FeSe Layers Research Articles

Peculiarities of superconductivity in the single-layerFeSe/SrTiO3interface

Observation of replica bands in the ARPES spectra of the single-layer FeSe on strontium titanate substrate revealed a phonon component among mechanisms behind high Tc superconductivity there. We study the interaction of the in-layer FeSe electrons with the electric potential of the longitudinal (LO) modes at surface of bulk SrTiO3. A two-dimensional system of charges at the FeSe/SrTiO3 interface includes both the itinerant and immobile electrons. The latter significantly change the interface characteristics, increasing screening at the substrate surface and reducing thereby the strength of the electron-LO phonon interactions. In that follows the dielectric constant serves as a free parameter and is found from the ARPES data for the replicas. The two-dimensional Coulomb screening is accounted for in the random phase approximation. It is shown that the model is applicable within the whole range of the parameters typical for current experiments. The estimates allow concluding that the LO-phonon mediated pairing alone cannot account for the temperatures of the superconducting transition Tc in the single-layer FeSe/SrTiO3 reported in these experiments. This does not exclude that the LO-phonons mechanism can become more significant in the differently and better prepared single layer FeSe films. Available experiments are briefly discussed. Thus far no data exist on the dependence of Tc on the concentration of electrons doped into the in-layer FeSe band.

New Superconducting Phase of Lix(C6H16N2)yFe2−zSe2 with Tc = 41 K Obtained through the Post-Annealing

Post-annealing effects on the crystal structure and superconductivity of the lithium- and hexamethylenediamine (HMDA)-intercalated superconductor Lix(C6H16N2)yFe2−zSe2 have been investigated. Through the post-annealing, a two-step reduction of the interlayer spacing between neighboring Fe layers, d, has been observed. It has been found that a new phase of Lix(C6H16N2)yFe2−zSe2 with d = 10.30(2) Å and Tc = 41 K different from the as-intercalated phase is stabilized owing to the possible stable inclination of HMDA intercalated between FeSe layers. This result supports the domic relation between Tc and d in the FeSe-based intercalation superconductors. The reason why Tc increases with a decrease in d through the post-annealing is discussed.

Molecular beam epitaxy growth of multilayer FeSe thin film on SrTiO3 (001)

Single-layer FeSe film grown on SrTiO3(001) surface (STO surface) by molecular beam epitaxy has aroused a great research boom ever since the discovery of its huge superconductive energy gap which indicates a possible critical temperature (Tc) higher than the liquid nitrogen temperature. The interface enhanced superconductivity with a Tc above 100 K is revealed in an in situ electrical transport measurement by using a four-point probe installed in a scanning tunneling microscope (STM). Consequent research interest in multi-layer FeSe films grown on STO surface is also increasing. The quality of thick FeSe film, however, has not been well studied yet in previous studies, although it is related to the sample properties including superconductivity. Here, reflection high-energy electron diffraction (RHEED) is used to monitor the growths of multi-layer FeSe thin films on STO surface under different growth conditions. Combing the RHEED results with STM observations taken at various FeSe coverages, we find that the intensity evolution of the RHEED pattern in the early growth stage can be well explained by the step density model but not by the widely known facet model. The intensity evolution of the FeSe(02) diffraction streak exhibits a single-peak oscillation in the growing of the first layer of FeSe. As the oscillation does not depend on the grazing angle of the high-energy electron beam, the FeSe(02) diffraction streak is very suitable for calibrating the FeSe growth rate. In contrast, the intensity of the specular spot exhibits different evolution pattern when the grazing angle of electron beam is changed. It is found in STM observations that only at an appropriate substrate temperature and a growth rate can the high-quality multi-layer FeSe films be grown on STO substrates. If the growth temperature is too high, the FeSe molecules nucleate into islands so that FeSe films with various thickness values eventually come into being on the STO surface. If the growth temperature is too low, a different phase of FeSe film is formed. The optimal growth temperature is in a range from 400 ℃ to 430 ℃, within which a two-layer FeSe film grown at a low rate (0.15 layer/min) coveres the whole STO surface with a negligible number of small FeSe islands. In contrast, a larger growth rate is necessary for growing thicker FeSe film. This is because FeSe islands tend to come into form at steps when the growth rate is too low, which is more distinct in a thicker FeSe film. An STM image of 80-layer FeSe film grown under an optimal condition, i.e., the substrate temperature of 420 ℃ and the growth rate of 2.3 layer/min, shows that it is in a perfect layer-by-layer growth mode. These experimental results are useful for growing high-quality multi-layer FeSe films on STO substrates, which could be critical for studying their physical properties and relevant physical phenomena.

