Evidence for eight node mixed-symmetry superconductivity in a correlated organic metal

We report a combined theoretical and experimental investigation of the superconducting state in the quasi-two-dimensional organic superconductor $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br. Applying spin-fluctuation theory to a low-energy material-specific Hamiltonian derived from ab initio density functional theory we calculate the quasiparticle density of states in the superconducting state. We find a distinct three-peak structure that results from a strongly anisotropic mixed-symmetry superconducting gap with eight nodes and twofold rotational symmetry. This theoretical prediction is supported by low-temperature scanning tunneling spectroscopy on in situ cleaved single crystals of $\kappa$-(ET)$_2$…

Guided vortex motion in Nb films on facetted substrate surfaces

Abstract Anisotropy of the pinning force in a superconductor can cause a guiding effect on the vortices, which leads to the appearance of new components in the galvanomagnetic quantities of the sample. In this case one can observe an additional odd magnetoresistive component with respect to magnetic field reversal. Furthermore, an even contribution to the Hall voltage is observed. Guided motion of vortices in Nb films on facetted α-Al2O3 (1 0 1 0) was found by measuring the longitudinal and transversal resistivities of three films with transport current directed parallel, perpendicular and at an angle of 45° with respect to the facet ridges. Field inversion was used to separate the even and…

Probing the superconducting state of UPd2Al3 thin films by tunneling spectroscopy

Abstract Giaever-type planar cross junctions of thin films of the heavy fermion superconductor UPd2Al3 and counter-electrodes of Au, Ag and Al were prepared. Tunneling barriers consisting of the native oxide layer of UPd2Al3 and artificial barriers of AlOx and UOx were investigated. The junctions without artificial barrier show a lack of reproducebility. On some junctions a BCS-like tunneling conductivity with a ratio of 2Δ 0 /k B T c ⋍ 3.8 was observed. Using oxidized Al deposited at room temperature as a barrier no substantial increase of the junction resistance occured. With Uraniumoxide a much higher resistance was obtained, but an association of the bias dependent junction conductivity…

Temperature and angular dependence of the upper critical field of UPd2Al3 thin films

Abstract Temperature and angular resolved measurements of the upper critical field Bc2 of UPd2Al3 thin films are presented and compared with new theoretical calculations for weak-coupling s- and d-wave superconductors in the clean limit. In order to take account of the observed anisotropy of the initial slopes near T c , B′ c 2 = −4.9 T/K for B ‖ c and B′ c 2 = −5.8 T/K for B ⊥ c , and the crossing of the phase boundaries in the temperature dependence of Bc2 near T ∗ ≈ 0.75 T/T c , the calculations include anisotropic orbital and paramagnetic pair breaking. Recent speculations upon the occurence of a non-uniform superconducting “Fulde-Ferrell-Larkin-Ovchinnikov”-state are also considered an…

Disorder-induced gap in the normal density of states of the organic superconductorκ-(BEDT-TTF)2Cu[N(CN)2]Br

The local density of states (DOS) of the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br, measured by scanning tunneling spectroscopy on in situ cleaved surfaces, reveals a logarithmic suppression near the Fermi edge persisting above the critical temperature T(c). The experimentally observed suppression of the DOS is in excellent agreement with a soft Hubbard gap as predicted by the Anderson-Hubbard model for systems with disorder. The electronic disorder also explains the diminished coherence peaks of the quasi-particle DOS below T(c).

Magnetotransport properties of epitaxial - and -oriented CeSb thin films

Abstract We present the electronic magnetotransport properties of (1 0 0) - and (1 1 1) -oriented CeSb thin films. The samples were grown epitaxially by molecular beam epitaxy onto sapphire (1 1 2 0) and (0 0 0 1) substrates. The temperature-dependent resistivity is in full correspondence with the behaviour known from bulk material. For (1 0 0) -oriented films the analysis of the temperature and magnetic field-dependent magnetoresistivity results in a H/T phase diagram analogous to that of bulk samples. For (1 1 1) -oriented films slight deviations were observed. In Hall-effect measurements of (1 0 0) -oriented samples a sign change of the Hall coefficient occurred which is in contrast to m…

