arXiv subjects

Dirk Johrendt

Publications and source records attributed to Dirk Johrendt.

At least 19 recordsLinked to original sources

arXiv2024

Effects of Chemical Pressure on Superconductivity in Electrochemically Intercalated (TMA)yFe2(Se1-xSx)2 (TMA = Tetramethylammonium)

The beta-modification of FeSe, which has an anti-PbO type structure, achieves superconductivity at 8 K without external doping or pressure and exhibits a nematic phase, which has been crucial for studies of unconventional pairing mechanisms. Although the critical temperature (Tc) in FeSe increases significantly with applied pressure, intercalation, and in thin films, the effect of replacing selenium with sulfur in FeSe1-xSx has remained unclear. To investigate the effects of chemical pressure, we have synthesized FeSe1-xSx crystals (up to x = 0.52) and intercalated them with tetramethylammonium ions (TMA+). Our results show that both Tc and unit cell volume decrease linearly with sulfur content in both host and intercalated (TMA)yFe2(Se1-xSx)2. The unexpected common rate of normalized decrease of Tc suggests that chemical pressure affects the electron-doped intercalates in the same way as the host compounds. This result highlights the unique role of chemical pressure and electron doping in tuning superconductivity in FeSe systems and provides new insights into their complex behavior.

cond-mat.supr-con↗

arXiv2023

Fidelity and variability in the interlayer electronic structure of the kagome superconductor CsV3Sb5

The AV3Sb5 (A = K, Rb, Cs) kagome materials host an interplay of emergent phenomena including superconductivity, charge density wave states, and non-trivial electronic structure topology. The band structures of these materials exhibit a rich variety of features like Dirac crossings, saddle points associated with van Hove singularities, and flat bands prompting significant investigations into the in-plane electronic behavior. However, recent findings including the charge density wave ordering and effects due to pressure or chemical doping point to the importance of understanding interactions between kagome layers. Probing this c-axis electronic structure via experimental methods remains challenging due to limitations of the crystals and, therefore, rigorous computational approaches are necessary to study the interlayer interactions. Here we use first-principles approaches to study the electronic structure of CsV3Sb5 with emphasis on the kz dispersion. We find that the inclusion of nonlocal and dynamical many-body correlation has a substantial impact on the interlayer band structure. We present new band behavior that additionally supports the integration of symmetry in accurately plotting electronic structures and influences further analysis like the calculation of topological invariants.

cond-mat.mtrl-sci↗

arXiv2023

Electronic nematicity without charge density waves in titanium-based kagome metal

Layered crystalline materials that consist of transition metal atoms on a kagome network have emerged as a versatile platform to study unusual electronic phenomena. For example, in the vanadium-based kagome superconductors AV3Sb5 (where A can stand for K, Cs, or Rb) there is a parent charge density wave phase that appears to simultaneously break both the translational and the rotational symmetry of the lattice. Here, we show a contrasting situation where electronic nematic order - the breaking of rotational symmetry without the breaking of translational symmetry - can occur without a corresponding charge density wave. We use spectroscopic-imaging scanning tunneling microscopy to study the kagome metal CsTi3Bi5 that is isostructural to AV3Sb5 but with a titanium atom kagome network. CsTi3Bi5 does not exhibit any detectable charge density wave state, but comparison to density functional theory calculations reveals substantial electronic correlation effects at low energies. Comparing the amplitudes of scattering wave vectors along different directions, we discover an electronic anisotropy that breaks the six-fold symmetry of the lattice, arising from both in-plane and out-of-plane titanium-derived d orbitals. Our work uncovers the role of electronic orbitals in CsTi3Bi5, suggestive of a hexagonal analogue of the nematic bond order in Fe-based superconductors.

