2.43 GPa in CsV$_3$Sb$_5$ single crystals. We demonstrate that the CDW gradually changes from a commensurate modulation with a star-of-David (SoD) pattern to an incommensurate one with a superimposed SoD and Tri-hexagonal (TrH) pattern stacking along the $c$-axis. Moreover, the linewidth $δν$ of $^{121/123}$Sb-NQR spectra increases with cooling down to $T_{\rm CDW}$, indicating the appearance of a short-range CDW order due to CDW fluctuations pinned by quenched disorders. The $δν$ shows a Curie-Weiss temperature dependence and tends to diverge at $P_{\rm c} \sim$ 1.9 GPa, suggesting that a CDW quantum critical point (QCP) exists at $P_{\rm c}$ where $T_{\rm c}$ shows the maximum. For $P > P_{\rm c}$, spin fluctuations are enhanced when the CDW is suppressed. Our results suggest that the maximal $T_{\rm c}$ at $P_{\rm c} \sim$ 1.9 GPa is related to the CDW QCP and the presence of spin fluctuations prevent the $T_{\rm c}$ from a rapid decrease otherwise after the CDW is completely suppressed. △ Less

Submitted 2 March, 2023; originally announced March 2023.

Comments: 17 pages, 8 figures

Journal ref: npj Quantum Materials 8, 23 (2023)

Exploiting Anyonic Behavior of Quasicrystals for Topological Quantum Computing

Authors: Marcelo Amaral, David Chester, Fang Fang, Klee Irwin

Abstract: We show that quasicrystals exhibit anyonic behavior that can be used for topological quantum computing. In particular, we study a correspondence between the fusion Hilbert spaces of the simplest non-abelian anyon, the Fibonacci anyons, and the tiling spaces of a class of quasicrystals, which includes the one dimensional Fibonacci chain and the two dimensional Penrose tiling. A possible encoding on… ▽ More We show that quasicrystals exhibit anyonic behavior that can be used for topological quantum computing. In particular, we study a correspondence between the fusion Hilbert spaces of the simplest non-abelian anyon, the Fibonacci anyons, and the tiling spaces of a class of quasicrystals, which includes the one dimensional Fibonacci chain and the two dimensional Penrose tiling. A possible encoding on tiling spaces of topological quantum information processing is also discussed. △ Less

Submitted 1 July, 2022; originally announced July 2022.

Journal ref: Symmetry 2022, 14(9), 1780

Charge Carrier Mediation and Ferromagnetism induced in MnBi6Te10 Magnetic Topological Insulators by antimony doping

Authors: Hangkai Xie, Fucong Fei, Fenzhen Fang, Bo Chen, Jingwen Guo, Yu Du, Wuyi Qi, Yufan Pei, Tianqi Wang, Muhammad Naveed, Shuai Zhang, Minhao Zhang, Xuefeng Wang, Fengqi Song

Abstract: A new kind of intrinsic magnetic topological insulators (MTI) MnBi2Te4 family have shed light on the observation of novel topological quantum effect such as quantum anomalous Hall effect (QAHE). However, the strong anti-ferromagnetic (AFM) coupling and high carrier concentration in the bulk hinder the practical applications. In closely related materials MnBi4Te7 and MnBi6Te10, the interlayer magne… ▽ More A new kind of intrinsic magnetic topological insulators (MTI) MnBi2Te4 family have shed light on the observation of novel topological quantum effect such as quantum anomalous Hall effect (QAHE). However, the strong anti-ferromagnetic (AFM) coupling and high carrier concentration in the bulk hinder the practical applications. In closely related materials MnBi4Te7 and MnBi6Te10, the interlayer magnetic coupling is greatly suppressed by Bi2Te3 layer intercalation. However, AFM is still the ground state in these compounds. Here by magnetic and transport measurements, we demonstrate that Sb substitutional dopant plays a dual role in MnBi6Te10, which can not only adjust the charge carrier type and the concentration, but also induce the solid into a ferromagnetic (FM) ground state. AFM ground state region which is also close to the charge neutral point can be found in the phase diagram of Mn(SbxBi1-x)6Te10 when x ~ 0.25. An intrinsic FM-MTI candidate is thus demonstrated, and it may take a step further for the realization of high-quality and high-temperature QAHE and the related topological quantum effects in the future. △ Less

Submitted 22 January, 2022; v1 submitted 15 November, 2021; originally announced November 2021.

Journal ref: J.Phys.D:Appl.Phys.55,104002(2021)

Revealing a charge-density-wave gap in the predicted weak topological insulator HoSbTe

Authors: J. L. Liu, R. Liu, M. Yang, L. Y. Cao, B. X. Gao, L. Wang, A. F. Fang, Y. G. Shi, Z. P. Yin, R. Y. Chen

Abstract: HoSbTe was predicted to be a weak topological insulator, whose spin-orbit coupling (SOC) gaps are reported to be as large as hundreds of meV. Utilizing infrared spectroscopy, we find that the compound is of metallic nature from 350 K down to 10 K. Particularly, both of its itinerant carrier density and scattering rate are demonstrated to decrease with temperature cooling, which is responsible for… ▽ More HoSbTe was predicted to be a weak topological insulator, whose spin-orbit coupling (SOC) gaps are reported to be as large as hundreds of meV. Utilizing infrared spectroscopy, we find that the compound is of metallic nature from 350 K down to 10 K. Particularly, both of its itinerant carrier density and scattering rate are demonstrated to decrease with temperature cooling, which is responsible for the appearance of a broad hump feature in the temperature dependent resistivity around 200 K. More importantly, we reveal the appearance of a charge density wave (CDW) gap in addition to the SOC related gap. The energy scale of the CDW gap is identified to be 364 meV at 10 K, which shift to 252 meV at 350 K. The coexistence of CDW and SOC gaps in the same compound paves a new avenue to explore more intriguing physics. △ Less

Submitted 5 October, 2021; originally announced October 2021.

Journal ref: Phys. Rev. B 105, 075111 (2022)

Possible star-of-David pattern charge density wave with additional modulation in the kagome superconductor CsV$_3$Sb$_5$

Authors: J. Luo, Z. Zhao, Y. Z. Zhou, J. Yang, A. F. Fang, H. T. Yang, H. J. Gao, R. Zhou, Guo-qing Zheng

Abstract: $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) is a novel kagome superconductor coexisting with the charge density wave (CDW) order. Identifying the structure of the CDW order is crucial for understanding the exotic normal state and superconductivity in this system. Here, we report $^{51}$V nuclear magnetic resonance (NMR) and $^{121/123}$Sb nuclear quadrupole resonance (NQR) studies on kagome-metal CsV$_3$Sb$_… ▽ More $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) is a novel kagome superconductor coexisting with the charge density wave (CDW) order. Identifying the structure of the CDW order is crucial for understanding the exotic normal state and superconductivity in this system. Here, we report $^{51}$V nuclear magnetic resonance (NMR) and $^{121/123}$Sb nuclear quadrupole resonance (NQR) studies on kagome-metal CsV$_3$Sb$_5$. Below the CDW transition temperature $T_\textrm{CDW} \sim$ 98 K, an abrupt change of spectra was observed, indicating that the transition is of the first order. By further analysing the spectra, we find that the CDW order is commensurate. And most remarkably, the obtained experimental results suggest that the charge modulation of the CDW order is of star-of-David pattern and accompanied by an additional charge modulation in bulk below $T^* \sim$ 40 K. Our results revealing the unconventional CDW order provide new insights into $A$V$_3$Sb$_5$. △ Less

Submitted 18 March, 2022; v1 submitted 23 August, 2021; originally announced August 2021.

Comments: 15 pages, 4 figures

Journal ref: npj Quantum Materials 7, 30 (2022)

Magnetic ordering phase transition and abnormal brittleness in dilute Fe-Mn solid solution

Authors: Wei Liu, Yunfeng Liang, Xiangyan Li, Yichun Xu, Yange Zhang, Wenliang Li, Q. F. Fang, Caetano R. Miranda, Chuan-Lu Yang, C. S. Liu, Xuebang Wu

Abstract: Experiments showed that solute Mn in bcc iron is in antiferromagnetic (AFM) coupling with iron neighbours below 2 at.% Mn, but is in ferromagnetic (FM) coupling at higher concentrations. Surprisingly, although Mn is an important alloying element in high-strength steels, it induces brittleness just at around 2 at.% Mn and higher concentrations. However, the mechanisms for the magnetic ordering phas… ▽ More Experiments showed that solute Mn in bcc iron is in antiferromagnetic (AFM) coupling with iron neighbours below 2 at.% Mn, but is in ferromagnetic (FM) coupling at higher concentrations. Surprisingly, although Mn is an important alloying element in high-strength steels, it induces brittleness just at around 2 at.% Mn and higher concentrations. However, the mechanisms for the magnetic ordering phase transition and the abnormal brittleness remain unclear. Based on magnetism-constrained/unconstrained calculations and ab initio molecular dynamics simulations within density functional theory, we show that while the AFM phase prevails at low Mn contents, the FM phase becomes dominant at 1.85 at.% Mn and elevated temperatures. Our results suggest that the AFM-FM phase transition with increasing Mn concentration can be ascribed to the thermal effect. Furthermore, we find that the brittleness of the Fe-Mn alloys at intermediate Mn content might be related to the stress variations within the grains accompanying the local magnetic ordering changes. △ Less

Submitted 8 June, 2024; v1 submitted 25 May, 2021; originally announced May 2021.