Tunneling into epitaxial UPd2Al3 thin films

Abstract UPd2Al3–AlOx–Pb Giaever-type tunneling junctions were prepared employing an in vacuo process. The high junction quality is evident by the observation of the well-known superconducting density of states of the Pb counter electrode. For HPbc2

Tunneling junctions of the heavy-fermion superconductor UPd2Al3

Abstract Tunneling spectroscopy on planar Giaever-type junctions is a powerful tool for the investigation of the superconducting state of metals. Since it is possible to prepare high-quality epitaxial thin films of the heavy-fermion compound UPd 2 Al 3 , this method can be used to examine the energy gap of this presumably unconventional superconductor. We prepared cross-junctions consisting of a UPd 2 Al 3 base electrode and a metal counter electrode (Au, Al or Ag). These small area contacts without artificial barriers have only low junction resistances and suffer from irreproducibility. On the other hand, on some of those junctions we observed BCS-like tunneling conductivity. In order to i…

Frequency-dependent conductivity of UPd2Al3 films

The transmission of UPd2Al3 films was studied (4 K < T < 300 K) in the frequency range from 4 to 32 cm−1 by using a coherent source interferometer which allows for measuring both, amplitude and phase. In addition we report on radio frequency and optical measurements. Below 20 K the conductivity and dielectric constant show strong deviations from the behavior of a normal metal which cannot simply be explained by a single renormalized Drude model with an enhanced mass and reduced scattering rate. Instead, we find evidence for the opening of a pseudogap with a gap energy of 6 cm−1 and an extremely narrow ω = 0 mode which is responsible for the large DC conductivity.

High-resolution transmission electron microscopic investigations of molybdenum thin films on faceted α-Al2O3

Epitaxially grown Mo films on a faceted corundum (α-Al2O3)mplane were investigated by transmission electron microscopy. Low- and high-resolution images were taken from a cross-section specimen cut perpendicular to the facets. It was possible to identify unambiguously the crystallographic orientation of these facets and explain the considerable deviation (∼10°) of the experimental interfacet angle, as measured with atomic force microscopy (AFM), from the expected value. For the first time, proof is given for a smooth \{10\bar{1}1\} facet and a curvy facet with orientation near to \{10\bar{1}\bar{2}\}. Moreover, the three-dimensional epitaxial relationship of an Mo film on a faceted corundumm…

Controlling the intralayer structure of Co/Pt superlattices

Abstract The effect of the surface quality of MgO (1 1 1) substrates seeded by a 3 nm thick Pt layer on the intralayer and interlayer structure of Co/Pt (1 1 1) superlattices was investigated. The superlattices were grown by DC magnetron sputtering. Pre-growth processing of the MgO shows that intensive polishing using a specific H 2 O/SiO 2 suspension prior to growth is mandatory for obtaining long-range structural coherence in the films. The comparison between superlattices grown on re-polished and as-supplied substrates shows that the substrate treatment enhances the crystalline in-plane orientation and suppresses abc / acb twin formation. Magnetization measurements were performed using t…

Strain induced renormalization of transport properties in UPt3 thin films

The growth of sputter deposited UPt3 thin films on Al2O3 (1012), LaAlO3 (111) and SrTiO3 (111) was investigated. We found strongly 0001-textured growth of UPt3 in a small compositional range of 23–25% uranium content. For Al2O3-and LaAlO3-substrates no in-plane order could be observed whereas epitaxial growth was initiated on SrTiO3 (111): The growth can be identified as Vollmer-Weber like resulting in the formation of large lateral strain as a consequence of the growth mode and a lattice misfit of −4.3% between UPt3 (0001) and SrTiO3 (111). Strong deviations from the typical heavy-fermion characteristics in electronic transport properties like resistivity, magnetoresitivity and Hall-effect…

Superconductivity mediated by spin fluctuations in the heavy-fermion compound UPd2 Al3

