Thickness dependence of superconductivity in FeSe films. (arXiv:2302.09627v1 [cond-mat.supr-con])

2023-02-21T02:29:33+00:00February 21st, 2023|Categories: Publications|Tags: |

The films of FeSe on substrates have attracted attention because of their unusually high-temperature (Tc) superconducting properties whose origins continue to be debated. To disentangle the competing effects of the substrate and interlayer and intralayer processes, we present here results of density functional theory (DFT)-based analysis of the electronic structure of unsupported FeSe films consisting of 1 to 5 layers (1L-5L). Furthermore, by solving the Bardeen-Schrieffer-Cooper (BCS) equation with spin-wave exchange attraction derived from the Hubbard model, we find the superconducting critical temperature Tc for 1L-5L and bulk FeSe systems in reasonable agreement with experimental data. Our results point to the importance of correlation effects in superconducting properties of single- and multi-layer FeSe films, independently of the role of substrate.

Published in: "arXiv Material Science".

Collapse of Metallicity and High-$T_c$ Superconductivity in the High-Pressure phase of FeSe$_{0.89}$S$_{0.11}$. (arXiv:2212.06824v1 [cond-mat.supr-con])

2022-12-14T02:30:08+00:00December 14th, 2022|Categories: Publications|Tags: |

We investigate the high-pressure phase of the iron-based superconductor FeSe$_{0.89}$S$_{0.11}$ using transport and tunnel diode oscillator studies. We construct detailed pressure-temperature phase diagrams that indicate that outside of the nematic phase, the superconducting critical temperature reaches a minimum before it is quickly enhanced towards 40 K above 4 GPa. The resistivity data reveal signatures of a fan-like structure of non-Fermi liquid behaviour which could indicate the existence of a putative quantum critical point buried underneath the superconducting dome around 4.3 GPa. Further increasing the pressure, the zero-field electrical resistivity develops a non-metallic temperature dependence and the superconducting transition broadens significantly. Eventually, the system fails to reach a fully zero-resistance state despite a continuous finite superconducting transition temperature, and any remaining resistance at low temperatures becomes strongly current-dependent. Our results suggest that the high-pressure, high-$T_c$ phase of iron chalcogenides is very fragile and sensitive to uniaxial effects of the pressure medium, cell design and sample thickness which can trigger a first-order transition. These high-pressure regions could be understood assuming a real-space phase separation caused by concomitant electronic and structural instabilities.

Published in: "arXiv Material Science".

Unconventional localization of electrons inside of a nematic electronic phase. (arXiv:2212.06196v1 [cond-mat.supr-con])

2022-12-14T02:29:59+00:00December 14th, 2022|Categories: Publications|Tags: |

The magnetotransport behaviour inside the nematic phase of bulk FeSe reveals unusual multiband effects that cannot be reconciled with a simple two-band approximation proposed by surface-sensitive spectroscopic probes. In order to understand the role played by the multiband electronic structure and the degree of two-dimensionality we have investigated the electronic properties of exfoliated flakes of FeSe by reducing their thickness. Based on magnetotransport and Hall resistivity measurements, we assess the mobility spectrum that suggests an unusual asymmetry between the mobilities of the electrons and holes with the electron carriers becoming localized inside the nematic phase. Quantum oscillations in magnetic fields up to 38 T indicate the presence of a hole-like quasiparticle with a lighter effective mass and a quantum scattering time three times shorter, as compared with bulk FeSe. The observed localization of negative charge carriers by reducing dimensionality can be driven by orbitally-dependent correlation effects, enhanced interband spin-fluctuations or a Lifshitz-like transition which affect mainly the electron bands. The electronic localization leads to a fragile two-dimensional superconductivity in thin flakes of FeSe, in contrast to the two-dimensional high-Tc induced with electron doping via dosing or using a suitable interface.

Published in: "arXiv Material Science".