Comments: 31 pages, 4 figures

Nodal superconducting gap in LiFeP revealed by NMR: contrast with LiFeAs

Authors: A. F. Fang, R. Zhou, H. Tukada, J. Yang, Z. Deng, X. C. Wang, C. Q. Jin, Guo-qing Zheng

Abstract: Identifying the uniqueness of FeP-based superconductors may shed new lights on the mechanism of superconductivity in iron-pnictides. Here, we report nuclear magnetic resonance(NMR) studies on LiFeP and LiFeAs which have the same crystal structure but different pnictogen atoms. The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state and can provide the information on the… ▽ More Identifying the uniqueness of FeP-based superconductors may shed new lights on the mechanism of superconductivity in iron-pnictides. Here, we report nuclear magnetic resonance(NMR) studies on LiFeP and LiFeAs which have the same crystal structure but different pnictogen atoms. The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state and can provide the information on the superconducting pairing symmetry through the temperature dependence of London penetration depth $λ_L$. We find that $λ_L$ saturates below $T \sim 0.2$ $T_c$ in LiFeAs, where $T_c$ is the superconducting transition temperature, indicating nodeless superconducting gaps. Furthermore, by using a two-gaps model, we simulate the temperature dependence of $λ_L$ and obtain the superconducting gaps of LiFeAs, as $Δ_1 = 1.2$ $k_B T_c$ and $Δ_2 = 2.8$ $k_B T_c$, in agreement with previous result from spin-lattice relaxation. For LiFeP, in contrast, the London penetration depth $λ_L$ does not show any saturation down to $T \sim 0.03 $ $T_c$, indicating nodes in the superconducting energy gap function. Finally, we demonstrate that the strong spin fluctuations with diffusive characteristics exist in LiFeP, as in some cuprate high temperature superconductors. △ Less

Submitted 4 March, 2021; originally announced March 2021.

Comments: 14 pages, 6 figures, to appear in Chinese Phys. B

Journal ref: Chinese Phys. B 30 047403 (2021)

Golden, Quasicrystalline, Chiral Packings of Tetrahedra

Authors: Fang Fang, Garrett Sadler, Julio Kovacs, Klee Irwin

Abstract: Since antiquity, the packing of convex shapes has been of great interest to many scientists and mathematicians. Recently, particular interest has been given to packings of three-dimensional tetrahedra. Dense packings of both crystalline and semi-quasicrystalline have been reported. It is interesting that a semiquasicrystalline packing of tetrahedra can emerge naturally within a thermodynamic simul… ▽ More Since antiquity, the packing of convex shapes has been of great interest to many scientists and mathematicians. Recently, particular interest has been given to packings of three-dimensional tetrahedra. Dense packings of both crystalline and semi-quasicrystalline have been reported. It is interesting that a semiquasicrystalline packing of tetrahedra can emerge naturally within a thermodynamic simulation approach. However, this packing is not perfectly quasicrystalline and the packing density, while dense, is not maximal. Here we suggest that a "golden rotation" between tetrahedral facial junctions can arrange tetrahedra into a perfect quasicrystalline packing. △ Less

Submitted 7 January, 2019; originally announced January 2019.

Unveiling the superconducting mechanism of Ba$_{0.51}$K$_{0.49}$BiO$_3$

Authors: C. H. P. Wen, H. C. Xu, Q. Yao, R. Peng, X. H. Niu, Q. Y. Chen, Z. T. Liu, D. W. Shen, Q. Song, X. Lou, Y. F. Fang, X. S. Liu, Y. H. Song, Y. J. Jiao, T. F. Duan, H. H. Wen, P. Dudin, G. Kotliar, Z. P. Yin, D. L. Feng

Abstract: Bismuthates were the first family of oxide high-temperature superconductors, exhibiting superconducting transition temperatures (Tc) up to 32K, but the superconducting mechanism remains under debate despite more than 30 years of extensive research. Our angle-resolved photoemission spectroscopy studies on Ba$_{0.51}$K$_{0.49}$BiO$_3$ reveal an unexpectedly 34% larger bandwidth than in conventional… ▽ More Bismuthates were the first family of oxide high-temperature superconductors, exhibiting superconducting transition temperatures (Tc) up to 32K, but the superconducting mechanism remains under debate despite more than 30 years of extensive research. Our angle-resolved photoemission spectroscopy studies on Ba$_{0.51}$K$_{0.49}$BiO$_3$ reveal an unexpectedly 34% larger bandwidth than in conventional density functional theory calculations. This can be reproduced by calculations that fully account for long-range Coulomb interactions --- the first direct demonstration of bandwidth expansion due to the Fock exchange term, a long-accepted and yet uncorroborated fundamental effect in many body physics. Furthermore, we observe an isotropic superconducting gap with 2Δ$_0$/k$_B$ T$_c$ = 3.51 $\pm$ 0.05, and strong electron-phonon interactions with a coupling constant λ$\sim$ 1.3 $\pm$ 0.2. These findings solve a long-standing mystery --- Ba$_{0.51}$K$_{0.49}$BiO$_3$ is an extraordinary Bardeen-Cooper-Schrieffer (BCS) superconductor, where long-range Coulomb interactions expand the bandwidth, enhance electron-phonon coupling, and generate the high Tc. Such effects will also be critical for finding new superconductors. △ Less

Submitted 28 February, 2018; originally announced February 2018.

Comments: 14 pages, 4 Postscript figures

Journal ref: Phys. Rev. Lett. 121, 117002 (2018)

Evidence for negative thermal expansion in the superconducting precursor phase SmFeAsO

Authors: H. D. Zhou, P. M. Sarte, B. S. Conner, L. Balicas, C. R. Wiebe, X. H. Chen, T. Wu, G. Wu, R. H. Liu, H. Chen, D. F. Fang

Abstract: The fluorine-doped rare-earth iron oxypnictide series SmFeAsO$_{1-x}$F$_x$ (0 $\leq x \leq$ 0.10) was investigated with high resolution powder x-ray scattering. In agreement with previous studies, the parent compound SmFeAsO exhibits a tetragonal-to-orthorhombic structural distortion at T$\rm{_{S}}$~=~130~K which is rapidly suppressed by $x \simeq$ 0.10 deep within the superconducting dome. The ch… ▽ More The fluorine-doped rare-earth iron oxypnictide series SmFeAsO$_{1-x}$F$_x$ (0 $\leq x \leq$ 0.10) was investigated with high resolution powder x-ray scattering. In agreement with previous studies, the parent compound SmFeAsO exhibits a tetragonal-to-orthorhombic structural distortion at T$\rm{_{S}}$~=~130~K which is rapidly suppressed by $x \simeq$ 0.10 deep within the superconducting dome. The change in unit cell symmetry is followed by a previously unreported magnetoelastic distortion at 120~K. The temperature dependence of the thermal expansion coefficient $α_{V}$ reveals a rich phase diagram for SmFeAsO: (i) a global minimum at 125 K corresponds to the opening of a spin-density wave instability as measured by pump-probe femtosecond spectroscopy whilst (ii) a global maximum at 110 K corresponds to magnetic ordering of the Sm and Fe sublattices as measured by magnetic x-ray scattering. At much lower temperatures than T$\rm{_{N}}$, SmFeAsO exhibits a significant negative thermal expansion on the order of -40~ppm~$\cdot$~K$^{-1}$ in contrast to the behavior of other rare-earth oxypnictides such as PrFeAsO and the actinide oxypnictide NpFeAsO where the onset of $α<$ 0 only appears in the vicinity of magnetic ordering. Correlating this feature with the temperature and doping dependence of the resistivity and the unit cell parameters, we interpret the negative thermal expansion as being indicative of the possible condensation of itinerant electrons accompanying the opening of a SDW gap, consistent with transport measurements. △ Less

Submitted 23 February, 2018; originally announced February 2018.