It is well known that any weak attractive electron–electron interaction in metals can in principle cause the formation of Cooper pairs, which then condense into a superconducting ground state1. In conventional superconductors, this attractive interaction is mediated by lattice vibrations (phonons). But for the heavy-fermion and high-temperature superconductors, alternative pairing interactions are considered to be possible2. For example, the low-temperature properties of heavy-fermion systems are dominated by antiferromagnetic spin fluctuations, which have been considered theoretically3 as a possible cause for Cooper-pair formation. This picture recently received some experimental support: …

MgO surface microstructure and crystalline coherence of Co/Pt superlattices

We report the growth of Co/Pt(111) superlattices sputtered onto MgO(111) substrates of different surface quality. Long-range structural coherence in the Pt-seed layers and the superlattices could only be obtained on specially re-polished substrates. In seed layers and superlattices grown on miscut re-polished substrates suppression of abc versus acb stacking was observed.

Strong-coupling effects in the heavy-fermion superconductor UPd2Al3

Abstract Recent results of superconducting tunneling spectroscopy on epitaxial thin films of the antiferromagnetic heavy-fermion superconductor UPd 2 Al 3 are presented. Strong-coupling effects in the tunneling density of states are analyzed within the framework of the anisotropic Eliashberg theory for a pair-coupling mechanism based on the exchange of antiferromagnetic spin excitations. The multi-sheeted Fermi surface of UPd 2 Al 3 is taken into account.

Correlation gap in the heavy-fermion antiferromagnetUPd2Al3

The optical properties of the heavy-fermion compound ${\mathrm{UPd}}_{2}{\mathrm{Al}}_{3}$ have been measured in a frequency range from 0.04 to 5 meV $(0.3--40{\mathrm{cm}}^{\ensuremath{-}1})$ at temperatures $2\mathrm{K}lTl300\mathrm{K}.$ Below the coherence temperature ${T}^{*}\ensuremath{\approx}50\mathrm{K},$ a hybridization gap opens around 10 meV. As the temperature decreases further $(Tl~20\mathrm{K}),$ a well-pronounced pseudogap of approximately 0.2 meV develops in the optical response; we relate this to the antiferromagnetic ordering which occurs below ${T}_{N}\ensuremath{\approx}14\mathrm{K}.$ The frequency-dependent mass and scattering rate give evidence that the enhancement of …

Magnetic and magnetoelastic properties of epitaxial (2 1 1)-oriented RFe2 (R=Dy, Tb) thin films

Abstract Epitaxial, (2 1 1)-oriented thin films of RFe 2 (R: rare earth; here: Dy, Tb) were deposited by molecular beam epitaxy on faceted and non-faceted α - Al 2 O 3 ( 1 0 1 ¯ 0 ) (m-plane) substrates utilizing a 15 A thin Fe seed layer and a 500 A Nb buffer layer. Detailed X-ray diffraction analyses revealed a twin-free epitaxial (2 1 1)-oriented growth of buffer layer and film. The magnetostrictive layer RFe 2 , as well as the template layers Nb and Mo exhibited the same crystallographic in-plane orientation with regard to the substrate. The same epitaxial relationship was found for films prepared on faceted and on non-faceted substrates. This implies a coherent crystal overgrowth of th…

Artificial granularity in two-dimensional arrays of nanodots fabricated by focused-electron-beam-induced deposition.

We have prepared 2D arrays of nanodots embedded in an insulating matrix by means of focused-electron-beam-induced deposition using the W(CO)(6) precursor. By varying the deposition parameters, i.e. the electron beam current and energy and the raster constant, we obtain an artificial granular material with tunable electrical properties. The analysis of the temperature dependence of the conductivity and of the current-voltage characteristic suggests that the transport mechanism is governed by electron tunneling between artificial grains. In order to understand the nature of the granularity and thus the microstructural origin of the electronic transport behavior, we perform TEM and micro-Raman…

Correlation of quenched structural disorder and magnetism inTiFe2Laves-phase thin films