cond-mat.str-el↗

arXiv2022

The kagomé metals RbTi$_3$Bi$_5$ and CsTi$_3$Bi$_5$

The kagomé metals RbTi$_3$Bi$_5$ and CsTi$_3$Bi$_5$ were synthesized both as polycrystalline powders by heating the elements an argon atmosphere and as single crystals grown using a self-flux method. The compounds crystallize in the hexagonal crystal system isotypically to KV$_3$Sb$_5$ (P6/mmm, Z = 1, CsTi3Bi5: a = 5.7873(1) Å, c = 9.2062(1) Å; RbTi3Bi5: a = 5.773(1) Å, c = 9.065(1) Å). Titanium atoms form a kagomé net with bismuth atoms in the hexagons as well as above and below the triangles. The alkali metal atoms are coordinated by 12 bismuth atoms and form AlB$_2$-like slabs between the kagomé layers. Magnetic susceptibility measurements with CsTi$_3$Bi$_5$ and RbTi$_3$Bi$_5$ single crystals reveal Pauli-paramagnetism and traces of superconductivity caused by CsBi$_2$/RbBi$_2$ impurities. Magnetotransport measurements reveal conventional Fermi liquid behavior and quantum oscillations indicative of a single dominant orbit at low temperature. DFT calculations show the characteristic metallic kagomé band structure similar to that of CsV$_3$Sb$_5$ with reduced band filling. A symmetry analysis of the band structure does not reveal an obvious and unique signature of a nontrivial topology.

cond-mat.mtrl-sci↗

arXiv2021

Electrochemical synthesis and crystal structure of the organic ion intercalated superconductor (TMA)0.5Fe2Se2 with Tc = 43 K

Intercalation of organic cations in superconducting iron selenide can significantly increase the critical temperature (Tc). We present an electrochemical method using beta-FeSe crystals (Tc ~ 8 K) floating on a mercury cathode to intercalate tetramethylammonium ions (TMA+) quantitatively to obtain bulk samples of (TMA)0.5Fe2Se2 with Tc = 43 K. The layered crystal structure is closely related to the ThCr2Si2-type with disordered TMA+ ions between the FeSe layers. Although the organic ions are not detectable by X-ray diffraction, packing requirements as well as first principle DFT calculations constrain the specified structure. Our synthetic route enables electrochemical intercalations of other organic cations with high yields to greatly optimize the superconducting properties, and to expand this class of high-Tc materials.

cond-mat.supr-con↗

arXiv2017

FeSe(en)0.3 - Separated FeSe layers with stripe-type crystal structure by intercalation of neutral spacer molecules

Solvothermal intercalation of ethylenediamine molecules into FeSe separates the layers by 1078 pm and creates a different stacking. FeSe(en)0.3 is not superconducting although each layer exhibits the stripe-type crystal structure and the Fermi surface topology of superconducting FeSe. FeSe(en)0.3 requires electron-doping for high-Tc similar to monolayers of FeSe@SrTiO3, whose much higher Tc may arise from the proximity of the oxide surface.

cond-mat.supr-con↗

arXiv2015

Structural transition and superconductivity in hydrothermally synthesized FeX (X = S, Se)

Tetragonal beta-FeSe obtained by hydrothermal reaction is not superconducting and transforms to a triclinic structure at 60 K unlike superconducting FeSe from solid state synthesis, which becomes orthorhombic at 90 K. In contrast, tetragonal iron sulphide FeS from hydrothermal synthesis is superconducting at 4.8 K but undergoes no structural transition. Our results suggest that the absence of superconductivity in hydrothermally synthesized FeSe may be associated to the low-temperature structure with zigzag chains of iron atoms, which is different from the known orthorhombic Cmme structure of superconducting FeSe.

cond-mat.supr-con↗

arXiv2015

Synthesis, Crystal Structure and Magnetism of Eu3Sc2O5Fe2As2

The iron arsenide Eu3Fe2O5Fe2As2 was synthesized at 1173-1373 K in a resistance furnace and characterized by X-ray powder diffraction with Rietveld analysis: Sr3Fe2O5Cu2S2-type, I4/mmm, a = 406.40(1) pm, c = 2646.9(1) pm. Layers of edge-sharing FeAs4/4 tetrahedra are separated by perovskite-like oxide blocks. No structural transition occurs in the temperature range from 10 to 300 K. Magnetic measurements have revealed Curie-Weiss behavior with an effective magnetic moment of 7.79 muB per europium atom in agreement with the theoretical value of 7.94 muB for Eu2+. A drop in the magnetic susceptibility at 5 K indicates possible antiferromagnetic ordering. 151Eu and 57Fe Mössbauer spectroscopic measurements have confirmed a beginning cooperative magnetic phenomenon by showing significantly broadened spectra at 4.8 K compared to those at 78 K.