Comments: 7 pages, 5 figures

Journal ref: H D Zhou et al. 2018 J. Phys.: Condens. Matter 30 095601

Electronic structure of FeS

Authors: J. Miao, X. H. Niu, D. F. Xu, Q. Yao, Q. Y. Chen, T. P. Ying, S. Y. Li, Y. F. Fang, J. C. Zhang, S. Ideta, K. Tanaka, B. P. Xie, D. L. Feng, Fei Chen

Abstract: Here we report the electronic structure of FeS, a recently identified iron-based superconductor. Our high-resolution angle-resolved photoemission spectroscopy studies show two hole-like ($α$ and $β$) and two electron-like ($η$ and $δ$) Fermi pockets around the Brillouin zone center and corner, respectively, all of which exhibit moderate dispersion along $k_z$. However, a third hole-like band (… ▽ More Here we report the electronic structure of FeS, a recently identified iron-based superconductor. Our high-resolution angle-resolved photoemission spectroscopy studies show two hole-like ($α$ and $β$) and two electron-like ($η$ and $δ$) Fermi pockets around the Brillouin zone center and corner, respectively, all of which exhibit moderate dispersion along $k_z$. However, a third hole-like band ($γ$) is not observed, which is expected around the zone center from band calculations and is common in iron-based superconductors. Since this band has the highest renormalization factor and is known to be the most vulnerable to defects, its absence in our data is likely due to defect scattering --- and yet superconductivity can exist without coherent quasiparticles in the $γ$ band. This may help resolve the current controversy on the superconducting gap structure of FeS. Moreover, by comparing the $β$ bandwidths of various iron chalcogenides, including FeS, FeSe$_{1-x}$S$_x$, FeSe, and FeSe$_{1-x}$ Te$_x$, we find that the $β$ bandwidth of FeS is the broadest. However, the band renormalization factor of FeS is still quite large, when compared with the band calculations, which indicates sizable electron correlations. This explains why the unconventional superconductivity can persist over such a broad range of isovalent substitution in FeSe$_{1-x}$Te$_{x}$ and FeSe$_{1-x}$S$_{x}$. △ Less

Submitted 25 March, 2017; originally announced March 2017.

Comments: 10 pages, 5 figures

Report number: 10.1103/PhysRevB.95.205127

Journal ref: Phys. Rev. B 95, 205127 (2017)

Condensing magnons in a degenerate ferromagnetic spinor Bose gas

Authors: Fang Fang, Ryan Olf, Shun Wu, Holger Kadau, Dan M. Stamper-Kurn

Abstract: We observe the condensation of magnon excitations within an $F=1$ $^{87}$Rb spinor Bose-Einstein condensed gas. Magnons are pumped into a longitudinally spin-polarized gas, allowed to equilibrate to a non-degenerate distribution, and then cooled evaporatively at near-constant net longitudinal magnetization whereupon they condense. We find magnon condensation to be described quantitatively as the c… ▽ More We observe the condensation of magnon excitations within an $F=1$ $^{87}$Rb spinor Bose-Einstein condensed gas. Magnons are pumped into a longitudinally spin-polarized gas, allowed to equilibrate to a non-degenerate distribution, and then cooled evaporatively at near-constant net longitudinal magnetization whereupon they condense. We find magnon condensation to be described quantitatively as the condensation of free particles in an effective potential that is uniform within the ferromagnetic condensate volume, evidenced by the number and distribution of magnons at the condensation transition. Transverse magnetization images reveal directly the spontaneous, inhomogeneous symmetry breaking by the magnon quasi-condensate, including signatures of Mermin-Ho spin textures that appear as phase singularities in the magnon condensate wavefunction. △ Less

Submitted 16 November, 2015; originally announced November 2015.

Comments: 5 pages, 3 figures

Journal ref: Phys. Rev. Lett. 116, 095301 (2016)

Plasmon Weyl Degeneracies in Magnetized Plasma

Authors: Wenlong Gao, Biao Yang, Mark Lawrence, Fengzhou Fang, Benjamin Béri, Shuang Zhang

Abstract: In this letter, we report the presence of novel type of plasmon Weyl points in a naturally existing material - magnetized plasma. In such a medium, conventional, purely longitudinal bulk plasma oscillations exists only along the direction of applied magnetic field (z direction). With strong enough magnetic field, there exist helical propagating modes along z direction with circular polarizations.… ▽ More In this letter, we report the presence of novel type of plasmon Weyl points in a naturally existing material - magnetized plasma. In such a medium, conventional, purely longitudinal bulk plasma oscillations exists only along the direction of applied magnetic field (z direction). With strong enough magnetic field, there exist helical propagating modes along z direction with circular polarizations. The orthogonality between the longitudinal bulk plasmon mode and the transverse helical propagating modes guarantees their crossing at the bulk plasmon frequency. These crossing points, embedded in the bulk plasmon dispersion line, serve as monopoles in the k space - the so called Weyl points. These Weyl points lead to salient observable features. These include the highly intriguing observation that, at a magnetized plasma surface which is parallel to the applied magnetic field, reflection of an electromagnetic wave with in-plane wave-vector close to the Weyl points exhibits chiral behavior only in half of the k plane, which is bounded by the projection of the bulk plasmon dispersion line. We also verify the presence of 'Fermi arcs' connecting the two Weyl points with opposite chiralities when magnetized plasma interfaces with trivial photonic materials. Our study introduces the concept of Weyl photonics into homogeneous strongly dispersive photonic materials, which could pave way for realizing new topological photonic devices. △ Less

Submitted 16 November, 2015; originally announced November 2015.

Comments: 13 pages, 5 figures

Journal ref: Nat. Commun. 7, 12435 (2016)

Self-blocking of interstitial clusters near metallic grain boundaries

Authors: Xiangyan Li, Wei Liu, Yichun Xu, C. S. Liu, B. C. Pan, Yunfeng Liang, Q. F. Fang, Jun-Ling Chen, G. -N. Luo, Zhiguang Wang, Y. Dai

Abstract: Nano-crystallize materials have been known for decades to potentially owe the novel self-healing ability for radiation damage, which has been demonstrated to be especially linked to preferential occupation of interstitials at grain boundary (GB) and promoted vacancy-interstitial annihilation. A major obstacle to better understanding the healing property is the lack of an atomistic picture of the i… ▽ More Nano-crystallize materials have been known for decades to potentially owe the novel self-healing ability for radiation damage, which has been demonstrated to be especially linked to preferential occupation of interstitials at grain boundary (GB) and promoted vacancy-interstitial annihilation. A major obstacle to better understanding the healing property is the lack of an atomistic picture of the interstitial states near GBs, due to severely separation of the timescale of interstitial segregation from other events and abundance of interstitials at the GB. Here, we report a generic "self-blocking" effect of the interstitial cluster (SIAn) near the metallic GB in W, Mo and Fe. Upon creating a SIAn near the GB, it is immediately trapped by the GB during the GB structural relaxation and blocks there, impeding GB's further spontaneous trapping of the SIAn in the vicinity and making these SIAns stuck nearby the GB. The SIAn in the stuck state surprisingly owes an exceptionally larger number of annihilation sites with vacancies near the GB than the SIAn trapped at the GB due to maintaining its bulk configuration basically. Besides, it also has an unexpectedly long-ranged repelling interaction with the SIA in the bulk region, which may further affect the GB's trap of the SIAn. The self-blocking effect might shed light on more critical and extended role of the GB in healing radiation-damage in NCs than previously recognized the GB's limited role based on GB's trap for the SIA and resulted vacancy-SIA recombination. △ Less

Submitted 27 May, 2015; originally announced May 2015.

Comments: 29 pages, 6 figures

Thermometry and cooling of a Bose-Einstein condensate to 0.02 times the critical temperature

Authors: Ryan Olf, Fang Fang, G. Edward Marti, Andrew MacRae, Dan M. Stamper-Kurn

Abstract: Ultracold gases promise access to many-body quantum phenomena at convenient length and time scales. However, it is unclear whether the entropy of these gases is low enough to realize many phenomena relevant to condensed matter physics, such as quantum magnetism. Here we report reliable single-shot temperature measurements of a degenerate $^{87}$Rb gas by imaging the momentum distribution of therma… ▽ More Ultracold gases promise access to many-body quantum phenomena at convenient length and time scales. However, it is unclear whether the entropy of these gases is low enough to realize many phenomena relevant to condensed matter physics, such as quantum magnetism. Here we report reliable single-shot temperature measurements of a degenerate $^{87}$Rb gas by imaging the momentum distribution of thermalized magnons, which are spin excitations of the atomic gas. We record average temperatures as low as $0.022(1)_\text{stat}(2)_\text{sys}$ times the Bose-Einstein condensation temperature, indicating an entropy per particle, $S/N\approx0.001\, k_B$ at equilibrium, that is well below the critical entropy for antiferromagnetic ordering of a Bose-Hubbard system. The magnons themselves can reduce the temperature of the system by absorbing energy during thermalization and by enhancing evaporative cooling, allowing low-entropy gases to be produced within deep traps. △ Less

Submitted 22 May, 2015; originally announced May 2015.