Nematic state of the FeSe superconductor. (arXiv:2203.03909v1 [cond-mat.supr-con])

2022-03-09T02:29:22+00:00March 9th, 2022|Categories: Publications|Tags: |

We study the crystal structure of the tetragonal iron selenide FeSe and its nematic phase transition to the low-temperature orthorhombic structure using synchrotron x-ray and neutron scattering analyzed in both real and reciprocal space. We show that in the local structure the orthorhombic distortion associated with the electronically driven nematic order is more pronounced at short length scales. It also survives up to temperatures above 90 K where reciprocal-space analysis suggests tetragonal symmetry. Additionally, the real-space pair distribution function analysis of the synchrotron x-ray diffraction data reveals a tiny broadening of the peaks corresponding to the nearest Fe-Fe, nearest Fe-Se, and the next-nearest Fe-Se bond distances as well as the tetrahedral torsion angles at a short length scale of 20 angstr”om. This broadening appears below 20 K and is attributed to a pseudogap. However, we did not observe any further reduction in local symmetry below orthorhombic down to 3 K. Our results suggest that the superconducting gap anisotropy in FeSe is not associated with any symmetry-lowering short-range structural correlations.

Published in: "arXiv Material Science".

Crystal structure and properties of iron-based spin-chain compound Ba9Fe3Se15. (arXiv:2102.03708v1 [cond-mat.supr-con])

2021-02-09T02:29:49+00:00February 9th, 2021|Categories: Publications|Tags: |

We report the synthesis of a new quasi one-dimensional (1D) iron selenide. Ba9Fe3Se15 was synthesized at high temperature and high pressure of 5.5 GPa and systematically studied via structural, magnetic and transport measurements at ambient and at high-pressures. Ba9Fe3Se15 crystallizes in a monoclinic structure and consists of face-sharing FeSe6 octahedral chains along the c axis. At ambient pressure it exhibits an insulating behavior with a band gap ~460 meV and undergoes a ferrimagnet-like phase transition at 14 K. Under high pressure, a complete metallization occurs at ~29 GPa, which is accompanied by a spin state crossover from high spin (HS) state to low spin (LS) state. The LS appears for pressures P >36 GPa.

Published in: "arXiv Material Science".

Step-edge assisted large scale FeSe monolayer growth on epitaxial Bi2Se3 thin films. (arXiv:1912.13280v1 [cond-mat.mtrl-sci])

2020-01-01T02:29:32+00:00January 1st, 2020|Categories: Publications|Tags: , , |

The interest in Fe-chalcogenide unconventional superconductors is intense after the critical temperature of FeSe was reported enhanced by more than one order of magnitude in the monolayer limit at the interface to an insulating oxide substrate. In heterostructures comprising interfaces of FeSe with topological insulators, additional interesting physical phenomena is predicted to arise e.g. in form of {it topological superconductivity}. So far superconductive properties of Fe-chalcogenide monolayers were mostly studied by local scanning tunneling spectroscopy experiments, which can detect pseudo-gaps in the density of states as an indicator for Cooper pairing. Direct macroscopic transport properties which can prove or falsify a superconducting phase were rarely reported due to the difficulty to grow films with homogeneous material properties. Here we report on a promising growth method to fabricate continuous carpets of monolayer thick FeSe on molecular beam epitaxy grown Bi$_2$Se$_3$ topological insulator thin films. In contrast to previous works using atomically flat cleaved bulk Bi$_2$Se$_3$ crystal surfaces we observe a strong influence of the high step-edge density (terrace width about 10~nm) on MBE-grown Bi$_2$Se$_3$ substrates, which significantly promotes the growth of coalescing FeSe domains with small tetragonal crystal distortion without compromising the underlying Bi$_2$Se$_3$ crystal structure.

Published in: "arXiv Material Science".

FeSe quantum dots for in vivo multiphoton biomedical imaging

2019-12-06T22:36:44+00:00December 6th, 2019|Categories: Publications|Tags: |

An immense demand in biomedical imaging is to develop efficient photoluminescent probes with high biocompatibility and quantum yield, as well as multiphoton absorption performance to improve penetration depth and spatial resolution. Here, iron selenide (FeSe) quantum dots (QDs) are reported to meet these criteria. The synthesized QDs exhibit two- and

Published in: "Science Advances".