cond-mat.str-el↗

arXiv2015

Oxygen-annealing effects on superconductivity in polycrystalline Fe1-xTe1-ySey

Superconductivity in anti-PbO-type iron chalcogenides Fe1-xTe1-ySey (x = 0, 0.1, y = 0.1 0.4) depends on the amount (x) of interstitial iron atoms located between the FeTe1-ySey layers. Non-superconducting samples of nominal Fe1.1Te1-ySey convert to superconductors with critical temperatures up to 14 K after annealing at 300°C in an oxygen atmosphere. The process is irreversible upon subsequent hydrogen annealing. Magnetic measurements are consistent with the formation of iron oxides suggesting that oxygen annealing preferably extracts interstitial iron from Fe1-xTe1-ySey which interfere with superconductivity.

cond-mat.supr-con↗

arXiv2014

[(Li0.8Fe0.2)OH]FeS and the ferromagnetic superconductors [(Li0.8Fe0.2)OH]Fe(S1-xSex) (0<x<1)

[(Li0.8Fe0.2)OH]FeS and the series [(Li0.8Fe0.2)OH]Fe(S1-xSex) (0<x<1) were synthesized by hydrothermal methods and characterized by X-ray single crystal and powder diffraction, EDX and chemical analysis. Selenium-rich compounds show the coexistence of magnetic ordering with superconductivity known from the pure selenium compound. Sulphur doping decreases the critical temperature through chemical pressure until superconductivity is completely absent in [(Li0.8Fe0.2)OH]FeS, while the ferromagnetism in the [(Li0.8Fe0.2)OH] layers persists. The Li:Fe ratio in the hydroxide layer, and thus the charge transfer of 0.2 electrons from the hydroxide to the iron chalcogenide layers remains unchanged in [(Li0.8Fe0.2)OH]Fe(S1-xSex), which indicates that the chemical pressure effect of the smaller sulphide ions impedes superconductivity in [(Li0.8Fe0.2)OH]FeS

cond-mat.supr-con↗

arXiv2014

Coexistence of 3d-ferromagnetism and superconductivity in [(Li(1-x)Fex)OH](Fe(1-y)Liy)Se

Superconducting [(Li(1-x)Fex)OH](Fe(1-y)Liy)Se (x ~ 0.2, y ~ 0.08) was synthesized by hydrothermal methods and structurally characterized by single crystal X-ray diffraction. The crystal structure contains anti-PbO type (Fe(1-y)Liy)Se layers separated by layers of (Li(1-x)Fex)OH. Electrical resistivity and magnetic susceptibility measurements reveal superconductivity at 43 K. An anomaly in the diamagnetic shielding indicates ferromagnetic ordering near 10 K while superconductivity is retained. The ferromagnetism emerges from the iron atoms in the (Li(1-x)Fex)OH layer. Isothermal magnetization measurements confirm the superposition of ferromagnetic with superconducting hysteresis. The internal ferromagnetic field is larger than the lower, but smaller than the upper critical field of the superconductor, which gives evidence for a spontaneous vortex phase where both orders coexist. 57Fe-Mössbauer spectra, 7Li-NMR spectra, and muSR experiments consistently support this rare situation, especially in a bulk material where magnetism emerges from a 3d-element.

cond-mat.supr-con↗

arXiv2014

Superconductivity by transition metal doping in Ca$_{10}$(Fe$_{1-x}$M$_x$As)$_{10}$(Pt$_3$As$_8$) (M = Co, Ni, Cu)

We report the successful substitution of cobalt, nickel, and copper for iron in the 1038 phase parent compound Ca$_{10}$(FeAs)$_{10}$(Pt$_3$As$_8$) yielding Ca$_{10}$(Fe$_{1-x}$Co$_x$As)$_{10}$(Pt$_3$As$_8$), Ca$_{10}$(Fe$_{1-x}$Ni$_x$As)$_{10}$(Pt$_3$As$_8$), and Ca$_{10}$(Fe$_{1-x}$Cu$_x$As)$_{10}$(Pt$_3$As$_8$), respectively. Superconductivity is induced in Co and Ni doped compounds reaching critical temperatures up to 15 K, similar to known Pt substituted Ca$_{10}$(Fe$_{1-x}$Pt$_x$As)$_{10}$(Pt$_3$As$_8$), whereas no superconductivity was detected in Ca$_{10}$(Fe$_{1-x}$Cu$_x$As)$_{10}$(Pt$_3$As$_8$). The obtained Tc(x) phase diagrams are very similar to those of other iron arsenide superconductors indicating rather universal behavior despite the more complex structures of the 1038-type compounds, where the physics is primarily determined by the FeAs layer.

