Showing 1–50 of 72 results for author: Jin, C Q

Superconductivity Discovered in Niobium Polyhydride at High Pressures

Authors: X. He, C. L. Zhang, Z. W. Li, K. Lu, S. J. Zhang, B. S. Min, J. Zhang, L. C. Shi, S. M. Feng, Q. Q. Liu, J. Song, X. C. Wang, Y. Peng, L. H. Wang, V. B. Prakapenka, S. Chariton, H. Z. Liu, C. Q. Jin

Abstract: Niobium polyhydride was synthesized at high pressure and high temperature conditions by using diamond anvil cell combined with in situ high pressure laser heating techniques. High pressure electric transport experiments demonstrate that superconducting transition occurs with critical temperature(Tc) 42 K at 187 GPa. The shift of Tc as function of external applied magnetic field is in consistent to… ▽ More Niobium polyhydride was synthesized at high pressure and high temperature conditions by using diamond anvil cell combined with in situ high pressure laser heating techniques. High pressure electric transport experiments demonstrate that superconducting transition occurs with critical temperature(Tc) 42 K at 187 GPa. The shift of Tc as function of external applied magnetic field is in consistent to the nature of superconductivity while the upper critical field at zero temperature Hc2(0) is estimated to~16.8 Tesla while the GL coherent length ~57 angstrom is estimated. The structure investigation using synchrotron radiation implies that the observed superconductivity may come from Fm-3m phase of NbH3. △ Less

Submitted 5 September, 2024; v1 submitted 11 August, 2024; originally announced August 2024.

Journal ref: Materials Today Physics 40, 101298 (2024)

Superconductivity with Tc 116K discovered in antimony polyhydrides

Authors: K. Lu, X. He, C. L. Zhang, Z. W. Li, S. J. Zhang, B. S. Min, J. Zhang, J. F. Zhao, L. C. Shi, Y. Peng, S. M. Feng, Q. Q. Liu, J. Song, R. C. Yu, X. C. Wang, Y. Wang, M. Bykov, C. Q. Jin

Abstract: Superconductivity (SC) was experimentally observed for the first time in antimony polyhydride. The diamond anvil cell combined with laser heating system was used to synthesize the antimony polyhydride sample at high pressure and high temperature conditions. In-situ high pressure transport measurements as function of temperature with applied magnet are performed to study the SC properties. It was f… ▽ More Superconductivity (SC) was experimentally observed for the first time in antimony polyhydride. The diamond anvil cell combined with laser heating system was used to synthesize the antimony polyhydride sample at high pressure and high temperature conditions. In-situ high pressure transport measurements as function of temperature with applied magnet are performed to study the SC properties. It was found that the antimony polyhydride samples show superconducting transition with critical temperature Tc 116 K at 184 GPa. The investigation of SC at magnetic field revealed that the superconducting coherent length ~40 angstroms based on Ginzburg Landau (GL) equation. Antimony polyhydride superconductor has the second highest Tc in addition to sulfur hydride among the polyhydrides of elements from main group IIIA to VIIA in periodic table. △ Less

Submitted 21 August, 2024; v1 submitted 6 October, 2023; originally announced October 2023.

Journal ref: National Science Review 11, nwad241 (2024)

Superconductivity Observed in Tantalum Polyhydride at High Pressure

Authors: X. He, C. L. Zhang, Z. W. Li, S. J. Zhang, B. S. Min, J. Zhang, K. Lu, J. F. Zhao, L. C. Shi, Y. Peng, X. C. Wang, S. M. Feng, J. Song, L. H. Wang, V. B. Prakapenka, S. Chariton, H. Z. Liu, C. Q. Jin

Abstract: We report experimental discovery of tantalum polyhydride superconductor. It was synthesized at high pressure and high temperature conditions using diamond anvil cell combined with in-situ high pressure laser heating techniques. The superconductivity was investigated via resistance measurements at pressures. The highest superconducting transition temperature Tc was found to be ~30 K at 197 GPa in t… ▽ More We report experimental discovery of tantalum polyhydride superconductor. It was synthesized at high pressure and high temperature conditions using diamond anvil cell combined with in-situ high pressure laser heating techniques. The superconductivity was investigated via resistance measurements at pressures. The highest superconducting transition temperature Tc was found to be ~30 K at 197 GPa in the sample that was synthesized at the same pressure with ~2000 K heating. The transitions are shifted to low temperature upon applying magnetic fields that supports the superconductivity nature. The upper critical field at zero temperature μ0Hc2(0) of the superconducting phase is estimated to be ~20 T that corresponds to GL coherent length ~40 angstroms. Our results suggest that the superconductivity may arise from I-43d phase of TaH3. It is, for the first time to our best knowledge, experimental realization of superconducting hydrides for the VB group of transitional metals. △ Less

Submitted 7 June, 2023; v1 submitted 28 December, 2022; originally announced December 2022.

Comments: Published by CPL

Journal ref: Chinese Physics Letters 40, 057404 (2023) (Express Letter)

Superconductivity above 80 K in polyhydrides of hafnium

Authors: C. L. Zhang, X. He, Z. W. Li, S. J. Zhang, B. S. Min, J. Zhang, K. Lu, J. F. Zhao, L. C. Shi, Y. Peng, X. C. Wang, S. M. Feng, R. C. Yu, L. H. Wang, V. B. Prakapenka, S. Chariton, H. Z. Liu, C. Q. Jin

Abstract: Studies on polyhydrides are attracting growing attentions recently due to their potential high temperature superconductivity (SC). We here report the discovery of SC in hafnium polyhydrides at high pressures. The hafnium superhydrides are synthesized at high pressure and high temperature conditions using diamond anvil cell in combination with in-situ high pressure laser heating technique. The SC w… ▽ More Studies on polyhydrides are attracting growing attentions recently due to their potential high temperature superconductivity (SC). We here report the discovery of SC in hafnium polyhydrides at high pressures. The hafnium superhydrides are synthesized at high pressure and high temperature conditions using diamond anvil cell in combination with in-situ high pressure laser heating technique. The SC was investigated by in-situ high pressure resistance measurements in applied magnetic fields. A superconducting transition with onset Tc ~83 K was observed at 243 GPa. The upper critical field Hc2(0) was estimated to be 24 Tesla by GL theory and the consequent superconducting coherent length to be ~37 angstrom. Our results suggest that the superconducting phase is from C2/m-HfH14. This is the first 5d transition metal polyhydride superconductor with Tc above the liquid nitrogen temperature. △ Less

Submitted 22 August, 2022; v1 submitted 11 August, 2022; originally announced August 2022.

Comments: Accepted by the journal of Material Today Physics (2022)

Magnetic Properties and Electronic Configurations of Mn Ions in the Diluted Magnetic Semiconductor Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ Studied by X-ray Magnetic Circular Dichroism and Resonant Inelastic X-ray Scattering

Authors: H. Suzuki, G. Q. Zhao, J. Okamoto, S. Sakamoto, Z. -Y. Chen, Y. Nonaka, G. Shibata, K. Zhao, B. J. Chen, W. -B. Wu, F. -H. Chang, H. -J. Lin, C. -T. Chen, A. Tanaka, M. Kobayashi, Bo Gu, S. Maekawa, Y. J. Uemura, C. Q. Jin, D. J. Huang, A. Fujimori

Abstract: The magnetic properties and the electronic excitations of the new diluted magnetic semiconductor Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ have been studied by x-ray magnetic circular dichroism (XMCD) and resonant inelastic x-ray scattering (RIXS) at the Mn $L_{2,3}$ edge. The sum rule analysis of the XMCD spectra yields the net spin moment of $0.45μ_{\text{B}}$/Mn and the small orbital… ▽ More The magnetic properties and the electronic excitations of the new diluted magnetic semiconductor Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ have been studied by x-ray magnetic circular dichroism (XMCD) and resonant inelastic x-ray scattering (RIXS) at the Mn $L_{2,3}$ edge. The sum rule analysis of the XMCD spectra yields the net spin moment of $0.45μ_{\text{B}}$/Mn and the small orbital moment of $0.05μ_{\text{B}}$/Mn. This indicates that the Mn atoms are in the high-spin configurations of $d^{5}$, whereas the presence of competing ferromagnetic and antiferromagnetic interactions between the Mn ions reduces the net spin moment. RIXS spectra show broad peaks from 1 to 6 eV energy loss, which originate from the $d$-$d$ crystal field excitations of the Mn ions. Based on a comparison of the RIXS line shapes with those of Ga$_{1-x}$Mn$_{x}$As, we conclude that the ground state of Mn in Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ consists not only of the charge-transferred $3d^{5}\underline{L}$ electron configuration ($\underline{L}$: ligand hole) with weakly bound holes as in Ga$_{1-x}$Mn$_{x}$As, but also of the pure $3d^{5}$ configuration with free holes. △ Less

Submitted 31 January, 2022; originally announced January 2022.

Journal ref: J. Phys. Soc. Jpn. 91, 064710 (2022)

Superconductivity in Zirconium Polyhydrides with Tc above 70 K

Authors: C. L. Zhang, X. He, Z. W. Li, S. J. Zhang, S. M. Feng, X. C. Wang, R. C. Yu, C. Q. Jin

Abstract: We report the experimental discovery of superconductivity with critical temperature Tc above 70 K in zirconium polyhydrides, the record high so far for 3d transition metal hydrides. The superconducting zirconium polyhydrides are synthesized at the extreme conditions above 200 GPa & 2000 K by using diamond anvil cell combined with in-situ laser heating. Superconductivity was observed in the samples… ▽ More We report the experimental discovery of superconductivity with critical temperature Tc above 70 K in zirconium polyhydrides, the record high so far for 3d transition metal hydrides. The superconducting zirconium polyhydrides are synthesized at the extreme conditions above 200 GPa & 2000 K by using diamond anvil cell combined with in-situ laser heating. Superconductivity was observed in the samples maintained at the same pressure upon quenched from heating. △ Less

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

Journal ref: Science Bulletin 67, 907(2022); Short Communications;

Record High Tc Element Superconductivity Achieved In Titanium

Authors: C. L. Zhang, X. He, C. Liu, Z. W. Li, K. Lu, S. J. Zhang, S. M. Feng, X. C. Wang, Y. Peng, Y. W. Long, R. C. Yu, L. H. Wang, V. B. Prakapenka, S. Chariton, Q. Li, H. Z. Liu, C. F. Chen, C. Q. Jin

Abstract: It is challenging to search for high Tc superconductivity (SC) in transition metal elements wherein d electrons are usually not favored by conventional BCS theory. Here we report discovery of surprising SC up to 310 GPa with Tc above 20 K in wide pressure range from 108 GPa to 240 GPa in titanium. The maximum Tc^onset above 26 K and zero resistance Tc^zero of 21 K are record high values hitherto a… ▽ More It is challenging to search for high Tc superconductivity (SC) in transition metal elements wherein d electrons are usually not favored by conventional BCS theory. Here we report discovery of surprising SC up to 310 GPa with Tc above 20 K in wide pressure range from 108 GPa to 240 GPa in titanium. The maximum Tc^onset above 26 K and zero resistance Tc^zero of 21 K are record high values hitherto achieved among element superconductors. The Hc2(0) is estimated to be about 32 Tesla with coherence length 32 angstrom. The results show strong s-d transfer and d-band dominance, indicating correlation driven contributions to high Tc SC in dense titanium. This finding is in sharp contrast to the theoretical predications based on pristine electron-phonon coupling scenario. The study opens a fresh promising avenue for rational design and discovery of high Tc superconductors among simple materials via pressure tuned unconventional mechanism. △ Less

Submitted 21 September, 2022; v1 submitted 23 December, 2021; originally announced December 2021.