The study of intrinsic defect state of FeSe with scanning tunneling microscopy. (arXiv:1908.03427v1 [cond-mat.supr-con])

2019-08-12T02:29:40+00:00August 12th, 2019|Categories: Publications|Tags: , |

We apply high resolution scanning tunneling microscopy to study intrinsic defect states of bulk FeSe. Four types of intrinsic defects including the type I dumbbell, type II dumbbell, top-layer Se vacancy and inner-layer Se-site defect are extensively analyzed by scanning tunneling spectroscopy. From characterized depression and enhancement of density of states measured in a large energy range, the type I dumbbell and type II dumbbell are determined to be the Fe vacancy and Se$_mathrm{Fe}$ defect, respectively. The top-layer Se vacancy and possible inner-layer Se-site vacancy are also determined by spectroscopy analysis. The determination of defects are compared and largely confirmed in the annular dark-field scanning transmission electron microscopy measurement of the exfoliated FeSe. The detailed mapping of defect states in our experiment lays the foundation for a comparison with complex theoretical calculations in the future.

Published in: "arXiv Material Science".

Suppression of superconductivity and enhanced critical field anisotropy in thin flakes of FeSe. (arXiv:1907.13174v1 [cond-mat.supr-con])

2019-08-01T02:29:38+00:00August 1st, 2019|Categories: Publications|Tags: |

FeSe is a unique superconductor that can be manipulated to enhance its superconductivity using different routes while its monolayer form grown on different substrates reaches a record high temperature for a two-dimensional system. In order to understand the role played by the substrate and the reduced dimensionality on superconductivity, we examine the superconducting properties of exfoliated FeSe thin flakes by reducing the thickness from bulk down towards 9 nm. Magnetotransport measurements performed in magnetic fields up to 16T and temperatures down to 2K help to build up complete superconducting phase diagrams of different thickness flakes. While the thick flakes resemble the bulk behaviour, by reducing the thickness the superconductivity of FeSe flakes is suppressed. In the thin limit we detect signatures of a crossover towards two-dimensional behaviour from the observation of the vortex-antivortex unbinding transition and strongly enhanced anisotropy. Our study provides detailed insights into the evolution of the superconducting properties from three-dimensional bulk behaviour towards the two-dimensional limit of FeSe in the absence of a dopant substrate.

Published in: "arXiv Material Science".

Spectroscopic Imaging of Quasiparticle Bound States Induced by Strong Nonmagnetic Scatterings in One-Unit-Cell $mathrm{FeSe}/{mathrm{SrTiO}}_{3}$

2019-07-15T18:46:11+00:00July 15th, 2019|Categories: Publications|Tags: |

Author(s): Chaofei Liu, Ziqiao Wang, Yi Gao, Xiaoqiang Liu, Yi Liu, Qiang-Hua Wang, and Jian WangThe absence of holelike Fermi pockets in the heavily electron-doped iron selenides (HEDISs) challenges the s±-wave pairing originally proposed for iron pnictides, which consists of opposite signs of the gap function on electron and hole pockets. While the HEDIS compounds have been investigated exten…[Phys. Rev. Lett. 123, 036801] Published Mon Jul 15, 2019

Published in: "Physical Review Letters".

Synergetic Behavior in 2D Layered Material/Complex Oxide Heterostructures. (arXiv:1905.02930v1 [cond-mat.mtrl-sci])

2019-05-09T02:29:20+00:00May 9th, 2019|Categories: Publications|Tags: , , , |

The marriage between a two-dimensional layered material (2DLM) and a complex transition metal oxide (TMO) results in a variety of physical and chemical phenomena that would not have been achieved in either material alone. Interesting recent discoveries in systems such as graphene/SrTiO3, graphene/LaAlO3/SrTiO3, graphene/ferroelectric oxide, MoS2/SrTiO3, and FeSe/SrTiO3 heterostructures include voltage scaling in field-effect transistors, charge state coupling across an interface, quantum conductance probing of the electrochemical activity, novel memory functions based on charge traps, and greatly enhanced superconductivity. In this progress report, we review various properties and functionalities appearing in numerous different 2DLM/TMO heterostructure systems. The results imply that the multidimensional heterostructure approach based on the disparate material systems leads to an entirely new platform for the study of condensed matter physics and materials science. The heterostructures are also highly relevant technologically, as each constituent material is a promising candidate for next-generation opto-electronic devices.

Published in: "arXiv Material Science".