cond-mat.supr-con↗

arXiv2014

Superconductivity by rare earth doping in the 1038-type compounds (Ca$_{1-x}$RE$_x$)$_{10}$(FeAs)$_{10}$(Pt$_3$As$_8$) with RE = Y, La-Nd, Sm-Lu

We report superconductivity in polycrystalline samples of the 1038-type compounds (Ca$_{1-x}$RE$_x$)$_{10}$(FeAs)$_{10}$(Pt$_3$As$_8$) up to T$_c$ = 35 K with RE = Y, La-Nd, Sm, Gd-Lu. The critical temperatures are independent of the trivalent rare earth element used, yielding an universal T$_c$($x$) phase diagram for electron doping in all these systems. The absence of superconductivity in Eu$^{2+}$ doped samples, as well as the close resemblance of (Ca$_{1-x}$RE$_x$)$_{10}$(FeAs)$_{10}$(Pt$_3$As$_8$) to the 1048 compound substantiate that the electron doping scenario in the RE-1038 and 1048 phases is completely analogous to other iron-based superconductors with simpler crystal structures.

cond-mat.supr-con↗

arXiv2014

Microscopic Coexistence of Magnetism and Superconductivity in charge compensated Ba1-xKx(Fe1-yCoy)2As2

We present a detailed investigation of the electronic phase diagram of effectively charge compensated Ba1-xKx(Fe1-yCoy)2As2 with x/2 = y. Our experimental study by means of x-ray diffraction, Mössbauer spectroscopy, muon spin relaxation and ac susceptibility measurements on polycrystalline samples is complemented by density functional electronic structure calculations. For low substitution levels of x/2 = y \< 0.13, the system displays an orthorhombically distorted and antiferromagnetically ordered ground state. The low temperature structural and magnetic order parameters are successively reduced with increasing substitution level. We observe a linear relationship between the structural and the magnetic order parameter as a function of temperature and substitution level for x/2 = y \< 0.13. At intermediate substitution levels in the range between 0.13 and 0.19, we find superconductivity with a maximum Tc of 15 K coexisting with static magnetic order on a microscopic length scale. For higher substitution levels x/2 = y \> 0.25 a tetragonal non-magnetic ground state is observed. Our DFT calculations yield a signifcant reduction of the Fe 3d density of states at the Fermi energy and a strong suppression of the ordered magnetic moment in excellent agreement with experimental results. The appearance of superconductivity within the antiferromagnetic state can by explained by the introduction of disorder due to non-magnetic impurities to a system with a constant charge carrier density. Our experimental study by means of x-ray diffraction, Mössbauer spectroscopy, muon spin relaxation and ac susceptibility measurements on polycrystalline samples is complemented by density functional electronic structure calculations.

cond-mat.supr-con↗

arXiv2013

Superconductivity in Ba1-xKxTi2Sb2O (0 < x < 1) controlled by charge doping

The solid solution of antimonide-oxides Ba1-xKxTi2Sb2O (0 < x < 1) has been synthesized by solid-state reactions and characterized by X-ray powder diffraction (CeCr2Si2C-type structure; P4/mmm, Z = 1). The crystal structure consists of Ti2Sb2O-layers that are stacked with layers of barium atoms along the c-axis. BaTi2Sb2O is a known superconductor with a critical temperature (Tc) of 1.2 K. Substitution of barium through potassium raises Tc up to 6.1 K at 12 % potassium, while no superconductivity emerges with concentrations higher than 20 %. Anomalies in electrical transport and magnetic susceptibility indicate charge density wave (CDW) instabilities. The CDW transition temperatures (Ta) decrease from 50 K in the parent compound to 28 K at 10 % potassium substitution. No CDW transition was detected at higher concentrations, and no evidence for a reduction of the lattice symmetry below Ta was found. The lattice parameters vary linearly while the unit cell volume increases with higher potassium concentrations. The phase diagrams Tc(x) and Ta(x) of Ba1-xKxTi2Sb2O are remarkably similar to the known series Ba1-xNaxTi2Sb2O (0 < x < 0.33) in spite of the reverse volume effect. From this we conclude that the charge and not the volume determines the phase diagrams of these superconducting antimony oxides.