Journal ref: Nature Communications 13, 5411 (2022)

Superconductivity above 200 K Observed in Superhydrides of Calcium

Authors: Z. Li, X. He, C. L. Zhang, X. C. Wang, S. J. Zhang, Y. T. Jia, S. M. Feng, K. Lu, J. F. Zhao, J. Zhang, B. S. Min, Y. W. Long, R. C. Yu, L. H. Wang, M. Y. Ye, Z. S. Zhang, V. Prakapenka, S. Chariton, P. A. Ginsberg, J. Bass, S. H. Yuan, H. Z. Liu, C. Q. Jin

Abstract: Searching for superconductivity with Tc near room temperature is of great interest both for fundamental science & many potential applications. Here we report the experimental discovery of superconductivity with maximum critical temperature (Tc) above 210 K in calcium superhydrides, the new alkali earth hydrides experimentally showing superconductivity above 200 K in addition to sulfur hydride & ra… ▽ More Searching for superconductivity with Tc near room temperature is of great interest both for fundamental science & many potential applications. Here we report the experimental discovery of superconductivity with maximum critical temperature (Tc) above 210 K in calcium superhydrides, the new alkali earth hydrides experimentally showing superconductivity above 200 K in addition to sulfur hydride & rare-earth hydride system. The materials are synthesized at the synergetic conditions of 160~190 GPa and ~2000 K using diamond anvil cell combined with in-situ laser heating technique. The superconductivity was studied through in-situ high pressure electric conductance measurements in an applied magnetic field for the sample quenched from high temperature while maintained at high pressures. The upper critical field Hc(0) was estimated to be ~268 T while the GL coherent length is ~11 angstrom. The in-situ synchrotron X-ray diffraction measurements suggest that the synthesized calcium hydrides are primarily composed of CaH6 while there may also exist other calcium hydrides with different hydrogen contents. △ Less

Submitted 9 June, 2022; v1 submitted 31 March, 2021; originally announced March 2021.

Journal ref: Nature Communications 13, 2863 (2022)

Nature of the ferromagnetic-antiferromagnetic transition in Y$_{1-x}$La$_{x}$TiO$_{3}$

Authors: S. Hameed, S. El-Khatib, K. P. Olson, B. Yu, T. J. Williams, T. Hong, Q. Sheng, K. Yamakawa, J. Zang, Y. J. Uemura, G. Q. Zhao, C. Q. Jin, L. Fu, Y. Gu, F. Ning, Y. Cai, K. M. Kojima, J. W. Freeland, M. Matsuda, C. Leighton, M. Greven

Abstract: We explore the magnetically-ordered ground state of the isovalently-substituted Mott-insulator Y$_{1-x}$La$_{x}$TiO$_{3}$ for $x$ $\leq$ 0.3 via single crystal growth, magnetometry, neutron diffraction, x-ray magnetic circular dichroism (XMCD), muon spin rotation ($μ$SR) and small-angle neutron scattering (SANS). We find that the decrease in the magnetic transition temperature on approaching the f… ▽ More We explore the magnetically-ordered ground state of the isovalently-substituted Mott-insulator Y$_{1-x}$La$_{x}$TiO$_{3}$ for $x$ $\leq$ 0.3 via single crystal growth, magnetometry, neutron diffraction, x-ray magnetic circular dichroism (XMCD), muon spin rotation ($μ$SR) and small-angle neutron scattering (SANS). We find that the decrease in the magnetic transition temperature on approaching the ferromagnetic (FM) - antiferromagnetic (AFM) phase boundary at the La concentration $x_c$ $\approx$ 0.3 is accompanied by a strong suppression of both bulk and local ordered magnetic moments, along with a volume-wise separation into magnetically-ordered and paramagnetic regions. The thermal phase transition does not show conventional second-order behavior, since neither a clear signature of dynamic critical behavior nor a power-law divergence of the magnetic correlation length is found for the studied substitution range; this finding becomes increasingly obvious with substitution. Finally, from SANS and magnetometry measurements, we discern a crossover from easy-axis to easy-plane magneto-crystalline anisotropy with increasing La substitution. These results indicate complex changes in magnetic structure upon approaching the phase boundary. △ Less

Submitted 15 March, 2021; originally announced March 2021.

Comments: 16 pages, 16 figures

Journal ref: Phys. Rev. B 104, 024410 (2021)

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)

The Hund's Superconductor Li(Fe,Co)As

Authors: H. Miao, Y. L. Wang, J. -X. Yin, J. Zhang, S. Zhang, M. Z. Hasan, R. Yang, X. C. Wang, C. Q. Jin, T. Qian, H. Ding, H. -L. Lee, G. Kotliar

Abstract: We combine transport, angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy to investigate several low energy manifestations of the Hund coupling in a canonical FeSC family Li(Fe,Co)As. We determine the doping dependence of the coherent-incoherent crossover temperature and the quasi-particle effective mass enhancement in the normal state. Our tunneling spectroscopy result i… ▽ More We combine transport, angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy to investigate several low energy manifestations of the Hund coupling in a canonical FeSC family Li(Fe,Co)As. We determine the doping dependence of the coherent-incoherent crossover temperature and the quasi-particle effective mass enhancement in the normal state. Our tunneling spectroscopy result in the superconducting state supports the idea that superconductivity emerging from Hund's metal state displays a universal maximal superconducting gap vs transition temperature (2$Δ_{max}/k_{B}T_{c}$) value, which is independent of doping level and $T_{c}$. △ Less

Submitted 23 November, 2020; originally announced November 2020.

Journal ref: Phys. Rev. B 103, 054503 (2021)

Electron Mass Enhancement near a Nematic Quantum Critical Point in NaFe$_{1-x}$Co$_{x}$As

Authors: C. G. Wang, Z. Li, J. Yang, L. Y. Xing, G. Y. Dai, X. C. Wang, C. Q. Jin, R. Zhou, Guo-qing Zheng

Abstract: A magnetic order can be completely suppressed at zero temperature($T$), by doping carriers or applying pressure, at a quantum critical point(QCP), around which physical properties change drastically. However, the situation is unclear for an electronic nematic order that breaks rotation symmetry. Here we report nuclear magnetic resonance(NMR) studies on NaFe$_{1-x}$Co$_{x}$As where magnetic and nem… ▽ More A magnetic order can be completely suppressed at zero temperature($T$), by doping carriers or applying pressure, at a quantum critical point(QCP), around which physical properties change drastically. However, the situation is unclear for an electronic nematic order that breaks rotation symmetry. Here we report nuclear magnetic resonance(NMR) studies on NaFe$_{1-x}$Co$_{x}$As where magnetic and nematic transitions are well separated. The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state, which is related to London penetration depth $λ_{\rm L}$ that measures the electron mass $m^*$. We discovered two peaks in the doping dependence of $λ_{\rm L}^2$($T\sim$0); one at $x_{\rm M}$=0.027 where the spin-lattice relaxation rate shows quantum critical behavior, and another at $x_{\rm c}$=0.032 around which the nematic transition temperature extrapolates to zero and the electrical resistivity shows a $T$-linear variation. Our results indicate that a nematic QCP lies beneath the superconducting dome at $x_{\rm c}$ where $m^*$ is enhanced. The impact of the nematic fluctuations on superconductivity is discussed. △ Less

Submitted 20 October, 2018; originally announced October 2018.

Comments: 13 pages, 6 figures

Journal ref: Phys. Rev. Lett. 121, 167004 (2018)

Single Crystal Growth and Spin Polarization Measurements of Diluted Magnetic Semiconductor (BaK)(ZnMn)2As2

Authors: G. Q. Zhao, C. J. Lin, Z. Deng, G. X. Gu, S. Yu, X. C. Wang, Z. Z. Gong, Y. J. Uemura, Y. Q. Li, C. Q. Jin

Abstract: Recently a new type diluted magnetic semiconductor (BaK)(ZnMn)2As2 (BZA) with high Cure temperature (Tc) was discovered showing independent spin and charge doping mechanism. This makes BZA a promising material for spintronics devices. Here we report for the first time the successful growth of BZA single crystal. An Andreev reflection junction that can be used to evaluate spin polarization was fabr… ▽ More Recently a new type diluted magnetic semiconductor (BaK)(ZnMn)2As2 (BZA) with high Cure temperature (Tc) was discovered showing independent spin and charge doping mechanism. This makes BZA a promising material for spintronics devices. Here we report for the first time the successful growth of BZA single crystal. An Andreev reflection junction that can be used to evaluate spin polarization was fabricated based on the BZA single crystal, a 66% spin polarization of the BZA single crystal was hence obtained by Andreev reflection spectroscopy analysis. △ Less

Submitted 10 October, 2018; originally announced October 2018.

Journal ref: Scientific Reports 7, 14473 (2017)

Superconductivity in a unique type of copper oxide

Authors: W. M. Li, J. F. Zhao, L. P. Cao, Z. Hu, Q. Z. Huang, X. C. Wang, Y. Liu, G. Q. Zhao, J. Zhang, Q. Q. Liu, R. Z. Yu, Y. W. Long H. Wu, H. J. Lin, C. T. Chen, Z. Li, Z. Z. Gong, Z. Guguchia, J. S. Kim, G. R. Stewart, Y. J. Uemura. S. Uchida, C. Q Jin

Abstract: The mechanism of superconductivity in cuprates remains one of the big challenges of condensed matter physics.High Tc cuprates crystallize into layered perovskite structure featuring copper oxygen octahedral coordination. Due to the Jahn Teller effect in combination with the strong static Coulomb interaction, the octahedra in high Tc cuprates are elongated along the c axis, leading to a 3dx2-y2 orb… ▽ More The mechanism of superconductivity in cuprates remains one of the big challenges of condensed matter physics.High Tc cuprates crystallize into layered perovskite structure featuring copper oxygen octahedral coordination. Due to the Jahn Teller effect in combination with the strong static Coulomb interaction, the octahedra in high Tc cuprates are elongated along the c axis, leading to a 3dx2-y2 orbital at the top of the band structure wherein the doped holes reside.This scenario gives rise to two dimensional characteristics in high Tc cuprates that favor d wave pairing symmetry. Here we report superconductivity in a cuprate Ba2CuO4-y wherein the local octahedron is in a very exceptional compressed version.The Ba2CuO4-y compound was synthesized at high pressure at high temperatures, and shows bulk superconductivity with critical temperature Tc above 70 K at ambient conditions. This superconducting transition temperature is more than 30 K higher than the Tc for the isostructural counterparts based on classical La2CuO4. X-ray absorption measurements indicate the heavily doped nature of the Ba2CuO4-y superconductor. In compressed octahedron the 3d3z2-r2 orbital will be lifted above the 3dx2-y2 orbital, leading to significant three dimensional nature in addition to the conventional 3dx2-y2 orbital. This work sheds important new light on advancing our comprehensive understanding of the superconducting mechanism of high Tc in cuprate materials. △ Less

Submitted 2 August, 2019; v1 submitted 28 August, 2018; originally announced August 2018.