Comments: 8 pages, 3 figures, including methods; Supplemental material: 2 pages, 1 figure

Unconventional charge-density wave in Sr3Ir4Sn13 cubic superconductor revealed by optical spectroscopy study

Authors: A. F. Fang, X. B. Wang, P. Zheng, N. L. Wang

Abstract: Sr3Ir4Sn13 is an interesting compound showing a coexistence of structural phase transition and superconductivity. The structural phase transition at 147 K leads to the formation of a superlattice. We performed optical spectroscopy measurements across the structural phase transition on single crystal sample of Sr3Ir4Sn13. The optical spectroscopy study reveals an unusual temperature induced spectra… ▽ More Sr3Ir4Sn13 is an interesting compound showing a coexistence of structural phase transition and superconductivity. The structural phase transition at 147 K leads to the formation of a superlattice. We performed optical spectroscopy measurements across the structural phase transition on single crystal sample of Sr3Ir4Sn13. The optical spectroscopy study reveals an unusual temperature induced spectral weight transfer over broad energy scale, yielding evidence for the presence of electron correlation effect. Below the structural phase transition temperature an energy gap-like suppression in optical conductivity was observed, leading to the removal of partial itinerant carriers near Fermi level. Unexpectedly, the suppression appears at much higher energy scale than that expected for a usual charge density wave phase transition. △ Less

Submitted 27 June, 2014; originally announced June 2014.

Comments: Accepted for Publication in Physical Review B

Journal ref: Phys. Rev. B 90, 035115 (2014)

Coherent magnon optics in a ferromagnetic spinor Bose-Einstein condensate

Authors: G. Edward Marti, Andrew MacRae, Ryan Olf, Sean Lourette, Fang Fang, Dan M. Stamper-Kurn

Abstract: We measure the mass, gap, and magnetic moment of a magnon in the ferromagnetic $F=1$ spinor Bose-Einstein condensate of $^{87}$Rb. We find an unusually heavy magnon mass of $1.038(2)_\mathrm{stat}(8)_\mathrm{sys}$ times the atomic mass, as determined by interfering standing and running coherent magnon waves within the dense and trapped condensed gas. This measurement is shifted significantly from… ▽ More We measure the mass, gap, and magnetic moment of a magnon in the ferromagnetic $F=1$ spinor Bose-Einstein condensate of $^{87}$Rb. We find an unusually heavy magnon mass of $1.038(2)_\mathrm{stat}(8)_\mathrm{sys}$ times the atomic mass, as determined by interfering standing and running coherent magnon waves within the dense and trapped condensed gas. This measurement is shifted significantly from theoretical estimates. The magnon energy gap of $h\times 2.5(1)_\mathrm{stat}(2)_\mathrm{sys}\;\mathrm{Hz}$ and the effective magnetic moment of $-1.04(2)_\mathrm{stat}(8)\,μ_\textrm{bare}$ times the atomic magnetic moment are consistent with mean-field predictions. The nonzero energy gap arises from magnetic dipole-dipole interactions. △ Less

Submitted 22 April, 2014; originally announced April 2014.

Comments: 5 pages, 3 figures

Journal ref: Phys. Rev. Lett. 113, 155302 (2014)

Structural phase transition induced by van Hove singularity in 5d transition metal compound IrTe2

Authors: T. Qian, H. Miao, Z. J. Wang, X. Liu, X. Shi, Y. B. Huang, P. Zhang, N. Xu, P. Richard, M. Shi, M. H. Upton, J. P. Hill, G. Xu, X. Dai, Z. Fang, H. C. Lei, C. Petrovic, A. F. Fang, N. L. Wang, H. Ding

Abstract: Comprehensive studies of the electronic states of Ir 5d and Te 5p have been performed to elucidate the origin of the structural phase transition in IrTe2 by combining angle-resolved photoemission spectroscopy and resonant inelastic X-ray scattering. While no considerable changes are observed in the configuration of the Ir 5d electronic states across the transition, indicating that the Ir 5d orbita… ▽ More Comprehensive studies of the electronic states of Ir 5d and Te 5p have been performed to elucidate the origin of the structural phase transition in IrTe2 by combining angle-resolved photoemission spectroscopy and resonant inelastic X-ray scattering. While no considerable changes are observed in the configuration of the Ir 5d electronic states across the transition, indicating that the Ir 5d orbitals are not involved in the transition, we reveal a van Hove singularity at the Fermi level (EF) related to the Te px+py orbitals, which is removed from EF at low temperatures. The wavevector connecting the adjacent saddle points is consistent with the in-plane projection of the superstructure modulation wavevector. These results can be qualitatively understood with the Rice-Scott "saddle-point" mechanism, while effects of the lattice distortions need to be additionally involved. △ Less

Submitted 19 November, 2013; originally announced November 2013.

Comments: 5 pages, 4 fitures

Journal ref: New Journal of Physics 16 (2014) 123038

An Icosahedral Quasicrystal as a Packing of Regular Tetrahedra

Authors: F. Fang, J. Kovacs, G. Sadler, K. Irwin

Abstract: We present the construction of a dense, quasicrystalline packing of regular tetrahedra with icosahedral symmetry. This quasicrystalline packing was achieved through two independent approaches. The first approach originates in the Elser-Sloane 4D quasicrystal. A 3D slice of the quasicrystal contains a few types of prototiles. An initial structure is obtained by decorating these prototiles with tetr… ▽ More We present the construction of a dense, quasicrystalline packing of regular tetrahedra with icosahedral symmetry. This quasicrystalline packing was achieved through two independent approaches. The first approach originates in the Elser-Sloane 4D quasicrystal. A 3D slice of the quasicrystal contains a few types of prototiles. An initial structure is obtained by decorating these prototiles with tetrahedra. This initial structure is then modified using the Elser-Sloane quasicrystal itself as a guide. The second approach proceeds by decorating the prolate and oblate rhombohedra in a 3-dimensional Ammann tiling. The resulting quasicrystal has a packing density of 59.783%. We also show a variant of the quasicrystal that has just 10 "plane classes" (compared with the 190 of the original), defined as the total number of distinct orientations of the planes in which the faces of the tetrahedra are contained. This small number of plane classes was achieved by a certain "golden rotation" of the tetrahedra. △ Less

Submitted 15 November, 2013; originally announced November 2013.

Large low-temperature magnetoresistance in SrFe2As2 single crystals

Authors: S. V. Chong, G. V. M. Williams, J. Kennedy, F. Fang, J. L. Tallon, K. Kadowaki

Abstract: We present the first report on a large low-temperature magnetoresistance (MR) of more than 1600% in a SrFe2As2 single crystal and 1300% in a low-energy Ca ion-implanted SrFe2As2 single crystal that occurs before the emergence of crystallographic strain-induced bulk superconductivity arising from a sample aging effect. In accordance to band structure calculations from literature, which consitently… ▽ More We present the first report on a large low-temperature magnetoresistance (MR) of more than 1600% in a SrFe2As2 single crystal and 1300% in a low-energy Ca ion-implanted SrFe2As2 single crystal that occurs before the emergence of crystallographic strain-induced bulk superconductivity arising from a sample aging effect. In accordance to band structure calculations from literature, which consitently show more than 2 bands are involved in the transport, we have modeled this large MR at high fields using a 3-carrier scenario rather than solely on quantum linear MR model generally used to explain the MR in iron-pnictides. At and below 20 K the large MR may be due to 3-carrier transport in an inhomogeneous state where there are superconducting and metallic regions. △ Less

Submitted 22 October, 2013; originally announced October 2013.