In recent bandstructure calculations for the $C14$ Laves-phase intermetallic compound ${\mathrm{TiFe}}_{2}$ two energetically nearly degenerate magnetic ground states (antiferromagnetic and ferromagnetic) were predicted. As a consequence of this near-degeneracy in the $C14$ stability range of ${\mathrm{Ti}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}x}$ $(x\ensuremath{\approx}1/3)$ the magnetic properties are strongly correlated with the sublattices' occupation by Ti and Fe. We analyzed the magnetic properties of a series of thin epitaxial films with varying composition in the $C14$ stability range. The temperature- and field-dependent magnetic properties of these samples were determined by dc superc…

Growth characteristics of sputter-deposited thin films

Thin films of the heavy-fermion superconductor were deposited on various substrate materials in various orientations by means of a quasi-multilayer sputter process. Strongly (0001)-textured growth of the hexagonal compound was found for a uranium content in the range of 23% to 28% on sapphire and with perfect in-plane order on the latter substrate material. Atomic force microscopy and scanning electron microscopy revealed a Vollmer - Weber-like growth mode resulting in the development of large compressive strain in films on . As a result the electronic transport properties - in particular the temperature dependence of the resistivity - were strongly renormalized. Strong deviations from the …

Nature of Heavy Quasiparticles in Magnetically Ordered Heavy FermionsUPd2Al3andUPt3

The optical conductivity of the heavy fermions $\mathrm{UPd}{}_{2}{\mathrm{Al}}_{3}$ and $\mathrm{UPt}{}_{3}$ has been measured in the energy range from 0.04 to 5 meV. In both compounds a well pronounced pseudogap of less than 1 meV develops in the optical response at low temperatures; we relate this to the antiferromagnetic ordering. From the energy dependence of the effective mass and scattering rate we conclude that the enhancement of the mass mainly occurs below the energy which is related to magnetic correlations between the local magnetic moments and the itinerant electrons. This implies that the magnetic order in these compounds is the prerequisite to the formation of the heavy quasi…

Formation of an intermolecular charge-transfer compound in UHV codeposited tetramethoxypyrene and tetracyanoquinodimethane

Ultrahigh vacuum (UHV)-deposited films of the mixed phase of tetramethoxypyrene and tetracyanoquinodimethane $({\text{TMP}}_{1}{\text{-TCNQ}}_{1})$ on gold have been studied using ultraviolet photoelectron spectroscopy (UPS), x-ray diffraction (XRD), infrared (IR) spectroscopy, and scanning tunneling spectroscopy (STS). The formation of an intermolecular charge-transfer (CT) compound is evident from the appearance of new reflexes in XRD (${d}_{1}=0.894\text{ }\text{nm}$ and ${d}_{2}=0.677\text{ }\text{nm}$). A softening of the CN stretching vibration (redshift by $7\text{ }{\text{cm}}^{\ensuremath{-}1}$) of TCNQ is visible in the IR spectra, being indicative of a CT on the order of $0.3e$ f…

Investigation of Many‐Body Effects in the Quasi‐Two‐Dimensional Electronic System of Organic Charge‐Transfer Salts

Growth of Fe nanostructures

Abstract Highly ordered arrays of epitaxial iron thin film nanostructures were grown by molecular beam epitaxy techniques on m-plane sapphire α-Al2O3 (1 0 1 0) substrates. Iron was deposited by electron beam evaporation under shallow incidence onto faceted sapphire substrates held at elevated temperatures of 450°C. Scanning electron microscopy suggests the formation of morphologically and electrically isolated nanowire structures on the ridges of the facets. The topology of the structures depends strongly on the iron deposition angle.

Step-Edge Induced Anisotropic Domain-Wall Propagation

We report the observation of anisotropic domain-wall propagation in ultrathin magnetic films with perpendicular anisotropy. A controlled density of step edges was introduced which allowed us to quantify its influence on the domain-wall pinning. For a sawtooth arrangement of the step edges the corresponding wall movement resulted in triangular shaped domains. All aspects of this anisotropic domain-wall evolution could be reproduced by a simulation based on a modified Ginzburg-Landau-type soft-spin model.