Correlation Between T c and Hyperfine Parameters of Fe in Layered Chalcogenide Superconductors

Although pairing mechanism in unconventional superconductors is still an open question, the density of states at the Fermi level is considered to be one of the factors affecting the superconducting transition temperature. Herein, we report on 57Fe-Mossbauer studies of β-FeSe, FeSe 0.5Te 0.5, and Rb 0.8Fe 1.6Se 2 superconductors as well as two intercalate products consisting of FeSe layers and a lithium-containing molecular spacer in between. In these materials, the hyperfine parameters of 57Fe are directly related to the 3d-electron density on Fe atoms and show strong correlation with superconducting properties.

Structure and superconductivity of (Li1−xFex)OHFeSe single crystals grown using AxFe2−ySe2 (A = K, Rb, and Cs) as precursors

We present results on the hydrothermal growth of ()OHFeSe single crystals using floating-zone-grown (A = K, Rb, and Cs) as precursors. The growth proceeds by the hydrothermal ion exchange of Li/Fe–O–H for K, Rb, and Cs, resulting in a stacking layer of ()OH sandwiched between the FeSe layers. Optimal growth parameters are achieved using high quality A0.80Fe1.81Se2 single crystals added to the mixtures of LiOH, H2O, Fe and C(NH2)2Se in an autoclave and subsequently heated to 120 °C for 2 d, to obtain highest quality single crystals. The obtained crystals have lateral dimensions up to centimeters, with the final size related to that of the precursor crystal used. All ()OHFeSe single crystals show a superconducting transition temperature Tc > 42 K, regardless of the phase of the precursor such as superconducting K0.80Fe1.81Se2 (Tc = 29.31 K) or non-superconducting Rb0.80Fe1.81Se2 or poor-superconducting Cs0.80Fe1.81Se2 (Tc = 28.67 K). The Tc and transition width ΔT vary in the obtained single crystals, due to the inhomogeneity of the ionic exchange. X-ray diffraction analysis demonstrates that the 245 insulating phase is absent in the ion-exchanged single crystals, while it is observed in the precursors. Comparative studies of the structure, magnetization, and superconductivity on the parent A0.80Fe1.81Se2 and the ion-exchanged ()OHFeSe crystals are discussed. A phase diagram including antiferromagnetic spin density wave and superconducting phases is also proposed.

Robustness of superconductivity to structural disorder inSr0.3(NH2)y(NH3)1−yFe2Se2

The superconducting properties of a recently discovered high-${T}_{\mathrm{c}}$ superconductor, Sr/ammonia-intercalated FeSe, have been measured using pulsed magnetic fields down to 4.2 K and muon spin spectroscopy down to 1.5 K. This compound exhibits intrinsic disorder resulting from random stacking of the FeSe layers along the $c$ axis that is not present in other intercalates of the same family. This arises because the coordination requirements of the intercalated Sr and ammonia moieties imply that the interlayer stacking (along $c$) involves a translation of either $\mathbf{a}/2$ or $\mathbf{b}/2$ that locally breaks tetragonal symmetry. The result of this stacking arrangement is that the Fe ions in this compound describe a body-centered-tetragonal lattice in contrast to the primitive arrangement of Fe ions described in all other Fe-based superconductors. In pulsed magnetic fields, the upper critical field ${H}_{\text{c2}}$ was found to increase on cooling with an upward curvature that is commonly seen in type-II superconductors of a multiband nature. Fitting the data to a two-band model and extrapolation to absolute zero gave a maximum upper critical field ${\ensuremath{\mu}}_{0}{H}_{\mathrm{c}2}(0)$ of $33(2)\phantom{\rule{4pt}{0ex}}\text{T}$. A clear superconducting transition with a diamagnetic shift was also observed in transverse-field muon measurements at ${T}_{\text{c}}\ensuremath{\approx}36.3(2)\phantom{\rule{4pt}{0ex}}\text{K}$. These results demonstrate that robust superconductivity in these intercalated FeSe systems does not rely on perfect structural coherence along the $c$ axis.