Comments: 6 pages, 4 figures

Journal ref: EPL 104 (2013) 17002

Atomistic Mechanism from Vacancy Trapped H/He Atoms to Initiation of Bubble in W under Low Energy Ions Irradiation

Authors: Yu-Wei You, Xiang-Shan Kong, Q. F. Fang, Jun-Ling Chen, G. -N. Luo, C. S. Liu, B. C. Pan, Y. Daid

Abstract: With the first principles calculations of H and He induced energetics change we demonstrate that in W the accumulation of H (up to 9) and He (up to 4) in a single vacancy (V) surprisingly reduce the formation energy of first and second nearest vacancy (as low as 0 eV), which gives the direct evidence of V-H (He) complex mutation mechanism from V-Hn (Hen) to V2-Hn (Hen) and with the potential to le… ▽ More With the first principles calculations of H and He induced energetics change we demonstrate that in W the accumulation of H (up to 9) and He (up to 4) in a single vacancy (V) surprisingly reduce the formation energy of first and second nearest vacancy (as low as 0 eV), which gives the direct evidence of V-H (He) complex mutation mechanism from V-Hn (Hen) to V2-Hn (Hen) and with the potential to lead to the growth of H (He)-vacancy complexes: an initial step to H and He bubble. This finding well explains the long-standing problem of why H and He bubbles being produced on W surface exposed to low-energy (far lower than displacement threshold energy) D or He ions irradiation. The further identified repulsive (attractive) interaction between V-H12 (V-He14) and additional H (He) illustrates the experimentally observed big difference of deposition depth of H (micron) and He (100 angstrom) bubbles in W even the migration rate of He is far larger than that of H. △ Less

Submitted 2 May, 2013; originally announced May 2013.

Comments: 34 pages 6 figures

An operational window for radiation-resistant materials based on sequentially healing grain interiors and boundaries

Authors: Xiangyan Li, Yichun Xu, C. S. Liu, B. C. Pan, Yunfeng Liang, Q. F. Fang, Jun- Ling Chen, G. -N. Luo, Zhiguang Wang, Y. Dai

Abstract: Design of nuclear materials with high radiation-tolerance has great significance1, especially for the next generation of nuclear energy systems2,3. Response of nano- and poly-crystals to irradiation depends on the radiation temperature, dose-rate and grain size4-13. However the dependencies had been studied and interpreted individually, and thus severely lacking is the ability to predict radiation… ▽ More Design of nuclear materials with high radiation-tolerance has great significance1, especially for the next generation of nuclear energy systems2,3. Response of nano- and poly-crystals to irradiation depends on the radiation temperature, dose-rate and grain size4-13. However the dependencies had been studied and interpreted individually, and thus severely lacking is the ability to predict radiation performance of materials in extreme environments. Here we propose an operational window for radiation-resistant materials, which is based on a perspective of interactions among irradiation-induced interstitials, vacancies, and grain boundaries. Using atomic simulations, we find that healing grain boundaries needs much longer time than healing grain interiors. Not been noticed before, this finding suggests priority should be thereafter given to recovery of the grain boundary itself. This large disparity in healing time is reflected in the spectra of defects-recombination energy barriers by the presence of one high-barrier peak in addition to the peak of low barriers. The insight gained from the study instigates new avenues for examining the role of grain boundaries in healing the material. In particular, we sketch out the radiation-endurance window in the parameter space of temperature, dose-rate and grain size. The window helps evaluate material performance and develop resistant materials against radiation damage. △ Less

Submitted 29 March, 2013; originally announced March 2013.

Comments: 22p ages, 4 figures

Periodic modification of the Boerdijk-Coxeter helix (tetrahelix)

Authors: Garrett Sadler, Fang Fang, Julio Kovacs, Klee Irwin

Abstract: The Boerdijk-Coxeter helix is a helical structure of tetrahedra which possesses no non-trivial translational or rotational symmetries. In this document, we develop a procedure by which this structure is modified to obtain both translational and rotational (upon projection) symmetries along/about its central axis. We report the finding of several, distinct periodic structures, and focus on two part… ▽ More The Boerdijk-Coxeter helix is a helical structure of tetrahedra which possesses no non-trivial translational or rotational symmetries. In this document, we develop a procedure by which this structure is modified to obtain both translational and rotational (upon projection) symmetries along/about its central axis. We report the finding of several, distinct periodic structures, and focus on two particular forms related to the pentagonal and icosahedral aggregates of tetrahedra as well as Buckminster Fuller's "jitterbug transformation". △ Less

Submitted 1 February, 2013; originally announced February 2013.

Effect of disorder in the charge-density-wave compounds LaTe$_{1.95}$ and CeTe$_{1.95-x}$Se$_x$ (x=0 and 0.16) as revealed by optical spectroscopy

Authors: Y. Huang, B. F. Hu, T. Dong, A. F. Fang, P. Zheng, N. L. Wang

Abstract: We present optical spectroscopy measurements on rare-earth ditelluride single crystals of LaTe$_{1.95}$ and CeTe$_{1.95-x}$Se$_x$ (x=0 and 0.16). The measurements reveal formation of charge density wave energy gaps at rather high energy levels, e.g. 2$Δ\sim$ 8500 \cm for LaTe$_{1.95}$, and 6800 \cm for CeTe$_{1.95}$. More strikingly, the study reveals that, different from the rare-earth tri-tellur… ▽ More We present optical spectroscopy measurements on rare-earth ditelluride single crystals of LaTe$_{1.95}$ and CeTe$_{1.95-x}$Se$_x$ (x=0 and 0.16). The measurements reveal formation of charge density wave energy gaps at rather high energy levels, e.g. 2$Δ\sim$ 8500 \cm for LaTe$_{1.95}$, and 6800 \cm for CeTe$_{1.95}$. More strikingly, the study reveals that, different from the rare-earth tri-tellurides, the Te vacancies and disorder effect play a key role in the low-energy charge excitations of ditelluride systems. Although an eminent peak is observed between 800 and 1500 \cm in conductivity spectra for LaTe$_{1.95}$, and CeTe$_{1.95-x}$Se$_x$ (x=0. 0.16), our analysis indicates that it could not be attributed to the formation of a small energy gap, instead it could be well accounted for by the localization modified Drude model. Our study also indicates that the low-tempreature optical spectroscopic features are distinctly different from a semiconducting CDW state with entirely gapped Fermi surfaces. △ Less

Submitted 8 November, 2012; originally announced November 2012.

Comments: 5 pages, 4 figures, Accepted for publication in Phys. Rev. B

Journal ref: Phys. Rev. B 86, 205123 (2012)

Nodeless superconductivity in Ir$_{1-x}$Pt$_x$Te$_2$ with strong spin-orbital coupling

Authors: S. Y. Zhou, X. L. Li, B. Y. Pan, X. Qiu, J. Pan, X. C. Hong, Z. Zhang, A. F. Fang, N. L. Wang, S. Y. Li

Abstract: The thermal conductivity $κ$ of superconductor Ir$_{1-x}$Pt$_{x}$Te$_2$ ($x$ = 0.05) single crystal with strong spin-orbital coupling was measured down to 50 mK. The residual linear term $κ_0/T$ is negligible in zero magnetic field. In low magnetic field, $κ_0/T$ shows a slow field dependence. These results demonstrate that the superconducting gap of Ir$_{1-x}$Pt$_{x}$Te$_2$ is nodeless, and the p… ▽ More The thermal conductivity $κ$ of superconductor Ir$_{1-x}$Pt$_{x}$Te$_2$ ($x$ = 0.05) single crystal with strong spin-orbital coupling was measured down to 50 mK. The residual linear term $κ_0/T$ is negligible in zero magnetic field. In low magnetic field, $κ_0/T$ shows a slow field dependence. These results demonstrate that the superconducting gap of Ir$_{1-x}$Pt$_{x}$Te$_2$ is nodeless, and the pairing symmetry is likely conventional s-wave, despite the existence of strong spin-orbital coupling and a quantum critical point. △ Less

Submitted 22 July, 2013; v1 submitted 19 September, 2012; originally announced September 2012.

Comments: 5 pages, 4 figures

Journal ref: EPL 104, 27010 (2013)

Single crystal growth and optical conductivity of SrPt$_2$As$_2$ superconductors

Authors: A. F. Fang, T. Dong, H. P. Wang, Z. G. Chen, B. Cheng, Y. G. Shi, P. Zheng, G. Xu, L. Wang, J. Q. Li, N. L. Wang

Abstract: SrPt$_2$As$_2$ single crystals with CaBe$_2$Ge$_2$-type structure were synthesized by self-melting technique. X-ray diffraction, transmission electron microscopy, electrical resistivity, specific heat and optical spectroscopy measurements were conducted to elucidate the properties of SrPt$_2$As$_2$. SrPt$_2$As$_2$ single crystals exhibit a superconducting transition at 5.2K, experiencing a structu… ▽ More SrPt$_2$As$_2$ single crystals with CaBe$_2$Ge$_2$-type structure were synthesized by self-melting technique. X-ray diffraction, transmission electron microscopy, electrical resistivity, specific heat and optical spectroscopy measurements were conducted to elucidate the properties of SrPt$_2$As$_2$. SrPt$_2$As$_2$ single crystals exhibit a superconducting transition at 5.2K, experiencing a structural phase transition well above room temperature (about 455K). The superconducting and structural phase transition temperatures are both reduced by 6% Iridium doping. Both pure SrPt$_2$As$_2$ and the doped single crystals are revealed to be highly metallic with rather high plasma frequencies. In particular, the optical spectroscopy measurement revealed two gap-like suppression features. We elaborated that the one at higher energy scale could be ascribed to the correlation effect, while the other one at lower energy scale is related to the structural phase transition, leading to the removal of a small portion of the Fermi surfaces. △ Less

Submitted 17 May, 2012; v1 submitted 26 April, 2012; originally announced April 2012.