cond-mat.supr-con↗

arXiv2013

Weak magnetism and the Mott-state of vanadium in superconducting Sr2VO3FeAs

We report neutron scattering data and DFT calculations of the stoichiometric iron-arsenide superconductor Sr2VO3FeAs. Rietveld refinements of neutron powder patterns confirm the ideal composition without oxygen deficiencies. Experiments with polarized neutrons prove weak magnetic ordering in the V-sublattice of Sr2VO3FeAs at ~ 45 K with a probable propagation vector q = (1/8,1/8,0). The ordered moment of ~ 0.1 muB is too small to remove the V-3d bands from the Fermi level by magnetic exchange splitting, and much smaller than predicted from a recent LDA+U study. By using DFT calculations with a GGA+EECE functional we recover the typical quasi-nested Fermi-surface even without magnetic moment. From this we suggest that the V-atoms are in a Mott-state where the electronic correlations are dominated by on-site Coulomb-repulsion which shifts the V-3d states away from the Fermi energy. Our results are consistent with photoemission data and clearly reveal that Sr2VO3FeAs is a typical iron-arsenide superconductor with quasi-nested hole- and electron-like Fermi surface sheets, and constitutes no new paradigm. We suggest that intrinsic electron-doping through V3+/V4+ mixed valence is responsible for the absence of SDW ordering.

cond-mat.supr-con↗

arXiv2013

Short range magnetic order and effective suppression of superconductivity by manganese-doping in LaFe1-xMnxAsO1-yFy

We present a study of the structural and physical properties of directly hole doped LaFe1-xMnxAsO (x = 0.0-0.2) and the influence of charge compensation / electron-doping by additional F doping in LaFe0.9Mn0.1AsO1-yFy (y = 0.1-0.5). High quality polycrystalline samples were prepared using a solid state metathesis reaction. The unit cell increases upon Mn doping, but decreases again when additional F is inserted. The semiconducting character of LaFe1-xMnxAsO decreases with additional F doping. Muon spin relaxation (muSR) measurements reveal short range magnetic order in LaFe1-xMnxAsO and a suppression of magnetism by additional electron-doping with fluoride in LaFe0.9Mn0.1AsO1-yFy. Superconductivity remains absent even though the electronic preconditions are fulfilled in electron-doped LaFe0.9Mn0.1AsO1-yFy at x > 0.1, which is suggestive of effective pair breaking by Mn in this system.

cond-mat.supr-con↗

arXiv2013

Ferromagnetism and the formation of interlayer As2-dimers in Ca(Fe1-xNix)2As2

The compounds Ca(Fe1-xNix)2As2 with the tetragonal ThCr2Si2-type structure (space group I4/mmm) show a continuous transition of the interlayer As-As distances from a non-bonding state in CaFe2As2 (dAs-As = 313 pm) to single-bonded As2-dimers in CaNi2As2 (dAs-As = 260 pm). Magnetic measurements reveal weak ferromagnetism which develops near the composition Ca(Fe0.5Ni0.5)2As2, while the compounds with lower and higher nickel concentrations both are Pauli-paramagnetic. DFT band structure calculations reveal that the As2-dimer formation is a consequence of weaker metal-metal in MAs4-layers (M = Fe1-xNix) of Ni-richer compounds, and depends not on depopulation or shift of As-As antibonding states as suggested earlier. Our results also indicate that the ferromagnetism of Ca(Fe0.5Ni0.5)2As2 and related compounds like SrCo2(Ge0.5P0.5)2 is probably not induced by dimer breaking as recently suggested, but arises from the high density of states generated by the transition metal 3d bands near the Fermi level without contribution of the dimers.

cond-mat.str-el↗