Enhanced superconducting transition temperature in hyper-interlayer-expanded FeSe despite the suppressed electronic nematic order and spin fluctuations

The superconducting critical temperature, $T_{\rm c}$, of FeSe can be dramatically enhanced by intercalation of a molecular spacer layer. Here we report on a $^{77}$Se, $^7$Li and $^1$H nuclear magnetic resonance (NMR) study of the powdered hyper-interlayer-expanded Li$_{x}($C$_2$H$_8$N$_2$)$_y$Fe$_{2-z}$Se$_2$ with a nearly optimal $T_{\rm c}=45$~K. The absence of any shift in the $^7$Li and $^1$H NMR spectra indicates a complete decoupling of interlayer units from the conduction electrons in FeSe layers, whereas nearly temperature-independent $^7$Li and $^1$H spin-lattice relaxation rates are consistent with the non-negligible concentration of Fe impurities present in the insulating interlayer space. On the other hand, strong temperature dependence of $^{77}$Se NMR shift and spin-lattice relaxation rate, $1/^{77}T_1$, is attributed to the hole-like bands close to the Fermi energy. $1/^{77}T_1$ shows no additional anisotropy that would account for the onset of electronic nematic order down to $T_{\rm c}$. Similarly, no enhancement in $1/^{77}T_1$ due to the spin fluctuations could be found in the normal state. Yet, a characteristic power-law dependence $1/^{77}T_1\propto T^{4.5}$ still comply with the Cooper pairing mediated by spin fluctuations.

(Li0.84Fe0.16)OHFe0.98Sesuperconductor: Ion-exchange synthesis of large single-crystal and highly two-dimensional electron properties

A large and high-quality single crystal (Li0.84Fe0.16)OHFe0.98Se, the optimal superconductor of newly reported (Li1-xFex)OHFe1-ySe system, has been successfully synthesized via a hydrothermal ion-exchange technique. The superconducting transition temperature (Tc) of 42 K is determined by magnetic susceptibility and electric resistivity measurements, and the zero-temperature upper critical magnetic fields are evaluated as 79 and 313 Tesla for the field along the c-axis and the ab-plane, respectively. The ratio of out-of-plane to in-plane electric resistivity,\r{ho}c/\r{ho}ab, is found to increases with decreasing temperature and to reach a high value of 2500 at 50 K, with an evident kink occurring at a characteristic temperature T*=120 K. The negative in-plane Hall coefficient indicates that electron carriers dominate in the charge transport, and the hole contribution is significantly reduced as the temperature is lowered to approach T*. From T* down to Tc, we observe the linear temperature dependences of the in-plane electric resistivity and the magnetic susceptibility for the FeSe layers. Our findings thus reveal that the normal state of (Li0.84Fe0.16)OHFe0.98Se becomes highly two-dimensional and anomalous prior to the superconducting transition, providing a new insight into the mechanism of high-Tc superconductivity.

Surface electronic structure and isotropic superconducting gap in(Li0.8Fe0.2)OHFeSe

Using angle-resolved photoemission spectroscopy (ARPES), we revealed the surface electronic structure and superconducting gap of (Li$_{0.8}$Fe$_{0.2}$)OHFeSe, an intercalated FeSe-derived superconductor without antiferromagnetic phase or Fe-vacancy order in the FeSe layers, and with a superconducting transition temperature ($T_c$) $\sim$ 40 K. We found that (Li$_{0.8}$Fe$_{0.2}$)OH layers dope electrons into FeSe layers. The electronic structure of surface FeSe layers in (Li$_{0.8}$Fe$_{0.2}$)OHFeSe resembles that of Rb$_x$Fe$_{2-y}$Se$_2$ except that it only contains half of the carriers due to the polar surface, suggesting similar quasiparticle dynamics between bulk (Li$_{0.8}$Fe$_{0.2}$)OHFeSe and Rb$_x$Fe$_{2-y}$Se$_2$. Superconducting gap is clearly observed below $T_c$, with an isotropic distribution around the electron Fermi surface. Compared with $A_x$Fe$_{2-y}$Se$_2$ (\textit{A}=K, Rb, Cs, Tl/K), the higher $T_c$ in (Li$_{0.8}$Fe$_{0.2}$)OHFeSe might be attributed to higher homogeneity of FeSe layers or to some unknown roles played by the (Li$_{0.8}$Fe$_{0.2}$)OH layers.