Comments: 7 pages, 6 figures

Journal ref: Phys. Rev. B 85, 184520 (2012)

Two successive field-induced spin-flop transitions in single-crystalline CaCo$_{2}$As$_{2}$

Authors: B. Cheng, B. F. Hu, R. H. Yuan, T. Dong, A. F. Fang, Z. G. Chen, G. Xu, Y. G. Shi, P. Zheng, J. L. Luo, N. L. Wang

Abstract: CaCo$_{2}$As$_{2}$, a ThCr$_{2}$Si$_{2}$-structure compound, undergoes an antiferromagnetic transition at \emph{T$_{N}$}=76K with the magnetic moments being aligned parallel to the \emph{c} axis. Electronic transport measurement reveals that the coupling between conducting carriers and magnetic order in CaCo$_{2}$As$_{2}$ is much weaker comparing to the parent compounds of iron pnictide. Applying… ▽ More CaCo$_{2}$As$_{2}$, a ThCr$_{2}$Si$_{2}$-structure compound, undergoes an antiferromagnetic transition at \emph{T$_{N}$}=76K with the magnetic moments being aligned parallel to the \emph{c} axis. Electronic transport measurement reveals that the coupling between conducting carriers and magnetic order in CaCo$_{2}$As$_{2}$ is much weaker comparing to the parent compounds of iron pnictide. Applying magnetic field along \emph{c} axis induces two successive spin-flop transitions in its magnetic state. The magnetization saturation behaviors with \emph{\textbf{H}$\parallel$c} and \emph{\textbf{H}$\parallel$ab} at 10K indicate that the antiferromagnetic coupling along \emph{c} direction is very weak. The interlayer antiferromagntic coupling constant \emph{J$_{c}$} is estimated to be about 2 meV. △ Less

Submitted 8 April, 2012; originally announced April 2012.

Comments: Accepted for publication in Phys. Rev. B. 5 pages, 6 figures

Structural phase transition in IrTe$_2$: A combined study of optical spectroscopy and band structure calculations

Authors: A. F. Fang, G. Xu, T. Dong, P. Zheng, N. L. Wang

Abstract: Ir$_{1-x}$Pt$_x$Te$_2$ is an interesting system showing competing phenomenon between structural instability and superconductivity. Due to the large atomic numbers of Ir and Te, the spin-orbital coupling is expected to be strong in the system which may lead to nonconventional superconductivity. We grew single crystal samples of this system and investigated their electronic properties. In particular… ▽ More Ir$_{1-x}$Pt$_x$Te$_2$ is an interesting system showing competing phenomenon between structural instability and superconductivity. Due to the large atomic numbers of Ir and Te, the spin-orbital coupling is expected to be strong in the system which may lead to nonconventional superconductivity. We grew single crystal samples of this system and investigated their electronic properties. In particular, we performed optical spectroscopic measurements, in combination with density function calculations, on the undoped compound IrTe$_2$ in an effort to elucidate the origin of the structural phase transition at 280 K. The measurement revealed a dramatic reconstruction of band structure and a significant reduction of conducting carriers below the phase transition. We elaborate that the transition is not driven by the density wave type instability but caused by the crystal field effect which further splits/separates the energy levels of Te (p$_x$, p$_y$) and Te p$_z$ bands. △ Less

Submitted 30 January, 2013; v1 submitted 19 March, 2012; originally announced March 2012.

Comments: 16 pages, 5 figures

Journal ref: Sci. Rep. 3, 1153 (2013)

Role of Alloying-Atom Size Factor and System Shape Factor in Energetics of bcc Fe under Macroscopic Deformation

Authors: Wei Liu, Wei-Lu Wang, Q. F. Fang, C. S. Liu, Qun-Ying Huang, Yi-Can Wu

Abstract: We present an \emph{ab initio} study of the effect of macroscopic deformation on energetics of twelve alloying elements in bcc Fe under three specially designed strain modes. We find that there exists a universal linear relation of describing the volume dependence of substitutional energy of alloying elements via introducing two factors --- the system shape factor ($f_{\scriptsize{ss}}$) and the s… ▽ More We present an \emph{ab initio} study of the effect of macroscopic deformation on energetics of twelve alloying elements in bcc Fe under three specially designed strain modes. We find that there exists a universal linear relation of describing the volume dependence of substitutional energy of alloying elements via introducing two factors --- the system shape factor ($f_{\scriptsize{ss}}$) and the size factor of alloying element $M$ ($Ω^{M}_{\scriptsize{sf}}$): $E_{\scriptsize{sub}} \sim f_{\scriptsize{ss}}Ω^{M}_{\scriptsize{sf}}V$. $Ω^{M}_{\scriptsize{sf}}$ well describes the effect of intrinsic alloying-atom size and the influence of chemical interaction with matrix atom, and $f_{\scriptsize{ss}}$ characterizes the degree of system lattice distortion under deformation. This relation is further validated using the published data of stained-modulated doping in GaP △ Less

Submitted 15 August, 2011; originally announced August 2011.

Comments: 4 pages 3 figures; Phys. Rev. Lett. 2011, 7, 21 under review

Optical study of the multiple charge density wave transitions in ErTe$_3$

Authors: B. F. Hu, B. Cheng, R. H. Yuan, T. Dong, A. F. Fang, W. T. Guo, Z. G. Chen, P. Zheng, Y. G. Shi, N. L. Wang

Abstract: We present an optical spectroscopy study on singe crystalline ErTe$_3$, a rare-earth-element tri-telluride which experiences two successive charge density wave (CDW) transitions at T$_{c1}$ = 267 K and T$_{c2}$ = 150 K. Two corresponding gap features, centered at 2770 \cm ($\sim$ 343 meV) and 890 \cm ($\sim$ 110 meV) respectively, are clearly seen in ordered state. A pronounced Drude component, wh… ▽ More We present an optical spectroscopy study on singe crystalline ErTe$_3$, a rare-earth-element tri-telluride which experiences two successive charge density wave (CDW) transitions at T$_{c1}$ = 267 K and T$_{c2}$ = 150 K. Two corresponding gap features, centered at 2770 \cm ($\sim$ 343 meV) and 890 \cm ($\sim$ 110 meV) respectively, are clearly seen in ordered state. A pronounced Drude component, which exists at all measurement temperatures, demonstrates the partial gap character of both CDW orders. About half of the unmodulated Fermi surface (FS) remains in the CDW state at the lowest measurement temperature. The study also reveals that fluctuation effect is still prominent in this two-dimensional (2D) material. △ Less

Submitted 1 August, 2011; originally announced August 2011.

Comments: 5 figures, 6 pages

Journal ref: Physical Review B 84, 155132 (2011)

First principle study of hydrogen behavior in hexagonal tungsten carbide

Authors: Xiang-Shan Kong, Yu-Wei You, C. S. Liu, Q. F. Fang, Jun-Ling Chen, G. -N. Luo

Abstract: Understanding the behavior of hydrogen in hexagonal tungsten carbide (WC) is of particular interest for fusion reactor design due to the presence of WC in the divertor of fusion reactors. Therefore, we use first-principles calculations to study the hydrogen behavior in WC. The most stable interstitial site for the hydrogen atom is the projection of the octahedral interstitial site on tungsten basa… ▽ More Understanding the behavior of hydrogen in hexagonal tungsten carbide (WC) is of particular interest for fusion reactor design due to the presence of WC in the divertor of fusion reactors. Therefore, we use first-principles calculations to study the hydrogen behavior in WC. The most stable interstitial site for the hydrogen atom is the projection of the octahedral interstitial site on tungsten basal plane, followed by the site near the projection of the octahedral interstitial site on carbon basal plane. The binding energy between two interstitial hydrogen atoms is negative, suggesting that hydrogen itself is not capable of trapping other hydrogen atoms to form a hydrogen molecule. The calculated results on the interaction between hydrogen and vacancy indicate that the hydrogen atom is energetically trapped by vacancy and the hydrogen molecule can not be formed in mono-vacancy. In addition, the hydrogen atom bound to carbon is only found in tungsten vacancy. We also study the migrations of hydrogen in WC and find that the interstitial hydrogen atom prefers to diffusion along the c axis. Our studies on the hydrogen behavior in WC provide some explanations for the experimental results of the thermal desorption process of energetic hydrogen ion implanted into WC. △ Less

Submitted 12 October, 2010; originally announced October 2010.