Comments: Total 28 pages incuding 9 Figures

Decoupled Pairing Amplitude and Electronic Coherence in Iron-Based Superconductors

Authors: H. Miao, W. H. Brito, Z. P. Yin, R. D. Zhong, G. D. Gu, P. D. Johnson, M. P. M. Dean, S. Choi, G. Kotliar, W. Ku, X. C. Wang, C. Q. Jin, S. -F. Wu, T. Qian, H. Ding

Abstract: Here we use angle-resolved photoemission spectroscopy to study superconductivity that emerges in two extreme cases, from a Fermi liquid phase (LiFeAs) and an incoherent bad-metal phase (FeTe0.55Se0.45). We find that although the electronic coherence can strongly reshape the single-particle spectral function in the superconducting state, it is decoupled from the maximum superconducting pairing ampl… ▽ More Here we use angle-resolved photoemission spectroscopy to study superconductivity that emerges in two extreme cases, from a Fermi liquid phase (LiFeAs) and an incoherent bad-metal phase (FeTe0.55Se0.45). We find that although the electronic coherence can strongly reshape the single-particle spectral function in the superconducting state, it is decoupled from the maximum superconducting pairing amplitude, which shows a universal scaling that is valid for all FeSCs. Our observation excludes pairing scenarios in the BCS and the BEC limit for FeSCs and calls for a universal strong coupling pairing mechanism for the FeSCs. △ Less

Submitted 20 April, 2018; originally announced April 2018.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 98, 020502 (2018)

Weak doping dependence of the antiferromagnetic coupling between nearest-neighbor Mn$^{2+}$ spins in (Ba$_{1-x}$K$_x$)(Zn$_{1-y}$Mn$_y$)$_2$As$_2$

Authors: M. A. Surmach, B. J. Chen Z. Deng, C. Q. Jin, J. K. Glasbrenner, I. I. Mazin, A. Ivanov, D. S. Inosov

Abstract: Dilute magnetic semiconductors (DMS) are nonmagnetic semiconductors doped with magnetic transition metals. The recently discovered DMS material (Ba$_{1-x}$K$_{x}$)(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ offers a unique and versatile control of the Curie temperature, $T_{\mathrm{C}}$, by decoupling the spin (Mn$^{2+}$, $S=5/2$) and charge (K$^{+}$) doping in different crystallographic layers. In an atte… ▽ More Dilute magnetic semiconductors (DMS) are nonmagnetic semiconductors doped with magnetic transition metals. The recently discovered DMS material (Ba$_{1-x}$K$_{x}$)(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ offers a unique and versatile control of the Curie temperature, $T_{\mathrm{C}}$, by decoupling the spin (Mn$^{2+}$, $S=5/2$) and charge (K$^{+}$) doping in different crystallographic layers. In an attempt to describe from first-principles calculations the role of hole doping in stabilizing ferromagnetic order, it was recently suggested that the antiferromagnetic exchange coupling $J$ between the nearest-neighbor Mn ions would experience a nearly twofold suppression upon doping 20\% of holes by potassium substitution. At the same time, further-neighbor interactions become increasingly ferromagnetic upon doping, leading to a rapid increase of $T_{\mathrm{C}}$. Using inelastic neutron scattering, we have observed a localized magnetic excitation at about 13 meV, associated with the destruction of the nearest-neighbor Mn-Mn singlet ground state. Hole doping results in a notable broadening of this peak, evidencing significant particle-hole damping, but with only a minor change in the peak position. We argue that this unexpected result can be explained by a combined effect of superexchange and double-exchange interactions. △ Less

Submitted 8 February, 2018; originally announced February 2018.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 97, 104418 (2018)

Ba(Zn,Co)2As2: a II-II-V Diluted Ferromagnetic Semiconductor with N-type Carriers

Authors: Shengli Guo, Huiyuan Man, Cui Ding, Yao Zhao, Licheng Fu, Yilun Gu, Guoxiang Zhi, Benjamin A. Frandsen, Sky C. Cheung, Zurab Guguchia, Kohtaro Yamakawa, Bin Chen, Hangdong Wang, Z. Deng, C. Q. Jin, Yasutomo J. Uemura, Fanlong Ning

Abstract: Diluted ferromagnetic semiconductors (DMSs) that combine the properties of semiconductors with ferromagnetism have potential application in spin-sensitive electronics (spintronics) devices. The search for DMS materials exploded after the observation of ferromagnetic ordering in III-V (Ga,Mn)As films. Recently, a series of DMS compounds isostructural to iron-based superconductors have been reported… ▽ More Diluted ferromagnetic semiconductors (DMSs) that combine the properties of semiconductors with ferromagnetism have potential application in spin-sensitive electronics (spintronics) devices. The search for DMS materials exploded after the observation of ferromagnetic ordering in III-V (Ga,Mn)As films. Recently, a series of DMS compounds isostructural to iron-based superconductors have been reported. Among them, the highest Curie temperature $T_C$ of 230 K has been achieved in (Ba,K)(Zn,Mn)$_2$As$_2$. However, most DMSs, including (Ga,Mn)As, are p-type, i.e., the carriers that mediate ferromagnetism are holes. For practical applications, DMS with n-type carriers are also advantageous. Here we report the successful synthesis of a II-II-V diluted ferromagnetic semiconductor with n-type carriers, Ba(Zn,Co)$_2$As$_2$. Magnetization measurements show that the ferromagnetic transition occurs up to $T_{C} \sim$ 45 K. Hall effect and Seebeck effect measurements jointly confirm that the dominant carriers are electrons. Through muon spin relaxation ($μ$SR), a volume sensitive magnetic probe, we have also confirmed that the ferromagnetism in Ba(Zn,Co)$_2$As$_2$ is intrinsic and the internal field is static. △ Less

Submitted 18 December, 2017; originally announced December 2017.

Comments: 8 pages, 6 figures. Developed work of 1403.4019

Journal ref: Phys. Rev. B 99, 155201 (2019)

Magneto-elastic coupling in Fe-based superconductors

Authors: S. -F. Wu, W. -L. Zhang, V. K. Thorsmølle, G. F. Chen, G. T. Tan, P. C. Dai, Y. G. Shi, C. Q. Jin, T. Shibauchi, S. Kasahara, Y. Matsuda, A. S. Sefat, H. Ding, P. Richard, G. Blumberg

Abstract: We used polarization-resolved Raman scattering to study the magneto-elastic coupling in the parent compounds of several families of Fe-based superconductors (BaFe2As2, EuFe2As2, NaFeAs, LiFeAs, FeSe and LaFeAsO). We observe an emergent Ag-symmetry As phonon mode in the XY scattering geometry whose intensity is significantly enhanced below the magneto-structural transition only for compounds showin… ▽ More We used polarization-resolved Raman scattering to study the magneto-elastic coupling in the parent compounds of several families of Fe-based superconductors (BaFe2As2, EuFe2As2, NaFeAs, LiFeAs, FeSe and LaFeAsO). We observe an emergent Ag-symmetry As phonon mode in the XY scattering geometry whose intensity is significantly enhanced below the magneto-structural transition only for compounds showing magnetic ordering. We conclude that the small lattice anisotropy is insufficient to induce the in-plane electronic polarizability anisotropy necessary for the observed phonon intensity enhancement, and interpret this enhancement below the Neel temperature in terms of the anisotropy of the magnetic moment and magneto-elastic coupling. We evidence a Fano line- shape in the XY scattering geometry resulting from a strong coupling between the Ag (As) phonon mode and the B2g symmetry-like electronic continuum. Strong electron-phonon coupling may be relevant to superconductivity. △ Less

Submitted 5 December, 2017; originally announced December 2017.

Journal ref: Phys. Rev. Research 2, 033140 (2020)

Room-Temperature Ferromagnetic Topological Phase in CrO$_{2}$: From Tripe Fermions to Weyl Fermions

Authors: R. Wang, Y. J. Jin, L. Duan, J. Z. Zhao, H. Xu, C. Q. Jin

Abstract: Ferromagnetic topological semimetals due to their band topology co-existing with intrinsic magnetization exerted important influences on early study of topological fermions. However, they have not been observed in experiments up to now. In this work, we propose that rutile CrO$_{2}$, a widely used half-metallic ferromagnetic material in magnetic recording taps, exhibits unexpected ferromagnetic to… ▽ More Ferromagnetic topological semimetals due to their band topology co-existing with intrinsic magnetization exerted important influences on early study of topological fermions. However, they have not been observed in experiments up to now. In this work, we propose that rutile CrO$_{2}$, a widely used half-metallic ferromagnetic material in magnetic recording taps, exhibits unexpected ferromagnetic topological features. Using first-principles calculations and symmetry analysis, we reveal that rutile CrO$_2$ hosts the triple nodal points in the absence of spin orbital coupling (SOC). By taking into account of SOC, each triple nodal point splits into two Weyl points, which are located on the magnetic axis with four-fold rotational symmetry. Notably, the Fermi arcs and accompanying quasiparticle interference patterns are clearly visible, which facilitate experimental observations. In addition, we find that another experimentally synthesized CrO$_{2}$ in CaCl$_2$ structure, also hosts the topologically nontrivial ferromagnetic phase. Our findings substantially advance the experimental realization of ferromagnetic topological semimetals. More importantly, room-temperature time-reversal-breaking Weyl fermions in CrO$_{2}$ may result in promising applications related to the topological phenomena in industry. △ Less

Submitted 29 November, 2017; originally announced November 2017.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 97, 195157 (2018)

Rejuvenation of metallic glasses under high pressure

Authors: C. Wang, Z. Z. Yang, T. Ma, Y. T. Sun, Y. Y. Yin, Y. Gong, L. Gu, P. Wen, P. W. Zhu, Y. W. Long, X. H. Yu, C. Q. Jin, W. H. Wang, H. Y. Bai

Abstract: Modulating energy states of metallic glasses (MGs) is significant in understanding the nature of glasses and control their properties. In this study, we show that rejuvenation in enthalpy can be achieved and preserved in bulk MGs by using high pressure (HP) annealing, which is a controllable method to continuously alter the energy states of MGs. Contrary to the decrease in enthalpy by conventional… ▽ More Modulating energy states of metallic glasses (MGs) is significant in understanding the nature of glasses and control their properties. In this study, we show that rejuvenation in enthalpy can be achieved and preserved in bulk MGs by using high pressure (HP) annealing, which is a controllable method to continuously alter the energy states of MGs. Contrary to the decrease in enthalpy by conventional annealing at ambient pressure, such rejuvenation can occur and be enhanced by increasing both of annealing temperature and pressure. By using double aberration corrected scanning transmission electron microscopy, it is revealed that the rejuvenation, which is attributed to coupling effect of high pressure and high temperature, originates from the microstructural change that involves "negative flow units" with a higher atomic packing density compared to that of the elastic matrix of MGs. The results demonstrate that HP annealing is an effective way to rejuvenate MGs into higher energy states, and it may assist in understanding the microstructural origin of the rejuvenation in MGs. △ Less

Submitted 26 September, 2016; originally announced September 2016.