Emergence of double-dome superconductivity in ammoniated metal-doped FeSe.

The pressure dependence of the superconducting transition temperature (Tc) and unit cell metrics of tetragonal (NH3)yCs0.4FeSe were investigated in high pressures up to 41 GPa. The Tc decreases with increasing pressure up to 13 GPa, which can be clearly correlated with the pressure dependence of c (or FeSe layer spacing). The Tc vs. c plot is compared with those of various (NH3)yMxFeSe (M: metal atoms) materials exhibiting different Tc and c, showing that the Tc is universally related to c. This behaviour means that a decrease in two-dimensionality lowers the Tc. No superconductivity was observed down to 4.3 K in (NH3)yCs0.4FeSe at 11 and 13 GPa. Surprisingly, superconductivity re-appeared rapidly above 13 GPa, with the Tc reaching 49 K at 21 GPa. The appearance of a new superconducting phase is not accompanied by a structural transition, as evidenced by pressure-dependent XRD. Furthermore, Tc slowly decreased with increasing pressure above 21 GPa, and at 41 GPa superconductivity disappeared entirely at temperatures above 4.9 K. The observation of a double-dome superconducting phase may provide a hint for pursuing the superconducting coupling-mechanism of ammoniated/non-ammoniated metal-doped FeSe.

Modulation effect of interlayer spacing on the superconductivity of electron-doped FeSe-based intercalates.

FeSe-based intercalates are regarded as promising candidates for high-critical temperature (Tc) superconductors. Here we present new Na- and Sr-intercalated FeSe superconductors with embedded linear diamines (H2N)CnH2n(NH2) (abbreviated as DA; n = 0, 2, 3, or 6) prepared using a low-temperature ammonothermal method to investigate the effect of interlayer spacing on the superconductivity of electron-doped FeSes. The embedded DA formed a monolayer or bilayer in the interlayer of FeSe. The interlayer spacing between nearest FeSe layers could be tuned from 0.87 to 1.14 nm without significant change in the Na/Sr content or the ratio of Fe to Se. Importantly, bilayer phases Na/ethylenediamine- and Sr/hydrazine-FeSe show improved structural stability compared to that of Na/NH3-FeSe. The series of Na- and Sr-intercalated FeSe samples exhibited nearly the same high Tc values of 41-46 and 34-38 K, respectively, irrespective of rather different interlayer spacing d. The peculiar insensitivity for both series can be ascribed to the negligible dispersions of bands along the c axis; i.e., Fermi surfaces are nearly two-dimensional when d is larger than a certain threshold value (dsat) of ∼0.9 nm. The Fermi surface shape is already optimal for Tc, and a larger d will not enhance Tc further. On the other hand, the difference in Tc between two series may be explained by the higher carrier doping level in Na/DA-FeSes compared to that in Sr/DA-FeSes, resulting in the increased density of states at the Fermi level and superconducting pairing strength.

Optimization of a non-arsenic iron-based superconductor for wire fabrication

We report on the optimization of synthesis of iron selenide-based superconducting powders and the fabrication of selenide-based wire. The powders were synthesized by an ammonothermal method, whereby Ba is intercalated between FeSe layers to produce Bax(NH3)yFe2Se2, with tetragonal structure similar to AFe2X2 (X: As, Se), ‘122’, superconductors. The optimal Tc (up to 38 K) and Meissner and shielding superconducting fractions are obtained from the shortest reaction time (t) of reactants in liquid ammonia (30 min). With the increase of t, a second crystalline 122 phase, with a smaller unit cell, emerges. A small amount of NH3 is released from the structure above ∼200 °C, which results in loss of superconductivity. However, in the confined space of niobium/Monel tubing, results indicate there is enough pressure for some of NH3 to remain in the crystal lattice, and thermal annealing can be performed at temperatures of up to 780 °C, increasing wire density and yielded a reasonable Tc ≈ 16 K. Here, we report of the first successful wire fabrication of non-arsenic high-Tc iron-based superconductor. Although bulk materials are estimated to carry critical current densities >100 kA cm−2 (4 K, self-field), the current transport within wires need to be optimized (Jc ∼ 1 kA cm−2).