Comments: 29 pages and 7 figures, submitted to Journal of Nuclear Materials, under review

Contraction and expansion effects on the substitution-defect properties of thirteen alloying elements in bcc Fe

Authors: Wei Liu, Wei-Lu Wang, C. S. Liu, Q. F. Fang, Qun-Ying Huang, Yi-Can Wu, Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P. O. Box 1129, Hefei 230031, P. R. China, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, P. R. China

Abstract: Proposed as blanket structural materials for fusion power reactors, reduced activation ferritic/martensitic (RAFM) steel undergoes volume expanding and contracting in a cyclic mode under service environment. Particularly, being subjected to significant fluxes of fusion neutrons RAFM steel suffers considerable local volume variations in the radiation damage involved regions. It is necessary to stud… ▽ More Proposed as blanket structural materials for fusion power reactors, reduced activation ferritic/martensitic (RAFM) steel undergoes volume expanding and contracting in a cyclic mode under service environment. Particularly, being subjected to significant fluxes of fusion neutrons RAFM steel suffers considerable local volume variations in the radiation damage involved regions. It is necessary to study the structure properties of the alloying elements in contraction and expansion states. In this paper we studied local substitution structures of thirteen alloying elements Al, Co, Cr, Cu, Mn, Mo, Nb, Ni, Si, Ta, Ti, V, and W in bcc Fe and calculated their substitutional energies in the volume variation range from -1.0% to 1.0%. From the structure relaxation results of the first five neighbor shells around the substitutional atom we find the relaxation in each neighbor shell keeps approximately uniform within the volume variation from -1.0% to 1.0% except those of Mn and the relaxation of the fifth neighbor shell is stronger than that of the third and forth, indicating that the lattice distortion due to the substitution atom is easier to spread in <111> direction than in other direction. The relaxation pattern and intensity are related to the size and electron structure of the substitutional atom. For some alloying elements, such as Mo, Nb, Ni, Ta, Ti and W, the substitutional energy decreases noticeably when the volume increases. Further analysis show that the substitutional energy comprises the energy variation originated from local structure relaxation and the chemical potential difference of the substitutional atom between its elemental crystalline state and the solid solution phase in bcc Fe. We think the approximately uniform relaxation of each neighbor shell around a substitutional atom give rise to a linear decrease in the substitutional energy with the increasing volume. △ Less

Submitted 17 August, 2010; originally announced August 2010.

Comments: Submitted to Journal of Nuclear Materials

First principle study of intrinsic defects in hexagonal tungsten carbide

Authors: Xiang-Shan Kong, Yu-Wei You, J. H. Xia, C. S. Liu, Q. F. Fang, G. -N. Luo, Qun-Ying Huang

Abstract: The characteristics of intrinsic defects are important for the understanding of self-diffusion processes, mechanical strength, brittleness, and plasticity of tungsten carbide, which present in the divertor of fusion reactors. Here, we use first-principles calculations to investigate the stability of point defects and their complexes in WC. Our calculation results confirm that the formation energie… ▽ More The characteristics of intrinsic defects are important for the understanding of self-diffusion processes, mechanical strength, brittleness, and plasticity of tungsten carbide, which present in the divertor of fusion reactors. Here, we use first-principles calculations to investigate the stability of point defects and their complexes in WC. Our calculation results confirm that the formation energies of carbon defects are much lower than that of tungsten defects. The outward relaxations around vacancy are found. Both interstitial carbon and interstitial tungsten atom prefer to occupy the carbon basal plane projection of octahedral interstitial site. The results of isolated carbon defect diffusion show that the carbon vacancy stay for a wide range of temperature because of extremely high diffusion barriers, while carbon interstitial migration is activated at lower temperatures for its considerable lower activation energy. These results provide evidence for the presumption that the 800K stage is attributed by the annealing out of carbon vacancies by long-range migration. △ Less

Submitted 16 August, 2010; originally announced August 2010.

Comments: Submitted to Journal of Nuclear Materials

Superconductivity induced by oxygen deficiency in Sr-doped LaOFeAs

Authors: G. Wu, H. Chen, Y. L. Xie, Y. J. Yan, T. Wu, R. H. Liu, X. F. Wang, D. F. Fang, J. J. Ying, X. H. Chen

Abstract: We synthesized Sr-doped $La_{0.85}Sr_{0.15}OFeAs$ sample with single phase, and systematically studied the effect of oxygen deficiency in the Sr-doped LaOFeAs system. It is found that substitution of Sr for La indeed induces the hole carrier evidenced by positive thermoelectric power (TEP), but no bulk superconductivity is observed. The superconductivity can be realized by annealing the as-grown… ▽ More We synthesized Sr-doped $La_{0.85}Sr_{0.15}OFeAs$ sample with single phase, and systematically studied the effect of oxygen deficiency in the Sr-doped LaOFeAs system. It is found that substitution of Sr for La indeed induces the hole carrier evidenced by positive thermoelectric power (TEP), but no bulk superconductivity is observed. The superconductivity can be realized by annealing the as-grown sample in vacuum to produce the oxygen deficiency. With increasing the oxygen deficiency, the superconducting transition temperature ($T_c$) increases and maximum $T_c$ reaches about 26 K the same as that in La(O,F)FeAs. TEP dramatically changes from positive to negative in the nonsuperconducting as-grown sample to the superconducting samples with oxygen deficiency. While $R_H$ is always negative for all samples (even for Sr-doped as grown sample). It suggests that the $La_{0.85}Sr_{0.15}O_{1-δ}FeAs$ is still electron-type superconductor. △ Less

Submitted 10 June, 2008; originally announced June 2008.

Comments: 4 pages, 4 figures

Journal ref: Phys. Rev. B 78, 092503 (2008).

Transport properties and superconductivity in $Ba_{1-x}M_xFe_2As_2$ (M=La and K) with double FeAs layers

Authors: G. Wu, R. H. Liu, H. Chen, Y. J. Yan, T. Wu, Y. L. Xie, J. J. Ying, X. F. Wang, D. F. Fang, X. H. Chen

Abstract: We synthesized the samples $Ba_{1-x}M_xFe_2As_2$ (M=La and K) with $ThCr_2Si_2$-type structure. These samples were systematically characterized by resistivity, thermoelectic power (TEP) and Hall coefficient ($R_H$). $BaFe_2As_2$ shows an anomaly in resistivity at about 140 K. Substitution of La for Ba leads to a shift of the anomaly to low temperature, but no superconducting transition is observ… ▽ More We synthesized the samples $Ba_{1-x}M_xFe_2As_2$ (M=La and K) with $ThCr_2Si_2$-type structure. These samples were systematically characterized by resistivity, thermoelectic power (TEP) and Hall coefficient ($R_H$). $BaFe_2As_2$ shows an anomaly in resistivity at about 140 K. Substitution of La for Ba leads to a shift of the anomaly to low temperature, but no superconducting transition is observed. Potassium doping leads to suppression of the anomaly in resistivity and induces superconductivity at 38 K as reported by Rotter et al.\cite{rotter}. The Hall coefficient and TEP measurements indicate that the TEP is negative for $BaFe_2As_2$ and La-doped $BaFe_2As_2$, indicating n-type carrier; while potassium doping leads to change of the sign in $R_H$ and TEP. It definitely indicates p-type carrier in superconducting $Ba_{1-x}K_xFe_2As_2$ with double FeAs layers, being in contrast to the case of $LnO_{1-x}F_xFeAs$ with single FeAs layer. A similar superconductivity is also observed in the sample with nominal composition $Ba_{1-x}K_xOFe_2As_2$. △ Less

Submitted 9 June, 2008; originally announced June 2008.

Comments: 4 pages, 4 figures

Journal ref: Europhysics Letters 84, 27010(2008).