Comments: 24 pages, 8 figures

Orbital-differentiated coherence-incoherence crossover identified by photoemission spectroscopy in LiFeAs

Authors: H. Miao, Z. P. Yin, S. F. Wu, J. M. Li, J. Ma, B. -Q. Lv, X. P. Wang, T. Qian, P. Richard, L. -Y. Xing, X. -C. Wang, C. Q. Jin, K. Haule, G. Kotliar, H. Ding

Abstract: In the iron-based superconductors (FeSCs), orbital differentiation is an important phenomenon, whereby correlations stronger on the dxy orbital than on the dxz/yz orbital yield quasi-particles with dxy} orbital character having larger mass renormalization and abnormal temperature evolution. However, the physical origin of this orbital differentiation is debated between the Hund's coupling induced… ▽ More In the iron-based superconductors (FeSCs), orbital differentiation is an important phenomenon, whereby correlations stronger on the dxy orbital than on the dxz/yz orbital yield quasi-particles with dxy} orbital character having larger mass renormalization and abnormal temperature evolution. However, the physical origin of this orbital differentiation is debated between the Hund's coupling induced unbinding of spin and orbital degrees of freedom and the Hubbard interaction instigated orbital selective Mott transition. Here we use angle-resolved photoemission spectroscopy to identify an orbital-dependent correlation-induced quasi-particle (QP) anomaly in LiFeAs. The excellent agreement between our photoemission measurements and first-principles many-body theory calculations shows that the orbital-differentiated QP lifetime anomalies in LiFeAs are controlled by the Hund's coupling. △ Less

Submitted 11 July, 2016; originally announced July 2016.

Journal ref: Phys. Rev. B 94, 201109 (2016)

Observation of non-Fermi liquid behavior in hole-doped LiFe$_{1-x}$V$_x$As

Authors: L. Y. Xing, X. Shi, P. Richard, X. C. Wang, Q. Q. Liu, B. Q. Lv, J. -Z. Ma, B. B. Fu, L. -Y. Kong, H. Miao, T. Qian, T. K. Kim, M. Hoesch, H. Ding, C. Q. Jin

Abstract: We synthesized a series of V-doped LiFe$_{1-x}$V$_x$As single crystals. The superconducting transition temperature $T_c$ of LiFeAs decreases rapidly at a rate of 7 K per 1\% V. The Hall coefficient of LiFeAs switches from negative to positive with 4.2\% V doping, showing that V doping introduces hole carriers. This observation is further confirmed by the evaluation of the Fermi surface volume meas… ▽ More We synthesized a series of V-doped LiFe$_{1-x}$V$_x$As single crystals. The superconducting transition temperature $T_c$ of LiFeAs decreases rapidly at a rate of 7 K per 1\% V. The Hall coefficient of LiFeAs switches from negative to positive with 4.2\% V doping, showing that V doping introduces hole carriers. This observation is further confirmed by the evaluation of the Fermi surface volume measured by angle-resolved photoemission spectroscopy (ARPES), from which a 0.3 hole doping per V atom introduced is deduced. Interestingly, the introduction of holes does not follow a rigid band shift. We also show that the temperature evolution of the electrical resistivity as a function of doping is consistent with a crossover from a Fermi liquid to a non-Fermi liquid. Our ARPES data indicate that the non-Fermi liquid behavior is mostly enhanced when one of the hole $d_{xz}/d_{yz}$ Fermi surfaces is well nested by the antiferromagnetic wave vector to the inner electron Fermi surface pocket with the $d_{xy}$ orbital character. The magnetic susceptibility of LiFe$_{1-x}$V$_x$As suggests the presence of strong magnetic impurities following V doping, thus providing a natural explanation to the rapid suppression of superconductivity upon V doping. △ Less

Submitted 29 September, 2016; v1 submitted 14 April, 2016; originally announced April 2016.

Comments: 7 pages, 5 figures. See published version for the latest update

Journal ref: Phys. Rev. B 94, 094524 (2016)

Superconductivity in HfTe5 Induced via Pressures

Authors: Y. Liu, Y. J. Long, L. X. Zhao, S. M. Nie, S. J. Zhang, Y. X. Weng, M. L. Jin, W. M. Li, Q. Q. Liu, Y. W. Long, R. C. Yu, X. L. Fen, Q. Li, H. M. Weng, X. Dai, Z. Fang, G. F. Chen, C. Q. Jin

Abstract: Recently, ZrTe5 and HfTe5 are theoretically studied to be the most promising layered topological insulators since they are both interlayer weakly bonded materials and also with a large bulk gap in the single layer. It paves a new way for the study of novel topological quantum phenomenon tuned via external parameters. Here, we report the discovery of superconductivity and properties evolution in Hf… ▽ More Recently, ZrTe5 and HfTe5 are theoretically studied to be the most promising layered topological insulators since they are both interlayer weakly bonded materials and also with a large bulk gap in the single layer. It paves a new way for the study of novel topological quantum phenomenon tuned via external parameters. Here, we report the discovery of superconductivity and properties evolution in HfTe5 single crystal induced via pressures. Our experiments indicated that anomaly resistance peak moves to low temperature first before reverses to high temperature followed by disappearance which is opposite to the low pressure effect on ZrTe5. HfTe5 became superconductive above ~5.5 GPa up to at least 35 GPa in the measured range. The highest superconducting transition temperature (Tc) around 5 K was achieved at 20 GPa. High pressure Raman revealed that new modes appeared around pressure where superconductivity occurs. Crystal structure studies shown that the superconductivity is related to the phase transition from Cmcm structure to monoclinic C2/m structure. The second phase transition from C2/m to P-1 structure occurs at 12 GPa. The combination of transport, structure measurement and theoretical calculations enable a completely phase diagram of HfTe5 at high pressures. △ Less

Submitted 1 March, 2016; originally announced March 2016.

Comments: 19 pages+8 Figures with supplementary materials

Orbital selectivity of layer resolved tunneling on iron superconductor Ba0.6K0.4Fe2As2

Authors: J. -X. Yin, X. -X. Wu, Jian Li, Zheng Wu, J. -H. Wang, C. -S. Ting, P. -H. Hor, X. J. Liang, C. L. Zhang, P. C. Dai, X. C. Wang, C. Q. Jin, G. F. Chen, J. P. Hu, Z. -Q. Wang, Ang Li, H. Ding, S. H. Pan

Abstract: We use scanning tunneling microscopy/spectroscopy (STM/S) to elucidate the Cooper pairing of the iron pnictide superconductor Ba0.6K0.4Fe2As2. By a cold-cleaving technique, we obtain atomically resolved termination surfaces with different layer identities. Remarkably, we observe that the low-energy tunneling spectrum related to superconductivity has an unprecedented dependence on the layer-identit… ▽ More We use scanning tunneling microscopy/spectroscopy (STM/S) to elucidate the Cooper pairing of the iron pnictide superconductor Ba0.6K0.4Fe2As2. By a cold-cleaving technique, we obtain atomically resolved termination surfaces with different layer identities. Remarkably, we observe that the low-energy tunneling spectrum related to superconductivity has an unprecedented dependence on the layer-identity. By cross-referencing with the angle-revolved photoemission results and the tunneling data of LiFeAs, we find that tunneling on each termination surface probes superconductivity through selecting distinct Fe-3d orbitals. These findings imply the real-space orbital features of the Cooper pairing in the iron pnictide superconductors, and propose a new and general concept that, for complex multi-orbital material, tunneling on different terminating layers can feature orbital selectivity. △ Less

Submitted 21 August, 2020; v1 submitted 16 February, 2016; originally announced February 2016.

Journal ref: Phys. Rev. B 102, 054515 (2020)

Orbital Order and Spin Nematicity in the Tetragonal Phase of Electron-doped Iron-Pnictides NaFe$_{1-x}$Co$_{x}$As

Authors: R. Zhou, L. Y. Xing, X. C. Wang, C. Q. Jin, Guo-qing Zheng

Abstract: In copper-oxide and iron-based high temperature (high-$T_{\rm c}$) superconductors, many physical properties exhibit in-plane anisotropy, which is believed to be caused by a rotational symmetry-breaking nematic order, whose origin and its relationship to superconductivity remain elusive. In many iron-pnictides, a tetragonal-to-orthorhombic structural transition temperature $T_{\rm s}$ coincides wi… ▽ More In copper-oxide and iron-based high temperature (high-$T_{\rm c}$) superconductors, many physical properties exhibit in-plane anisotropy, which is believed to be caused by a rotational symmetry-breaking nematic order, whose origin and its relationship to superconductivity remain elusive. In many iron-pnictides, a tetragonal-to-orthorhombic structural transition temperature $T_{\rm s}$ coincides with the magnetic transition temperature $T_{\rm N}$, making the orbital and spin degrees of freedom highly entangled. NaFeAs is a system where $T_{\rm s}$ = 54 K is well separated from $T_{\rm N}$ = 42 K, which helps simplify the experimental situation. Here we report nuclear magnetic resonance (NMR) measurements on NaFe$_{1-x}$Co$_x$As (0 $\leq x \leq$ 0.042) that revealed orbital and spin nematicity occurring at a temperature $T^{\rm *}$ far above $T_{\rm s}$ in the tetragonal phase. We show that the NMR spectra splitting and its evolution can be explained by an incommensurate orbital order that sets in below $T^{\rm *}$ and becomes commensurate below $T_{\rm s}$, which brings about the observed spin nematicity. △ Less

Submitted 20 January, 2016; originally announced January 2016.

Comments: 14 pages, 5 figures, to appear in Phys.Rev.B (Rapid Commun.)

Journal ref: Physical Review B 93, 060502(R) (2016)

Reversible Tuning of the Collapsed Tetragonal Phase Transition in CaFe2As2 by Separate Control of Chemical Pressure and Electron Doping

Authors: K. Zhao, C. Stingl, R. S. Manna, C. Q. Jin, P. Gegenwart

Abstract: Single crystals of Ca(Fe1-xRux)2As2 (0<x<0.065) and Ca1-yLay(Fe0.973Ru0.027)2As2 (0<y<0.2) have been synthesized and studied with respect to their structural, electronic and magnetic properties. The partial substitution of Fe by Ru induces a decrease of the c-axis constant leading for x<0.023 to a suppression of the coupled magnetic and structural (tetragonal to orthorhombic) transitions. At x_cr=… ▽ More Single crystals of Ca(Fe1-xRux)2As2 (0<x<0.065) and Ca1-yLay(Fe0.973Ru0.027)2As2 (0<y<0.2) have been synthesized and studied with respect to their structural, electronic and magnetic properties. The partial substitution of Fe by Ru induces a decrease of the c-axis constant leading for x<0.023 to a suppression of the coupled magnetic and structural (tetragonal to orthorhombic) transitions. At x_cr=0.023 a first order transition to a collapsed tetragonal (CT) phase is found, which behaves like a Fermi liquid and which is stabilized by further increase of x. The absence of superconductivity near x_cr is consistent with truly hydrostatic pressure experiments on undoped CaFe2As2. Starting in the CT regime at x=0.027 we investigate the additional effect of electron doping by partial replacement of Ca by La. Most remarkably, with increasing y the CT phase transition is destabilized and the system is tuned back into a tetragonal ground state at y>0.08. This effect is ascribed to a weakening of interlayer As-As bonds by electron doping. Upon further electron doping filamentary superconductivity with Tc of 41 K at y=0.2 is observed. △ Less

Submitted 23 November, 2015; v1 submitted 13 October, 2015; originally announced October 2015.