Structural evolution and phase diagram of the superconducting iron selenides $mathrm{L}{mathrm{i}}_{x}{({mathrm{C}}_{2}{mathrm{H}}_{8}{mathrm{N}}_{2})}_{y}mathrm{F}{mathrm{e}}_{2}mathrm{S}{mathrm{e}}_{2}(x=0∼0.8)$

2019-03-04T14:34:42+00:00March 4th, 2019|Categories: Publications|Tags: |

Author(s): Linlin Zhao, Da Wang, Qingzhen Huang, Hui Wu, Ruijin Sun, Xiao Fan, Yanpeng Song, Shifeng Jin, and Xiaolong ChenHere we report on the structural and electronic phase diagram of lithium and ethylenediamine intercalated FeSe in a wide range of dopant concentration (x=0∼0.8). Undoped (C2H8N2)yFe2Se2 crystallizes in an orthorhombic phase. With increasing lithium doping, an orthorhombic to tetragonal phase transit…[Phys. Rev. B 99, 094503] Published Mon Mar 04, 2019

Published in: "Physical Review B".

X-ray diffraction analysis to support phase identification in FeSe and Fe$_7$Se$_8$ epitaxial thin films. (arXiv:1901.09794v1 [cond-mat.mtrl-sci])

2019-01-29T02:29:32+00:00January 29th, 2019|Categories: Publications|Tags: |

X-ray diffraction (XRD) data and analysis for epitaxial iron selenide thin films grown by pulsed laser deposition (PLD) are presented. The films contain ${beta}$-FeSe and Fe$_7$Se$_8$ phases in a double epitaxy configuration with the ${beta}$-FeSe phase (001) oriented on the (001) MgO growth substrate. Fe$_7$Se$_8$ simultaneously takes on two different epitaxial orientations in certain growth conditions, exhibiting both (101)- and (001)- orientations. Each of these orientations are verified with the presented XRD data. Additionally, XRD data used to determine the PLD target composition as well as mosaic structure of the ${beta}$-FeSe phase are shown.

Published in: "arXiv Material Science".

Particulate Generation on Surface of Iron Selenide Films by Air Exposure. (arXiv:1901.08899v1 [cond-mat.supr-con])

2019-01-28T02:29:22+00:00January 28th, 2019|Categories: Publications|Tags: |

Nanometer-sized particular structures are generated on the surfaces of FeSe epitaxial films directly after exposure to air; this phenomenon was studied in the current work because these structures are an obstacle to field-induced superconductivity in electric double-layer transistors using FeSe channel layers. Chemical analyses using field-effect scanning Auger electron spectroscopy revealed no clear difference in the chemical composition between the particular structures and the other flat surface region. This observation limits the possible origins of the particulate formation to light elements in air such as O, C, H, and N.

Published in: "arXiv Material Science".

Insulator-like behavior coexisting with metallic electronic structure in strained FeSe thin films grown by molecular beam epitaxy

2019-01-24T14:35:05+00:00January 24th, 2019|Categories: Publications|Tags: |

Author(s): Kota Hanzawa, Yuta Yamaguchi, Yukiko Obata, Satoru Matsuishi, Hidenori Hiramatsu, Toshio Kamiya, and Hideo HosonoThis paper reports that ∼10-nm-thick iron selenide (FeSe) thin films exhibit insulator-like behavior in terms of the temperature dependence of their electrical resistivity even though bulk FeSe has a metallic electronic structure that has been confirmed by photoemission spectroscopy and first-princi…[Phys. Rev. B 99, 035148] Published Thu Jan 24, 2019

Published in: "Physical Review B".

Synergetic Behavior in 2D Layered Material/Complex Oxide Heterostructures

2019-01-01T22:34:47+00:00January 1st, 2019|Categories: Publications|Tags: , , , |

Heterostructures composed of a 2D layered material (2DLM) and complex transition metal oxides (TMO) provide a new platform for design, realization, and examination of artificial materials that are physically intriguing and technologically useful. An overview is presented for some of the examples, where various functional properties emerge at the novel 2DLM/TMO heterostructures. Abstract The marriage between a 2D layered material (2DLM) and a complex transition metal oxide (TMO) results in a variety of physical and chemical phenomena that cannot be achieved in either material alone. Interesting recent discoveries in systems such as graphene/SrTiO3, graphene/LaAlO3/SrTiO3, graphene/ferroelectric oxide, MoS2/SrTiO3, and FeSe/SrTiO3 heterostructures include voltage scaling in field‐effect transistors, charge state coupling across an interface, quantum conductance probing of the electrochemical activity, novel memory functions based on charge traps, and greatly enhanced superconductivity. In this context, various properties and functionalities appearing in numerous different 2DLM/TMO heterostructure systems are reviewed. The results imply that the multidimensional heterostructure approach based on the disparate material systems leads to an entirely new platform for the study of condensed matter physics and materials science. The heterostructures are also highly relevant technologically as each constituent material is a promising candidate for next‐generation optoelectronic devices.