Phase Diagram and Quantum Critical Point in Newly Discovered Superconductors: SmO_{1-x}F_xFeAs

Authors: R. H. Liu, G. Wu, T. Wu, D. F. Fang, H. Chen, S. Y. Li, K. Liu, Y. L. Xie, X. F. Wang, R. L. Yang, L. Ding, C. He, D. L. Feng, X. H. Chen

Abstract: The magnetic fluctuations associated with a quantum critical point (QCP) are widely believed to cause the non-Fermi liquid behaviors and unconventional superconductivities, for example, in heavy fermion systems and high temperature cuprate superconductors. Recently, superconductivity has been discovered in iron-based layered compound $LaO_{1-x}F_xFeAs$ with $T_c$=26 K\cite{yoichi}, and it compet… ▽ More The magnetic fluctuations associated with a quantum critical point (QCP) are widely believed to cause the non-Fermi liquid behaviors and unconventional superconductivities, for example, in heavy fermion systems and high temperature cuprate superconductors. Recently, superconductivity has been discovered in iron-based layered compound $LaO_{1-x}F_xFeAs$ with $T_c$=26 K\cite{yoichi}, and it competes with spin-density-wave (SDW) order\cite{dong}. Neutron diffraction shows a long-rang SDW-type antiferromagnetic (AF) order at $\sim 134$ K in LaOFeAs\cite{cruz,mcguire}. Therefore, a possible QCP and its role in this system are of great interests. Here we report the detailed phase diagram and anomalous transport properties of the new high-Tc superconductors $SmO_{1-x}F_xFeAs$ discovered by us\cite{chenxh}. It is found that superconductivity emerges at $x\sim$0.07, and optimal doping takes place in the $x\sim$0.20 sample with highest $T_c \sim $54 K. While $T_c$ increases monotonically with doping, the SDW order is rapidly suppressed, suggesting a QCP around $x \sim$0.14. As manifestations, a linear temperature dependence of the resistivity shows up at high temperatures in the $x<0.14$ regime, but at low temperatures just above $T_c$ in the $x>0.14$ regime; a drop in carrier density evidenced by a pronounced rise in Hall coefficient are observed, which mimic the high-$T_c$ cuprates. The simultaneous occurrence of order, carrier density change and criticality makes a compelling case for a quantum critical point in this system. △ Less

Submitted 7 May, 2008; v1 submitted 14 April, 2008; originally announced April 2008.

Comments: 10 pages, 4 figures

Journal ref: Phys. Rev. Lett 101, 087001(2008)

Superconductivity at 43 K in Samarium-arsenide Oxides $SmFeAsO_{1-x}F_x$

Authors: X. H. Chen, T. Wu, G. Wu, R. H. Liu, H. Chen, D. F. Fang

Abstract: Since the discovery of high-transition temperature ($T_c$) superconductivity in layered copper oxides, extensive efforts have been devoted to explore the higher $T_c$ superconductivity. However, the $T_c$ higher than 40 K can be obtained only in the copper oxide superconductors so far. The highest reported value of $T_c$ for non-copper-oxide bulk superconductivity is 39 K in $MgB_2$.\cite{jun} T… ▽ More Since the discovery of high-transition temperature ($T_c$) superconductivity in layered copper oxides, extensive efforts have been devoted to explore the higher $T_c$ superconductivity. However, the $T_c$ higher than 40 K can be obtained only in the copper oxide superconductors so far. The highest reported value of $T_c$ for non-copper-oxide bulk superconductivity is 39 K in $MgB_2$.\cite{jun} The $T_c$ of about 40 K is close to or above the theoretical value predicted from BCS theory.\cite{mcmillan} Therefore, it is very significant to search for non-copper oxide superconductor with the transition temperature higher than 40 K to understand the mechanism of high-$T_c$ superconductivity. Here we report the discovery of bulk superconductivity in samarium-arsenide oxides $SmFeAsO_{1-x}F_x$ with ZrCuAiAs type structure. Resistivity and magnetization measurements show strong evidences for transition temperature as high as 43 K. $SmFeAsO_{1-x}F_x$ is the first non-copper oxide superconductor with $T_c$ higher than 40 K. The $T_c$ higher than 40 K may be a strong argument to consider $SmFeAsO_{1-x}F_x$ as an unconventional superconductor. △ Less

Submitted 28 March, 2008; v1 submitted 25 March, 2008; originally announced March 2008.

Comments: 8 pages, 4 figures

Journal ref: Nature 453,761-762(2008)

Giant Anisotropy of Magnetoresistance and "Spin Valve" effect in Antiferromagnetic $Nd_{2-x}Ce_xCuO_{4}$

Authors: T. Wu, C. H. Wang, G. Wu, D. F. Fang, J. L. Luo, G T. Liu, X. H. Chen

Abstract: We have studied anisotropic magnetoresistance (MR) and magnetization with rotating magnetic field (B) within $CuO_2$ plane in lightly doped AF $Nd_{2-x}Ce_xCuO_{4}$. \emph{A giant anisotropy} in MR is observed at low temperature below 5 K. The c-axis resistivity can be tuned about one order of magnitude just by changing B direction within $CuO_2$ plane and a scaling behavior between out-of-plane… ▽ More We have studied anisotropic magnetoresistance (MR) and magnetization with rotating magnetic field (B) within $CuO_2$ plane in lightly doped AF $Nd_{2-x}Ce_xCuO_{4}$. \emph{A giant anisotropy} in MR is observed at low temperature below 5 K. The c-axis resistivity can be tuned about one order of magnitude just by changing B direction within $CuO_2$ plane and a scaling behavior between out-of-plane and in-plane MR is found. A "Spin valve" effect is proposed to understand the giant anisotropy of out-of-plane MR and the evolution of scaling parameters with the external field. It is found that the field-induced spin-flop transition of Nd$^{3+}$ layer under high magnetic field is the key to understand the giant anisotropy. These results suggest that a novel entanglement between charge and spin dominates the underlying physics. △ Less

Submitted 19 March, 2009; v1 submitted 1 July, 2007; originally announced July 2007.

Comments: 7 pages, 8 figures

Journal ref: Journal of Physics: Condensed Matter 20, 275226(2008)

Magnetic field induced spin-flop transition in Na$_x$CoO$_2$ (0.5$<$x$<$0.55)

Authors: T. Wu, D. F. Fang, G. Y. Wang, L. Zhao, G. Wu, X. G. Luo, C. H. Wang, X. H. Chen

Abstract: The isothermal magnetoresistance (MR) with magnetic field (H) parallel to and perpendicular to ab plane is systematically studied on the single crystal Na$_{0.52}$CoO$_2$ with charge ordering at $\sim 50$ K and an in-plane ferromagnetism below 25 K. The isothermal MR behavior with H $\parallel$ ab plane and H $\perp$ ab plane is quite different. When H $\parallel$ ab plane, the MR is always nega… ▽ More The isothermal magnetoresistance (MR) with magnetic field (H) parallel to and perpendicular to ab plane is systematically studied on the single crystal Na$_{0.52}$CoO$_2$ with charge ordering at $\sim 50$ K and an in-plane ferromagnetism below 25 K. The isothermal MR behavior with H $\parallel$ ab plane and H $\perp$ ab plane is quite different. When H $\parallel$ ab plane, the MR is always negative and the in-plane ferromagnetic behavior is enhanced. While the MR with H $\perp$ ab plane changes from negative to positive with decreasing temperature or increasing H, and the in-plane ferromagnetic behavior is suppressed. A striking feature is that the MR with H $\perp$ ab plane shows a hysteresis behavior below 25 K, which is absent for the case of H $\parallel$ ab plane. These results provide strong evidence for a spin-flop transition of small moments of Co$^{3.5-δ}$ sites induced by H $\perp$ ab plane, leading to a metamagnetic transition for small moments of Co$^{3.5-δ}$ sites. These complex magnetism suggests an unconventional superconductivity in Na$_x$CoO$_2$ system because the Na$_x$CoO$_2$ around x=0.5 is considered to be the parent compound of superconductivity. △ Less

Submitted 25 March, 2007; originally announced March 2007.

Comments: 6 pages, 6 figures

Field-induced Domain Reorientation and Polarization Rotation of <110> Oriented Pb(Mg1/3Nb2/3)O3-PbTiO3 Single Crystals

Authors: Ke-Pi Chen, Xiao-Wen Zhang, Fei Fang, Hao-Su Luo

Abstract: Polarization hysteresis loops, x-ray diffraction (XRD) and temperature dependent dielectric constant under different electric fields for <110> oriented 0.7 Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 single crystals were measured. The field-induced phase transition and the process of depolarization were discussed. The results show that with the electric field increasing, the crystal form changes along via polar-… ▽ More Polarization hysteresis loops, x-ray diffraction (XRD) and temperature dependent dielectric constant under different electric fields for <110> oriented 0.7 Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 single crystals were measured. The field-induced phase transition and the process of depolarization were discussed. The results show that with the electric field increasing, the crystal form changes along via polar-axis reorientation and polarization rotation. However, the depolarization process is not just the reversal of the polarization process. It is noticed that just from the temperature dependent dielectric behavior is not enough to judge the processes of the E-field induced phase transition. △ Less

Submitted 25 June, 2002; originally announced June 2002.

Comments: 15 pages, 4 figures