Intercalation effect on hyperfine parameters of Fe in FeSe superconductor with Tc = 42 K

57Fe-Mössbauer spectra of superconducting β-FeSe, the Li/NH3 intercalate product and a subsequent sample of this intercalate treated with moist He gas have been measured in the temperature range 4.7–290 K. A correlation is established between hyperfine parameters and critical temperature in these phases. A strong increase of the isomer shift upon intercalation is explained by a charge transfer from the Li/NH3 intercalate to the FeSe layers resulting in an increase of up to 42 K. A significant decrease of the quadrupole splitting above 240 K has been attributed to diffusive motion of Li+ ions within the interlamellar space.

First-principles study of magnetic frustration in FeSe epitaxial films onSrTiO3

The effects of electron doping and phonon vibrations on the magnetic properties of monolayer and bilayer FeSe epitaxial films on SrTiO$_3$ have been studied, respectively, using first-principles calculations with van der Waals correction. For monolayer FeSe epitaxial film, the combined effect of electron doping and phonon vibrations readily leads to magnetic frustration between the collinear antiferromagnetic state and the checkerboard antiferromagnetic N\'eel state. For bilayer FeSe epitaxial film, such magnetic frustration is much more easily induced by electron doping in its bottom layer than its top layer. The underlying physics is that the doped electrons are accumulated at the interface between the FeSe layers and the substrate. These results are consistent with existing experimental studies.

Electronic and magnetic properties of the new iron-based superconductor [Li1 − x Fe x OH]FeSe

We present the results of paramagnetic LDA band structure calculations: band dispersions, densities of states and Fermi surfaces, for the new iron based high-temperature superconductor LiOHFeSe. Main structural motif providing bands in the vicinity of the Fermi level is FeSe layer which is isostructural to the bulk FeSe prototype superconductor. The bands crossing the Fermi level and Fermi surfaces of the new compound are typical for other iron based superconductors. Experimentally it was shown that introduction of Fe ions into LiOH layer gives rise to ferromagnetic ordering of the Fe ions at T$_C$=10K. To study magnetic properties of [Li$_{0.8}$Fe$_{0.2}$OH]FeSe system we have performed LSDA calculations for $\sqrt 5 \times \sqrt 5$ superlattice and found ferromagnetism within the Li$_4$Fe(OH) layer. To estimate the Curie temperature we obtained Fe-Fe exchange interaction parameters for Heisenberg model from our LSDA calculations, leading to theoretical value of Curie temperature 10.4K in close agreement with experiment.

New high-T c iron-selenide superconductor with hydroxide spacer layers

Iron-based superconductors include pnictides and chalcogenides. So far, the chalcogenides are much fewer in number, thus it is particularly valuable to find new superconductors in iron chalcogenides, for the “ultimate” understanding of iron-based superconductivity. The tetragonal binary iron selenide (β-Fe1+δSe) is the structurally simplest iron-based superconductor with a super-conducting critical temperature (Tc) of 8.5 K at ambient pressure. Under a hydrostatic pressure of ∼7 GPa, the Tc can be greatly enhanced to 37 K. By intercalation of alkali metal ions into adjacent iron-selenide layers, which induces more electrons into the FeSe layers, the Tc can also be increased to above 30 K. Upon applying high pressures around ∼12 GPa, an unknown secondary superconducting phase re-emerges at 48 K. Unfortunately, there exist phase separations in these intercalated compounds with only the minority phase responsible for superconductivity, which prevents revealing the intrinsic properties. Another remarkable progress is the observation of superconductivity in one-unit-cell FeSe thin film on a Sr-TiO3 substrate with impressively high Tc values up to 65 K and more recently exceeding 100 K. These findings inspire the exploration of high-Tc superconductivity in bulk FeSe-based materials.

Coexistence of superconductivity and antiferromagnetism in (Li0.8Fe0.2)OHFeSe.