Comments: 15 Pages including figures, PRB in press

Journal ref: Phys. Rev. B 92, 235132 (2015)

Coexistence of Clean- and Dirty-limit Superconductivity in LiFeAs

Authors: Y. M. Dai, H. Miao, L. Y. Xing, X. C. Wang, C. Q. Jin, H. Ding, C. C. Homes

Abstract: The optical properties of LiFeAs with $T_c \simeq$ 18 K have been determined in the normal and superconducting states. The superposition of two Drude components yields a good description of the low-frequency optical response in the normal state. Below $T_c$, the optical conductivity reveals two isotropic superconducting gaps with $Δ_{1} \simeq 2.9$ $\pm$ 0.2 meV and $Δ_{2} \simeq 5.5$ $\pm$ 0.4 me… ▽ More The optical properties of LiFeAs with $T_c \simeq$ 18 K have been determined in the normal and superconducting states. The superposition of two Drude components yields a good description of the low-frequency optical response in the normal state. Below $T_c$, the optical conductivity reveals two isotropic superconducting gaps with $Δ_{1} \simeq 2.9$ $\pm$ 0.2 meV and $Δ_{2} \simeq 5.5$ $\pm$ 0.4 meV. A comparison between the superconducting-state Mattis-Bardeen and the normal-state Drude components, in combination with a spectral weight analysis, indicates that the spectral weight associated with a band which has a very small scattering rate is fully transferred to the superfluid weight upon the superconducting condensate. These observations provide clear evidence for the coexistence of clean- and dirty-limit superconductivity in LiFeAs. △ Less

Submitted 8 February, 2016; v1 submitted 16 September, 2015; originally announced September 2015.

Comments: 6 pages, 4 figures, Supplementary Material available

Journal ref: Phys. Rev. B 93, 054508 (2016)

Fermi Surfaces and $p$-$d$ Hybridization in the Diluted Magnetic Semiconductor Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ Studied by Soft X-ray Angle Resolved Photoemission Spectroscopy

Authors: H. Suzuki, G. Q. Zhao, K. Zhao, B. J. Chen, M. Horio, K. Koshiishi, J. Xu, M. Kobayashi, M. Minohara, E. Sakai, K. Horiba, H. Kumigashira, Bo Gu, S. Maekawa, Y. J. Uemura, C. Q. Jin, A. Fujimori

Abstract: The electronic structure of the new diluted magnetic semiconductor Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ ($x=0.30$, $y=0.15$) in single crystal form has been investigated by angle-resolved photoemission spectroscopy (ARPES). %High density of states of nondispersive bands composed of the Zn $3d$ orbitals are observed with ultraviolet incident light. Measurements with soft x-rays clari… ▽ More The electronic structure of the new diluted magnetic semiconductor Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ ($x=0.30$, $y=0.15$) in single crystal form has been investigated by angle-resolved photoemission spectroscopy (ARPES). %High density of states of nondispersive bands composed of the Zn $3d$ orbitals are observed with ultraviolet incident light. Measurements with soft x-rays clarify the host valence-band electronic structure primarily composed of the As $4p$ states. Two hole pockets around the $Γ$ point, a hole corrugated cylinder surrounding the $Γ$ and Z points, and an electron pocket around the Z point are observed, and explain the metallic transport of Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$. This is contrasted with Ga$_{1-x}$Mn$_{x}$As (GaMnAs), where it is located above the As $4p$ valence-band maximum (VBM) and no Fermi surfaces have been clearly identified. Resonance soft x-ray ARPES measurements reveal a nondispersive (Kondo resonance-like) Mn $3d$ impurity band near the Fermi level, as in the case of GaMnAs. However, the impurity band is located well below the VBM, unlike the impurity band in GaMnAs, which is located around and above the VBM. We conclude that, while the strong hybridization between the Mn $3d$ and the As $4p$ orbitals plays an important role in creating the impurity band and inducing high temperature ferromagnetism in both systems, the metallic transport may predominantly occur in the host valence band in Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ and in the impurity band in GaMnAs. △ Less

Submitted 19 September, 2015; v1 submitted 15 September, 2015; originally announced September 2015.

Journal ref: Phys. Rev. B 92, 235120 (2015)

Spin-Fluctuation-Induced Non-Fermi-Liquid Behavior with suppressed superconductivity in LiFe$_{1-x}$Co$_{x}$As

Authors: Y. M. Dai, H. Miao, L. Y. Xing, X. C. Wang, P. S. Wang, H. Xiao, T. Qian, P. Richard, X. G. Qiu, W. Yu, C. Q. Jin, Z. Wang, P. D. Johnson, C. C. Homes, H. Ding

Abstract: A series of LiFe$_{1-x}$Co$_{x}$As compounds with different Co concentrations have been studied by transport, optical spectroscopy, angle-resolved photoemission spectroscopy and nuclear magnetic resonance. We observed a Fermi liquid to non-Fermi liquid to Fermi liquid (FL-NFL-FL) crossover alongside a monotonic suppression of the superconductivity with increasing Co content. In parallel to the FL-… ▽ More A series of LiFe$_{1-x}$Co$_{x}$As compounds with different Co concentrations have been studied by transport, optical spectroscopy, angle-resolved photoemission spectroscopy and nuclear magnetic resonance. We observed a Fermi liquid to non-Fermi liquid to Fermi liquid (FL-NFL-FL) crossover alongside a monotonic suppression of the superconductivity with increasing Co content. In parallel to the FL-NFL-FL crossover, we found that both the low-energy spin fluctuations and Fermi surface nesting are enhanced and then diminished, strongly suggesting that the NFL behavior in LiFe$_{1-x}$Co$_{x}$As is induced by low-energy spin fluctuations which are very likely tuned by Fermi surface nesting. Our study reveals a unique phase diagram of LiFe$_{1-x}$Co$_{x}$As where the region of NFL is moved to the boundary of the superconducting phase, implying that they are probably governed by different mechanisms. △ Less

Submitted 16 September, 2015; v1 submitted 3 May, 2015; originally announced May 2015.

Comments: 10 pages, 11 figures

Journal ref: Phys. Rev. X 5, 031035 (2015)

Pressure-induced superconductivity in the three-dimensional Dirac semimetal Cd3As2

Authors: L. P. He, Y. T. Jia, S. J. Zhang, X. C. Hong, C. Q. Jin, S. Y. Li

Abstract: The recently discovered Dirac and Weyl semimetals are new members of topological materials. Starting from them, topological superconductivity may be achieved, e.g. by carrier doping or applying pressure. Here we report high-pressure resistance and X-ray diffraction study of the three-dimensional topological Dirac semimetal Cd3As2. Superconductivity with Tc ~ 2.0 K is observed at 8.5 GPa. The Tc ke… ▽ More The recently discovered Dirac and Weyl semimetals are new members of topological materials. Starting from them, topological superconductivity may be achieved, e.g. by carrier doping or applying pressure. Here we report high-pressure resistance and X-ray diffraction study of the three-dimensional topological Dirac semimetal Cd3As2. Superconductivity with Tc ~ 2.0 K is observed at 8.5 GPa. The Tc keeps increasing to about 4.0 K at 21.3 GPa, then shows a nearly constant pressure dependence up to the highest pressure 50.9 GPa. The X-ray diffraction measurements reveal a structure phase transition around 3.5 GPa. Our observation of superconductivity in pressurized topological Dirac semimetal Cd3As2 provides a new candidate for topological superconductor, as argued in a recent point contact study and a theoretical work. △ Less

Submitted 11 April, 2016; v1 submitted 9 February, 2015; originally announced February 2015.

Comments: 20 pages, 5 figures

Journal ref: npj Quantum Materials 1, 16014 (2016)

(Sr,Na)(Zn,Mn)2As2: A new diluted ferromagnetic semiconductor with the hexagonal CaAl2Si2 type structure

Authors: B. J. Chen, K. Zhao, Z. Deng, W. Han, J. L. Zhu, X. C. Wang, Q. Q. Liu, B. Frandsen, L. Liu, S. Cheung, F. L. Ning, T. J. S. Munsie, T. Medina, G. M. Luke, J. P. Carl, J. Munevar, Y. J. Uemura, C. Q. Jin

Abstract: A new diluted ferromagnetic semiconductor (Sr,Na)(Zn,Mn)2As2 is reported, in which charge and spin doping are decoupled via Sr/Na and Zn/Mn substitutions, respectively, being distinguished from classic (Ga,Mn)As where charge & spin doping are simultaneously integrated. Different from the recently reported ferromagnetic (Ba,K)(Zn,Mn)2As2, this material crystallizes into the hexagonal CaAl2Si2-type… ▽ More A new diluted ferromagnetic semiconductor (Sr,Na)(Zn,Mn)2As2 is reported, in which charge and spin doping are decoupled via Sr/Na and Zn/Mn substitutions, respectively, being distinguished from classic (Ga,Mn)As where charge & spin doping are simultaneously integrated. Different from the recently reported ferromagnetic (Ba,K)(Zn,Mn)2As2, this material crystallizes into the hexagonal CaAl2Si2-type structure. Ferromagnetism with a Curie temperature up to 20 K has been observed from magnetization. The muon spin relaxation measurements suggest that the exchange interaction between Mn moments of this new system could be different to the earlier DMS systems. This system provides an important means for studying ferromagnetism in diluted magnetic semiconductors. △ Less

Submitted 17 November, 2014; originally announced November 2014.

Comments: 18 Pages 4 Figures

Journal ref: Phys. Rev. B 90, 155202(2014)

Superconductivity in Strong Spin Orbital Coupling Compound Sb2Se3

Authors: P. P. Kong, F. Sun, L. Y. Xing, J. Zhu, S. J. Zhang, W. M. Li, Q. Q. Liu, X. C. Wang, S. M. Feng, X. H. Yu, J. L. Zhu, R. C. Yu, W. G. Yang, G. Y. Shen, Y. S. Zhao, R. Ahuja, H. K. Mao, C. Q. Jin

Abstract: Recently, A2B3 type strong spin orbital coupling compounds such as Bi2Te3, Bi2Se3 and Sb2Te3 were theoretically predicated to be topological insulators and demonstrated through experimental efforts. The counterpart compound Sb2Se3 on the other hand was found to be topological trivial, but further theoretical studies indicated that the pressure might induce Sb2Se3 into a topological nontrivial stat… ▽ More Recently, A2B3 type strong spin orbital coupling compounds such as Bi2Te3, Bi2Se3 and Sb2Te3 were theoretically predicated to be topological insulators and demonstrated through experimental efforts. The counterpart compound Sb2Se3 on the other hand was found to be topological trivial, but further theoretical studies indicated that the pressure might induce Sb2Se3 into a topological nontrivial state. Here, we report on the discovery of superconductivity in Sb2Se3 single crystal induced via pressure. Our experiments indicated that Sb2Se3 became superconductive at high pressures above 10 GPa proceeded by a pressure induced insulator to metal like transition at ~3 GPa which should be related to the topological quantum transition. The superconducting transition temperature (TC) increased to around 8.0 K with pressure up to 40 GPa while it keeps ambient structure. High pressure Raman revealed that new modes appeared around 10 GPa and 20 GPa, respectively, which correspond to occurrence of superconductivity and to the change of TC slop as the function of high pressure in conjunction with the evolutions of structural parameters at high pressures. △ Less

Submitted 17 November, 2014; originally announced November 2014.