Published in: "Advanced Materials".

Synergetic Behavior in 2D Layered Material/Complex Oxide Heterostructures

2018-12-29T22:35:39+00:00December 29th, 2018|Categories: Publications|Tags: , , , |

Heterostructures composed of a 2D layered material (2DLM) and complex transition metal oxides (TMO) provide a new platform for design, realization, and examination of artificial materials that are physically intriguing and technologically useful. An overview is presented for some of the examples, where various functional properties emerge at the novel 2DLM/TMO heterostructures. Abstract The marriage between a 2D layered material (2DLM) and a complex transition metal oxide (TMO) results in a variety of physical and chemical phenomena that cannot be achieved in either material alone. Interesting recent discoveries in systems such as graphene/SrTiO3, graphene/LaAlO3/SrTiO3, graphene/ferroelectric oxide, MoS2/SrTiO3, and FeSe/SrTiO3 heterostructures include voltage scaling in field‐effect transistors, charge state coupling across an interface, quantum conductance probing of the electrochemical activity, novel memory functions based on charge traps, and greatly enhanced superconductivity. In this context, various properties and functionalities appearing in numerous different 2DLM/TMO heterostructure systems are reviewed. The results imply that the multidimensional heterostructure approach based on the disparate material systems leads to an entirely new platform for the study of condensed matter physics and materials science. The heterostructures are also highly relevant technologically as each constituent material is a promising candidate for next‐generation optoelectronic devices.

Published in: "Advanced Materials".

Synergetic Behavior in 2D Layered Material/Complex Oxide Heterostructures

2018-12-28T08:34:25+00:00December 28th, 2018|Categories: Publications|Tags: , , , |

Heterostructures composed of a 2D layered material (2DLM) and complex transition metal oxides (TMO) provide a new platform for design, realization, and examination of artificial materials that are physically intriguing and technologically useful. An overview is presented for some of the examples, where various functional properties emerge at the novel 2DLM/TMO heterostructures. Abstract The marriage between a 2D layered material (2DLM) and a complex transition metal oxide (TMO) results in a variety of physical and chemical phenomena that cannot be achieved in either material alone. Interesting recent discoveries in systems such as graphene/SrTiO3, graphene/LaAlO3/SrTiO3, graphene/ferroelectric oxide, MoS2/SrTiO3, and FeSe/SrTiO3 heterostructures include voltage scaling in field‐effect transistors, charge state coupling across an interface, quantum conductance probing of the electrochemical activity, novel memory functions based on charge traps, and greatly enhanced superconductivity. In this context, various properties and functionalities appearing in numerous different 2DLM/TMO heterostructure systems are reviewed. The results imply that the multidimensional heterostructure approach based on the disparate material systems leads to an entirely new platform for the study of condensed matter physics and materials science. The heterostructures are also highly relevant technologically as each constituent material is a promising candidate for next‐generation optoelectronic devices.

Published in: "Advanced Materials".

Realization of continuous electron doping in bulk iron selenides and identification of a new superconducting zone

2018-12-12T16:33:55+00:00December 12th, 2018|Categories: Publications|Tags: |

Author(s): R. J. Sun, Y. Quan, S. F. Jin, Q. Z. Huang, H. Wu, L. Zhao, L. Gu, Z. P. Yin, and X. L. ChenIron selenide superconductors exhibit unique characteristics distinct from iron pnictides, especially in the electron-doped region. Here, the authors report the synthesis of a new family of Lix(C3N2H10)0.37FeSe materials, which features a continuous superconducting dome harboring a Lifshitz transition within the wide range of 0.06≤x≤0.68. Under electron doping, the anion height of FeSe layers deviates linearly away from the optimized values of pnictides. This feature leads to a new superconducting zone with unique electronic structures and strong orbital-selective electronic correlation.[Phys. Rev. B 98, 214508] Published Wed Dec 12, 2018

Published in: "Physical Review B".

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