Iron selenide superconductors exhibit a number of unique characteristics that are helpful for understanding the mechanism of superconductivity in high-Tc iron-based superconductors more generally. However, in the case of AxFe2Se2 (A = K, Rb, Cs), the presence of an intergrown antiferromagnetic insulating phase makes the study of the underlying physics problematic. Moreover, FeSe-based systems intercalated with alkali metal ions, NH3 molecules or organic molecules are extremely sensitive to air, which prevents the further investigation of their physical properties. It is therefore desirable to find a stable and easily accessible FeSe-based superconductor to study its physical properties in detail. Here, we report the synthesis of an air-stable material, (Li0.8Fe0.2)OHFeSe, which remains superconducting at temperatures up to ~40 K, by means of a novel hydrothermal method. The crystal structure is unambiguously determined by a combination of X-ray and neutron powder diffraction and nuclear magnetic resonance. Moreover, antiferromagnetic order is shown to coexist with superconductivity. This synthetic route opens a path for exploring superconductivity in other related systems, and confirms the appeal of iron selenides as a platform for understanding superconductivity in iron pnictides more broadly.

Effect of the iron valence in the two types of layers inLiFeO2Fe2Se2

We perform electronic structure calculations for the recently synthesized iron-based superconductor ${\mathrm{LiFeO}}_{2}{\mathrm{Fe}}_{2}{\mathrm{Se}}_{2}$. In contrast to other iron-based superconductors, this material comprises two different iron atoms in $3{d}^{5}$ and $3{d}^{6}$ configurations. In band theory, both contribute to the low-energy electronic structure. Spin-polarized density functional theory calculations predict an antiferromagnetic metallic ground state with different moments on the two Fe sites. However, several other almost degenerate magnetic configurations exist. Due to their different valences, the two iron atoms behave very differently when local quantum correlations are included through the dynamical mean-field theory. The contributions from the half-filled $3{d}^{5}$ atoms in the ${\mathrm{LiFeO}}_{2}$ layer are suppressed and the $3{d}^{6}$ states from the FeSe layer restore the standard iron-based superconductor fermiology.

Tuning the band structure and superconductivity in single-layer FeSe by interface engineering.

The interface between transition metal compounds provides a rich playground for emergent phenomena. Recently, significantly enhanced superconductivity has been reported for single-layer FeSe on Nb-doped SrTiO3 substrate. Yet it remains mysterious how the interface affects the superconductivity. Here we use in situ angle-resolved photoemission spectroscopy to investigate various FeSe-based heterostructures grown by molecular beam epitaxy, and uncover that electronic correlations and superconducting gap-closing temperature (Tg) are tuned by interfacial effects. Tg up to 75 K is observed in extremely tensile-strained single-layer FeSe on Nb-doped BaTiO3, which sets a record high pairing temperature for both Fe-based superconductor and monolayer-thick films, providing a promising prospect on realizing more cost-effective superconducting device. Moreover, our results exclude the direct correlation between superconductivity and tensile strain or the energy of an interfacial phonon mode, and highlight the critical and non-trivial role of FeSe/oxide interface on the high Tg, which provides new clues for understanding its origin.

Dichotomy of the electronic structure and superconductivity between single-layer and double-layer FeSe/SrTiO3 films.

The latest discovery of possible high-temperature superconductivity in the single-layer FeSe film grown on a SrTiO3 substrate has generated much attention. Initial work found that, while the single-layer FeSe/SrTiO3 film exhibits a clear signature of superconductivity, the double-layer film shows an insulating behaviour. Such a marked layer-dependent difference is surprising and the underlying origin remains unclear. Here we report a comparative angle-resolved photoemission study between the single-layer and double-layer FeSe/SrTiO3 films annealed in vacuum. We find that, different from the single-layer FeSe/SrTiO3 film, the double-layer FeSe/SrTiO3 film is hard to get doped and remains in the semiconducting/insulating state under an extensive annealing condition. Such a behaviour originates from the much reduced doping efficiency in the bottom FeSe layer of the double-layer FeSe/SrTiO3 film from the FeSe-SrTiO3 interface. These observations provide key insights in understanding the doping mechanism and the origin of superconductivity in the FeSe/SrTiO3 films.