Comments: 30 Pages 5 Figures with Supplementary Materials

Journal ref: Scientific Reports 4: 6679(2014)

(Ca,Na)(Zn,Mn)2As2: a new spin & charge doping decoupled diluted ferromagnetic semiconductor with hexagonal CaAl2Si2 structure

Authors: K. Zhao, B. J. Chen, Z. Deng, W. Han, G. Q. Zhao, J. L. Zhu, Q. Q. Liu, X. C. Wang, B. Frandsen, L. Liu, S. Cheung, F. L. Ning, T. J. S. Munsie, T. Medina, G. M. Luke, J. P. Carlo, J. Munevar, G. M. Zhang, Y. J. Uemura, C. Q. Jin

Abstract: Here we report the successful synthesis of a spin- & charge-decoupled diluted magnetic semiconductor (Ca,Na)(Zn,Mn)2As2, crystallizing into the hexagonal CaAl2Si2 structure. The compound shows a ferromagnetic transition with a Curie temperature up to 33 K with 10% Na doping, which gives rise to carrier density of np~10^20 cm^-3. The new DMS is a soft magnetic material with HC<400 Oe. The anomalous… ▽ More Here we report the successful synthesis of a spin- & charge-decoupled diluted magnetic semiconductor (Ca,Na)(Zn,Mn)2As2, crystallizing into the hexagonal CaAl2Si2 structure. The compound shows a ferromagnetic transition with a Curie temperature up to 33 K with 10% Na doping, which gives rise to carrier density of np~10^20 cm^-3. The new DMS is a soft magnetic material with HC<400 Oe. The anomalous Hall effect is observed below the ferromagnetic ordering temperature. With increasing Mn doping, ferromagnetic order is accompanied by an interaction between the local spin and mobile charge, giving rise to a minimum in resistivity at low temperatures and localizing the conduction electrons. The system provides an ideal platform for studying the interaction of the local spins and conduction electrons. △ Less

Submitted 29 October, 2014; originally announced October 2014.

Comments: 23 Pages 1 Table 4 Figures

Journal ref: J Appl Phys.116,163906(2014)

Non-Fermi Liquid Behavior Close to a Quantum Critical Point in a Ferromagnetic State without Local Moments

Authors: E. Svanidze, L. Liu, B. Frandsen, B. D. White, T. Besara, T. Goko, T. Medina, T. J. S. Munsie, G. M. Luke, D. Zheng, C. Q. Jin, T. Siegrist, M. B. Maple, Y. J. Uemura, E. Morosan

Abstract: A quantum critical point (QCP) occurs upon chemical doping of the weak itinerant ferromagnet Sc_{3.1}In. Remarkable for a system with no local moments, the QCP is accompanied by non-Fermi liquid (NFL) behavior, manifested in the logarithmic divergence of the specific heat both in the ferro- and the paramagnetic states. Sc_{3.1}In displays critical scaling and NFL behavior in the ferromagnetic stat… ▽ More A quantum critical point (QCP) occurs upon chemical doping of the weak itinerant ferromagnet Sc_{3.1}In. Remarkable for a system with no local moments, the QCP is accompanied by non-Fermi liquid (NFL) behavior, manifested in the logarithmic divergence of the specific heat both in the ferro- and the paramagnetic states. Sc_{3.1}In displays critical scaling and NFL behavior in the ferromagnetic state, akin to what had been observed only in f-electron, local moment systems. With doping, critical scaling is observed close to the QCP, as the critical exponents, and delta, gamma and beta have weak composition dependence, with delta nearly twice, and beta almost half of their respective mean-field values. The unusually large paramagnetic moment mu_PM~1.3 mu_B/F.U. is nearly composition-independent. Evidence for strong spin fluctuations, accompanying the QCP at x_c = 0.035 +- 0.005, may be ascribed to the reduced dimensionality of Sc_{3.1}In, associated with the nearly one-dimensional Sc-In chains. △ Less

Submitted 24 October, 2014; originally announced October 2014.

Comments: 11 pages, 13 figures

Photoemission and X-ray Absorption Studies of the Diluted Magnetic Semiconductor Ba$_{1-y}$K$_{y}$(Zn$_{1-x}$Mn$_{x}$)$_{2}$As$_{2}$ Isostructural to Fe-based Superconductors

Authors: H. Suzuki, K. Zhao, G. Shibata, Y. Takahashi, S. Sakamoto, K. Yoshimatsu, B. J. Chen, H. Kumigashira, F. -H. Chang, H. -J. Lin, D. J. Huang, C. T. Chen, Bo Gu, S. Maekawa, Y. J. Uemura, C. Q. Jin, A. Fujimori

Abstract: The electronic and magnetic properties of a new diluted magnetic semiconductor (DMS) Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$, which is isostructural to so-called 122-type Fe-based superconductors, are investigated by x-ray absorption spectroscopy (XAS) and resonance photoemission spectroscopy (RPES). Mn $L_{2,3}$-edge XAS indicates that the doped Mn atoms have the valence 2+ and strong… ▽ More The electronic and magnetic properties of a new diluted magnetic semiconductor (DMS) Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$, which is isostructural to so-called 122-type Fe-based superconductors, are investigated by x-ray absorption spectroscopy (XAS) and resonance photoemission spectroscopy (RPES). Mn $L_{2,3}$-edge XAS indicates that the doped Mn atoms have the valence 2+ and strongly hybridize with the $4p$ orbitals of the tetrahedrally coordinating As ligands. The Mn $3d$ partial density of states (PDOS) obtained by RPES shows a peak around 4 eV and relatively high between 0-2 eV below the Fermi level ($E_{F}$) with little contribution at $E_{F}$, similar to that of the archetypal DMS Ga$_{1-x}$Mn$_{x}$As. This energy level creates $d^{5}$ electron configuration with $S=5/2$ local magnetic moments at the Mn atoms. Hole carriers induced by K substitution for Ba atoms go into the top of the As $4p$ valence band and are weakly bound to the Mn local spins. The ferromagnetic correlation between the local spins mediated by the hole carriers induces ferromagnetism in Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ △ Less

Submitted 26 January, 2015; v1 submitted 9 October, 2014; originally announced October 2014.

Journal ref: Physical Review B 91, 140401(R) (2015)

The anomaly Cu doping effects on LiFeAs superconductors

Authors: L. Y. Xing, H. Miao, X. C. Wang, J. Ma, Q. Q. Liu, Z. Deng, H. Ding, C. Q. Jin

Abstract: The Cu substitution effect on the superconductivity of LiFeAs has been studied in comparison with Co/Ni substitution. It is found that the shrinking rate of the lattice parameter c for Cu substitution is much smaller than that of Co/Ni substitution. This is in conjugation with the observation of ARPES that shows almost the same electron and hole Fermi surfaces (FSs) size for undoped and Cu substit… ▽ More The Cu substitution effect on the superconductivity of LiFeAs has been studied in comparison with Co/Ni substitution. It is found that the shrinking rate of the lattice parameter c for Cu substitution is much smaller than that of Co/Ni substitution. This is in conjugation with the observation of ARPES that shows almost the same electron and hole Fermi surfaces (FSs) size for undoped and Cu substituted LiFeAs sample except for a very small hole band sinking below Fermi level with doping, indicating little doping effect at Fermi surface by Cu substitution, in sharp contrast to the much effective carrier doping effect by Ni or Co. △ Less

Submitted 25 September, 2014; v1 submitted 19 September, 2014; originally announced September 2014.

Comments: 19 pages + 5 Figures Accepted by JPCM

Specific Heat of Ca0.33Na0.67Fe2As2

Authors: J. S. Kim, K. Zhao, C. Q. Jin, G. R. Stewart

Abstract: The specific heat of single crystal hole-doped Ca0.33Na0.67Fe2As2, Tc(onset)=33.7 K, was measured from 0.4 to 40 K. The discontinuity in the specific heat at Tc, deltaC, divided by Tc is 105 +- 5 mJ/molK2, consistent with values found previously for hole-doped Ba0.6K0.4Fe2As2 and somewhat above the general trend for deltaC/Tc vs Tc for the iron based superconductors established by Bud'ko, Ni and C… ▽ More The specific heat of single crystal hole-doped Ca0.33Na0.67Fe2As2, Tc(onset)=33.7 K, was measured from 0.4 to 40 K. The discontinuity in the specific heat at Tc, deltaC, divided by Tc is 105 +- 5 mJ/molK2, consistent with values found previously for hole-doped Ba0.6K0.4Fe2As2 and somewhat above the general trend for deltaC/Tc vs Tc for the iron based superconductors established by Bud'ko, Ni and Canfield. The usefulness of measured valued of deltaC/Tc as an important metric for the quality of samples is discussed. △ Less

Submitted 29 June, 2014; originally announced June 2014.

Journal ref: Solid State Communications 193, 34-36 (2014)

MuSR Investigation and Suppression of TC by overdoped Li in Diluted Ferromagnetic Semiconductor Li1+y(Zn1-xMnx)P

Authors: Huiyuan Man, Xin Gong, Guoxiang Zhi, Shengli Guo, Cui Ding, Quan Wang, T. Goko, L. Liu, B. A. Frandsen, Y. J. Uemura, H. Luetkens, E. Morenzoni, C. Q. Jin, T. Munsie, G. M. Luke, Hangdong Wang, Bin Chen, F. L. Ning

Abstract: We use muon spin relaxation (muSR) to investigate the magnetic properties of a bulk form diluted ferromagnetic semiconductor (DFS) Li1.15(Zn0.9Mn0.1)P with T_C ~ 22 K. MuSR results confirm the gradual development of ferromagnetic ordering below T_C with a nearly 100% magnetic ordered volume. Despite its low carrier density, the relation between static internal field and Curie temperature observed… ▽ More We use muon spin relaxation (muSR) to investigate the magnetic properties of a bulk form diluted ferromagnetic semiconductor (DFS) Li1.15(Zn0.9Mn0.1)P with T_C ~ 22 K. MuSR results confirm the gradual development of ferromagnetic ordering below T_C with a nearly 100% magnetic ordered volume. Despite its low carrier density, the relation between static internal field and Curie temperature observed for Li(Zn,Mn)P is consistent with the trend found in (Ga,Mn)As and other bulk DFSs, indicating these systems share a common mechanism for the ferromagnetic exchange interaction. Li1+y(Zn1-xMnx)P has the advantage of decoupled carrier and spin doping, where Mn2+ substitution for Zn2+ introduces spins and Li+ off-stoichiometry provides carriers. This advantage enables us to investigate the influence of overdoped Li on the ferromagnetic ordered state. Overdoping Li suppresses both T_C and saturation moments for a certain amount of spins, which indicates that more carriers are detrimental to the ferromagnetic exchange interaction, and that a delicate balance between charge and spin densities is required to achieve highest T_C. △ Less

Submitted 14 June, 2014; originally announced June 2014.

Journal ref: Physical Review B 90, 085123 (2014)

Observation of strong electron pairing on bands without Fermi surfaces in LiFe1-xCoxAs

Authors: H. Miao, T. Qian, X. Shi, P. Richard, T. K. Kim, M. Hoesch, L. Y. Xing, X. C. Wang, C. Q. Jin, J. P. Hu, H. Ding

Abstract: In conventional BCS superconductors, the quantum condensation of superconducting electron pairs is understood as a Fermi surface (FS) instability, in which the low-energy electrons are paired by attractive interactions. Whether this explanation is still valid in high-Tc superconductors such as cuprates and iron-based superconductors remains an open question. In particular, a fundamentally differen… ▽ More In conventional BCS superconductors, the quantum condensation of superconducting electron pairs is understood as a Fermi surface (FS) instability, in which the low-energy electrons are paired by attractive interactions. Whether this explanation is still valid in high-Tc superconductors such as cuprates and iron-based superconductors remains an open question. In particular, a fundamentally different picture of the electron pairs, which are believed to be formed locally by repulsive interactions, may prevail. Here we report a high-resolution angle-resolved photoemission spectroscopy study on LiFe1-xCoxAs. We reveal a large and robust superconducting (SC) gap on a band sinking below the Fermi energy upon Co substitution. The observed FS-free SC order is also the largest over the momentum space, which rules out a proximity effect origin and indicates that the SC order parameter is not tied to the FS as a result of a FS instability. △ Less

Submitted 4 June, 2014; v1 submitted 4 June, 2014; originally announced June 2014.

Comments: 28 pages, 9 figures

Journal ref: Nat. Commun. 6:6056 (2015)

Extraordinary quasiparticle scattering and bandwidth-control by dopants in iron-based superconductors

Authors: Z. R. Ye, Y. Zhang, F. Chen, M. Xu, J. Jiang, X. H. Niu, C. H. P. Wen, L. Y. Xing, X. C. Wang, C. Q. Jin, B. P. Xie, D. L. Feng

Abstract: The diversities in crystal structures and ways of doping result in extremely diversified phase diagrams for iron-based superconductors. With angle-resolved photoemission spectroscopy (ARPES), we have systematically studied the effects of chemical substitution on the electronic structure of various series of iron-based superconductors. In addition to the control of Fermi surface topology by heterov… ▽ More The diversities in crystal structures and ways of doping result in extremely diversified phase diagrams for iron-based superconductors. With angle-resolved photoemission spectroscopy (ARPES), we have systematically studied the effects of chemical substitution on the electronic structure of various series of iron-based superconductors. In addition to the control of Fermi surface topology by heterovalent doping, we found two more extraordinary effects of doping: 1. the site and band dependencies of quasiparticle scattering; and more importantly 2. the ubiquitous and significant bandwidth-control by both isovalent and heterovalent dopants in the iron-anion layer. Moreover, we found that the bandwidth-control could be achieved by either applying the chemical pressure or doping electrons, but not by doping holes. Together with other findings provided here, these results complete the microscopic picture of the electronic effects of dopants, which facilitates a unified understanding of the diversified phase diagrams and resolutions to many open issues of various iron-based superconductors. △ Less

Submitted 27 April, 2014; originally announced April 2014.

Comments: 12 pages, 9 figures

Journal ref: Physical Review X 4, 031041 (2014)

Small and nearly isotropic hole-like Fermi surfaces in LiFeAs detected through de Haas van Alphen-effect

Authors: B. Zeng, D. Watanabe, Q. R. Zhang, G. Li, T. Besara, T. Siegrist L. Y. Xing, X. C. Wang, C. Q. Jin, P. Goswami, M. D. Johannes, L. Balicas

Abstract: LiFeAs is unique among the arsenic based Fe-pnictide superconductors because it is the only nearly stoichiometric compound which does not exhibit magnetic order. This is at odds with electronic structure calculations which and a very stable magnetic state and predict cylindrical hole- and electron-like Fermi surface sheets whose geometry suggests spin uctuations and a possible instability toward l… ▽ More LiFeAs is unique among the arsenic based Fe-pnictide superconductors because it is the only nearly stoichiometric compound which does not exhibit magnetic order. This is at odds with electronic structure calculations which and a very stable magnetic state and predict cylindrical hole- and electron-like Fermi surface sheets whose geometry suggests spin uctuations and a possible instability toward long-range ordering at the nesting vector. In fact, a complex magnetic phase-diagram is indeed observed in the isostructural NaFeAs compound. Previous angle resolved photoemission (ARPES) experiments revealed the existence of both hole and electron-like surfaces, but with rather distinct cross-sectional areas and an absence of the nesting that is thought to underpin both magnetic order and superconductivity in the pnictide family of superconductors. These ARPES observations were challenged by subsequent de Haas van Alphen (dHvA) measurements which detected a few, electron like Fermi surface sheets in rough agreement with the original band calculations. Here, we show a detailed dHvA study unveiling additional, small and nearly isotropic Fermi surface sheets in LiFeAs single crystals, which ought to correspond to hole-like orbits, as previously observed by ARPES. Therefore, our results conciliate the apparent discrepancy between ARPES and the previous dHvA results5. The small size of these Fermi surface pockets suggests a prominent role for the electronic correlations in LiFeAs. The absence of gap nodes, in combination with the coexistence of quasi-two-dimensional and three-dimensional Fermi surfaces, favor a s-wave pairing symmetry for LiFeAs. But similar electron-like Fermi surfaces combined with very different hole pockets between LiFeAs and LiFeP, suggest that the nodes in the gap function of LiFeP might be located on the hole-pockets. △ Less

Submitted 1 October, 2013; originally announced October 2013.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. B, 88, 144518 (2013)

New Diluted ferromagnetic semiconductor Li(Zn,Mn)P with decoupled charge and spin doping

Authors: Z. Deng, K. Zhao, B. Gu, W. Han, J. L. Zhu, X. C. Wang, X. Li, Q. Q. Liu, R. C. Yu, T. Goko, B. Frandsen, L. Liu, Jinsong Zhang, Yayu Wang, F. L. Ning, S. Maekawa, Y. J. Uemura, C. Q. Jin

Abstract: We report the discovery of a new diluted magnetic semiconductor, Li(Zn,Mn)P, in which charge and spin are introduced independently via lithium off-stoichiometry and the isovalent substitution of Mn2+ for Zn2+, respectively. Isostructural to (Ga,Mn)As, Li(Zn,Mn)P was found to be a p-type ferromagnetic semiconductor with excess Lithium providing charge doping. First principles calculations indicate… ▽ More We report the discovery of a new diluted magnetic semiconductor, Li(Zn,Mn)P, in which charge and spin are introduced independently via lithium off-stoichiometry and the isovalent substitution of Mn2+ for Zn2+, respectively. Isostructural to (Ga,Mn)As, Li(Zn,Mn)P was found to be a p-type ferromagnetic semiconductor with excess Lithium providing charge doping. First principles calculations indicate that excess Li is favored to partially occupy the Zn site, leading to hole doping. Ferromagnetism is mediated in semiconducting samples of relative low mobile carriers with a small coercive force, indicating an easy spin flip. △ Less

Submitted 22 August, 2013; originally announced August 2013.

Comments: 18pages, 1 table, 3 Figues

Journal ref: Phys Rev B 88, 81203(R)(2013)

Visualization of Electron Nematicity and Unidirectional Antiferroic Fluctuations at High Temperatures in NaFeAs

Authors: E. P. Rosenthal, E. F. Andrade, C. J. Arguello, R. M. Fernandes, L. Y. Xing, X. C. Wang, C. Q. Jin, A. J. Millis, A. N. Pasupathy

Abstract: The driving forces behind electronic nematicity in the iron pnictides remain hotly debated. We use atomic-resolution variable-temperature scanning tunneling spectroscopy to provide the first direct visual evidence that local electronic nematicity and unidirectional antiferroic (stripe) fluctuations persist to temperatures almost twice the nominal structural ordering temperature in the parent pnict… ▽ More The driving forces behind electronic nematicity in the iron pnictides remain hotly debated. We use atomic-resolution variable-temperature scanning tunneling spectroscopy to provide the first direct visual evidence that local electronic nematicity and unidirectional antiferroic (stripe) fluctuations persist to temperatures almost twice the nominal structural ordering temperature in the parent pnictide NaFeAs. Low-temperature spectroscopic imaging of nematically-ordered NaFeAs shows anisotropic electronic features that are not observed for isostructural, non-nematic LiFeAs. The local electronic features are shown to arise from scattering interference around crystalline defects in NaFeAs, and their spatial anisotropy is a direct consequence of the structural and stripe-magnetic order present at low temperature. We show that the anisotropic features persist up to high temperatures in the nominally tetragonal phase of the crystal. The spatial distribution and energy dependence of the anisotropy at high temperatures is explained by the persistence of large amplitude, short-range, unidirectional, antiferroic (stripe) fluctuations, indicating that strong density wave fluctuations exist and couple to near-Fermi surface electrons even far from the structural and density wave phase boundaries. △ Less

Submitted 12 July, 2013; originally announced July 2013.

Comments: Main Text: 12 pages, 6 figures. Supplementary Information: 8 pages, 7 figures

Journal ref: Nature Physics 10, 225-232 (2014)

Superconductivity in Topological Insulator Sb2Te3 Induced by Pressure

Authors: J. Zhu, J. L. Zhang, P. P. Kong, S. J. Zhang, X. H. Yu, J. L. Zhu, Q. Q. Liu, X. Li, R. C. Yu, R. Ahuja, W. G. Yang, G. Y. Shen, H. K. Mao, H. M. Weng, X. Dai, Z. Fang, Y. S. Zhao, C. Q. Jin

Abstract: Topological superconductivity is one of most fascinating properties of topological quantum matters that was theoretically proposed and can support Majorana Fermions at the edge state. Superconductivity was previously realized in a Cu-intercalated Bi2Se3 topological compound or a Bi2Te3 topological compound at high pressure. Here we report the discovery of superconductivity in the topological compo… ▽ More Topological superconductivity is one of most fascinating properties of topological quantum matters that was theoretically proposed and can support Majorana Fermions at the edge state. Superconductivity was previously realized in a Cu-intercalated Bi2Se3 topological compound or a Bi2Te3 topological compound at high pressure. Here we report the discovery of superconductivity in the topological compound Sb2Te3 when pressure was applied. The crystal structure analysis results reveal that superconductivity at a low-pressure range occurs at the ambient phase. The Hall coefficient measurements indicate the change of p-type carriers at a low-pressure range within the ambient phase, into n-type at higher pressures, showing intimate relation to superconducting transition temperature. The first principle calculations based on experimental measurements of the crystal lattice show that Sb2Te3 retains its Dirac surface states within the low-pressure ambient phase where superconductivity was observed, which indicates a strong relationship between superconductivity and topology nature. △ Less

Submitted 27 June, 2013; originally announced June 2013.

Comments: 23 pages, 1 table, 6 figures

Journal ref: Scientific Reports 3: 2016(2013)

(La1-xBax)(Zn1-xMnx)AsO: A Two Dimensional "1111" Diluted Magnetic Semiconductor in Bulk Form

Authors: Cui Ding, Huiyuan Man, Chuan Qin, Jicai Lu, Yunlei Sun, Quan Wang, Biqiong Yu, Chunmu Feng, T. Goko, C. J. Arguello, L. Liu, B. J. Frandsen, Y. J. Uemura, Hangdong Wang, H. Luetkens, E. Morenzoni, W. Han, C. Q. Jin, T. Munsie, T. J. Williams, R. M. D'Ortenzio, T. Medina, G. M. Luke, T. Imai, F. L. Ning

Abstract: We report the synthesis and characterization of a bulk diluted magnetic semiconductor (La1-xBax)(Zn1-xMnx)AsO (0 <= x <= 0.2) with a layered crystal structure identical to that of the "1111" FeAs superconductors. No ferromagnetic order occurs for (Zn,Mn) substitution in the parent compound LaZnAsO without charge doping. Together with carrier doping via (La,Ba) sub- stitution, a small amount of Mn… ▽ More We report the synthesis and characterization of a bulk diluted magnetic semiconductor (La1-xBax)(Zn1-xMnx)AsO (0 <= x <= 0.2) with a layered crystal structure identical to that of the "1111" FeAs superconductors. No ferromagnetic order occurs for (Zn,Mn) substitution in the parent compound LaZnAsO without charge doping. Together with carrier doping via (La,Ba) sub- stitution, a small amount of Mn substituting for Zn results in ferromagnetic order with TC up to ~40 K, although the system remains semiconducting. Muon spin relaxation measurements confirm the development of ferromagnetic order in the entire volume, with the relationship between the internal field and TC consistent with the trend found in (Ga,Mn)As, the "111" Li(Zn,Mn)As, and the "122" (Ba,K)(Zn,Mn)2As2 systems. △ Less

Submitted 19 June, 2013; originally announced June 2013.

Comments: Accepted for publication in Physical Review B-Rapid Communications

Journal ref: Physical Review B 88, 041102(R) (2013)

Tuning Jeff = 1/2 Insulating State via Electron Doping and Pressure in Double-Layered Iridate Sr3Ir2O7

Authors: L. Li, P. P. Kong, T. F. Qi, C. Q. Jin, S. J. Yuan, L. E. DeLong, P. Schlottmann, G. Cao

Abstract: Sr3Ir2O7 exhibits a novel Jeff=1/2 insulating state that features a splitting between Jeff=1/2 and 3/2 bands due to spin-orbit interaction. We report a metal-insulator transition in Sr3Ir2O7 via either dilute electron doping (La3+ for Sr2+) or application of high pressure up to 35 GPa. Our study of single-crystal Sr3Ir2O7 and (Sr1-xLax)3Ir2O7 reveals that application of high hydrostatic pressure P… ▽ More Sr3Ir2O7 exhibits a novel Jeff=1/2 insulating state that features a splitting between Jeff=1/2 and 3/2 bands due to spin-orbit interaction. We report a metal-insulator transition in Sr3Ir2O7 via either dilute electron doping (La3+ for Sr2+) or application of high pressure up to 35 GPa. Our study of single-crystal Sr3Ir2O7 and (Sr1-xLax)3Ir2O7 reveals that application of high hydrostatic pressure P leads to a drastic reduction in the electrical resistivity by as much as six orders of magnitude at a critical pressure, PC = 13.2 GPa, manifesting a closing of the gap; but further increasing P up to 35 GPa produces no fully metallic state at low temperatures, possibly as a consequence of localization due to a narrow distribution of bonding angles θ. In contrast, slight doping of La3+ ions for Sr2+ ions in Sr3Ir2O7 readily induces a robust metallic state in the resistivity at low temperatures; the magnetic ordering temperature is significantly suppressed but remains finite for (Sr0.95La0.05)3Ir2O7 where the metallic state occurs. The results are discussed along with comparisons drawn with Sr2IrO4, a prototype of the Jeff = 1/2 insulator. △ Less

Submitted 18 April, 2013; originally announced April 2013.

Comments: five figures

New Diluted Ferromagnetic Semiconductor isostructural to 122 type iron pnictide superconductor with TC up to 180 K

Authors: K. Zhao, Z. Deng, X. C. Wang, W. Han, J. L. Zhu, X. Li, Q. Q. Liu, R. C. Yu, T. Goko, B. Frandsen, Lian Liu, Fanlong Ning, Y. J. Uemura, H. Dabkowska, G. M. Luke, H. Luetkens, E. Morenzoni, S. R. Dunsiger, A. Senyshyn, P. Böni, C. Q. Jin

Abstract: Diluted magnetic semiconductors (DMS) have received much attention due to its potential applications to spintronics devices. A prototypical system (Ga,Mn)As has been widely studied since 1990s. The simultaneous spin and charge doping via hetero-valence (Ga3+,Mn2+) substitution, however, resulted in severely limited solubility without availability of bulk specimens. Previously we synthesized a new… ▽ More Diluted magnetic semiconductors (DMS) have received much attention due to its potential applications to spintronics devices. A prototypical system (Ga,Mn)As has been widely studied since 1990s. The simultaneous spin and charge doping via hetero-valence (Ga3+,Mn2+) substitution, however, resulted in severely limited solubility without availability of bulk specimens. Previously we synthesized a new diluted ferromagnetic semiconductor of bulk Li(Zn,Mn)As with Tc up to 50K, where isovalent (Zn,Mn) spin doping was separated from charge control via Li concentrations. Here we report the synthesis of a new diluted ferromagnetic semiconductor (Ba1-xKx)(Zn1-yMny)2As2, isostructural to iron 122 system, where holes are doped via (Ba2+, K1+), while spins via (Zn2+,Mn2+) substitutions. Bulk samples with x=0.1-0.3 and y=0.05-0.15 exhibit ferromagnetic order with TC up to 180K, comparable to that of record high Tc for Ga(MnAs), significantly enhanced than Li(Zn,Mn)As. Moreover the (Ba,K)(Zn,Mn)2As2 shares the same 122 crystal structure with semiconducting BaZn2As2, antiferromagnetic BaMn2As2, and superconducting (Ba,K)Fe2As2, which makes them promising to the development of multilayer functional devices. △ Less

Submitted 28 March, 2013; originally announced March 2013.

Comments: 12 pages, 3 Figures, 3 tables

Journal ref: Nature Communications 4:1442 | DOI: 10.1038(2013)

Superconductivity of topological insulator Bi2Se3 at high pressures

Authors: P. P. Kong, J. L. Zhang, S. J. Zhang, J. Zhu, Q. Q. Liu, R. C. Yu, Z. Fang, C. Q. Jin, W. G. Yang, X. H. Yu, J. L. Zhu, Y. S. Zhao

Abstract: The pressure induced superconductivity and structural evolution for Bi2Se3 single crystal have been studied. The emergence of superconductivity with onset transition temperature (Tc) about 4.4K is observed around 12GPa. Tc increases rapidly to the highest 8.5K at 16GPa, decreases to 6.5K at 21GPa, then keep almost constant. It is found that Tc versus pressure is closely related to the carrier dens… ▽ More The pressure induced superconductivity and structural evolution for Bi2Se3 single crystal have been studied. The emergence of superconductivity with onset transition temperature (Tc) about 4.4K is observed around 12GPa. Tc increases rapidly to the highest 8.5K at 16GPa, decreases to 6.5K at 21GPa, then keep almost constant. It is found that Tc versus pressure is closely related to the carrier density which increases by more than two orders of magnitude from 2GPa to 23GPa. High pressure synchrotron radiation measurements reveal structure transitions occur around 12GPa, 20GPa, and above 29GPa, respectively. A phase diagram of superconductivity versus pressure is obtained. △ Less

Submitted 1 March, 2013; originally announced March 2013.

Comments: 16 pages

Specific Heat vs Field of LiFe1-xCuxAs

Authors: J. S. Kim, G. R. Stewart, L. Y. Xing, X. C. Wang, C. Q. Jin

Abstract: LiFeAs is one of the new class of iron superconductors with a bulk onset Tc in the 15-17 K range. We report on the specific heat characterization of single crystal material prepared from self-flux growth techniques with significantly improved properties, including a much decreased residual gamma (proportional to C/T as T->0) in the superconducting state. Thus, in contrast to previous explanations… ▽ More LiFeAs is one of the new class of iron superconductors with a bulk onset Tc in the 15-17 K range. We report on the specific heat characterization of single crystal material prepared from self-flux growth techniques with significantly improved properties, including a much decreased residual gamma (proportional to C/T as T->0) in the superconducting state. Thus, in contrast to previous explanations of a finite residual gamma in LiFeAs being due to intrinsic states in the superconducting gap, the present work shows that such a finite residual gamma in LiFeAs is instead a function of sample quality. Further, since LiFeAs has been characterized as nodeless with multiple superconducting gaps, we report here on its specific heat properties in zero and applied magnetic fields to compare to similar results on nodal iron superconductors. For comparison, we also investigate LiFe0.98Cu0.02As, which has the reduced Tc of 9 K and Hc2 of 15 T. Interestingly, although presumably both LiFeAs and LiFe0.98Cu0.02As are nodeless, they clearly show a non-linear dependence of the electronic density of states (proportional to the specific heat gamma) at the Fermi energy in the mixed state with applied field similar to the Volovik effect for nodal superconductors. However, rather than nodal behavior, the satisfactory comparison with a recent theory for gamma(H) for a two isotropic gap superconductor in the presence of impurities argues for nodeless behavior. Thus, in terms of specific heat in magnetic field, LiFeAs can serve as the prototypical multiband, nodeless iron superconductor. △ Less

Submitted 17 October, 2012; originally announced October 2012.

Comments: Accepted for publication in Journal of Physics: Condensed Matter

Effect of Li-deficiency impurities on the electron-overdoped LiFeAs superconductor

Authors: Meng Wang, Miaoyin Wang, Hu Miao, S. V. Carr, D. L. Abernathy, M. B. Stone, X. C. Wang, Lingyi Xing, C. Q. Jin, Xiaotian Zhang, Jiangping Hu, Tao Xiang, Hong Ding, Pengcheng Dai

Abstract: We use transport, inelastic neutron scattering, and angle resolved photoemission experiments to demonstrate that the stoichiometric LiFeAs is an intrinsically electron-overdoped superconductor similar to those of the electron-overdoped NaFe1-xTxAs and BaFe2-xTxAs2 (T = Co,Ni). Furthermore, we show that although transport properties of the stoichiometric superconducting LiFeAs and Li-deficient nons… ▽ More We use transport, inelastic neutron scattering, and angle resolved photoemission experiments to demonstrate that the stoichiometric LiFeAs is an intrinsically electron-overdoped superconductor similar to those of the electron-overdoped NaFe1-xTxAs and BaFe2-xTxAs2 (T = Co,Ni). Furthermore, we show that although transport properties of the stoichiometric superconducting LiFeAs and Li-deficient nonsuperconducting Li1-xFeAs are different, their electronic and magnetic properties are rather similar. Therefore, the nonsuperconducting Li1-xFeAs is also in the electron overdoped regime, where small Li deficiencies near the FeAs octahedra can dramatically suppress superconductivity through the impurity scattering effect. △ Less

Submitted 4 August, 2012; originally announced August 2012.

Comments: 5 figures,5 pages

Journal ref: Phys. Rev. B 86, 144511(2012)