Antiferromagnetism and the node structure of the superconducting order parameter of UPd Al

The node structure of the superconducting order parameter of the heavy-fermion system is analyzed within the weak-coupling theory. A pairing interaction induced by the exchange of antiferromagnetic spin excitations is assumed as suggested by recent inelastic neutron scattering experiments and tunneling spectroscopy. The multi-sheeted Fermi surface is taken into account. Based on a model susceptibility for the simple antiferromagnetic structure of , line nodes result at the rim of the magnetic Brillouin zone.

Probe Coherence Volume and the Interpretation of Scattering Experiments

Epitaxial thin films of intermetallic compounds

Publisher Summary The potential of epitaxial thin films of intermetallic compounds in basic and applied research is emerging. Although the growth of semiconductor heterostructures and compounds based on molecular beam epitaxy (MBE) and related methods has come through a 30-year history of ongoing refinement and sophistication, still much has to be learned concerning the growth and characterization of even moderately complex metallic thin film structures. MBE represents a well-defined crystallization technique based on the reactions among molecular or atomic beams of the constituent elements on a substrate or template at elevated temperatures in an ultrahigh vacuum (UHV) environment. Owing t…

Characterisation of structured thin films made from complex materials by photoabsorption spectromicroscopy

Al3 and YBa2Cu3O7/PrBa2Cu3O7. To investigate devices built from these complex materials we applied element-sensitive photoemission electron microscopy (PEEM). Information about the chemical composition of the imaged sample can be obtained by PEEM via tuning the photon energy to X-ray absorption edges. To apply spectromicroscopy we acquired microscopic images using photon energies near and at the edges. Such images give the lateral distribution of a specific element. Microspectroscopy is performed by recording the intensity of the true secondary electrons in selected spots during a sweep of the photon energy. The main aim of our work was to observe oxygen-related defects and changes in the c…

Thin film preparation of the low charge carrier density Kondo system CeSb

Abstract We report the thin film preparation of CeSb by means of molecular beam epitaxy (MBE) onto sapphire (1 1 −2 0) substrates. Above a substrate temperature of about 300°C CeSb crystallizes in (0 0 1) orientation. The growth mode of the films changes from a fiber textured to an epitaxial mode for deposition temperatures above 900°C. Although the specific resistivity is enhanced the characteristic energy scales, like the Kondo temperature TK and the magnetic ordering temperature TN, are not changed significantly.

Superconducting tunneling spectroscopy on epitaxial UPd2Al3 thin films

Abstract Results of superconducting tunneling spectroscopy experiments performed on two different types of thin film planar junctions of the heavy-fermion compound UPd 2 Al 3 are presented. Cross-type junctions consist of the heavy-fermion base electrode, an insulating layer of native surface oxide and a metal counter electrode (Au or Ag). The contact resistance was only weakly temperature dependent down to the superconducting transition. In the superconducting regime a strongly reduced zero bias conductivity indicated the junction being of the superconductor-insulator-normal metal type. The observed tunneling density of states is clearly due to the superconducting energy gap of UPd 2 Al 3 …

Preparation and structural analysis of Fe2+xTi1−x thin films in the C14 Laves phase stability range

Abstract We report the epitaxial growth of single phase (0 0 1)-oriented thin films of Fe2+xTi1−x in the C14 Laves phase stability range using molecular beam epitaxy (MBE). The growth was studied by reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD). The onset temperature for epitaxial growth and the temperature range for improved crystalline coherence were identified. From X-ray reflectometry analysis the rms roughness was estimated to 0.5 nm for typical film thicknesses of 22 nm. As revealed by scanning tunneling microscopy (STM), this roughness is discrete and due to step edges corresponding to the full c-axis length of Fe2Ti. The epitaxial growth implies an …

Heavy-fermion superconductivity induced by antiferromagnetic spin fluctuations

Superconductivity is caused by an attractive interaction between electrons at the Fermi level that induces the pairing of time-reversed electron states to Cooper pairs. Conventionally this attractive interaction is mediated by phonons. Theoretically, non-phonon mediated coupling seems to be likely for heavy-fermion superconductors whose low-temperature dynamics is dominated by antiferromagnetic spin correlations. However, evidence for spin-fluctuation coupling has not yet been experimentally observed. One of the most direct methods of investigation of the superconducting state is tunneling spectroscopy. We prepared cross-type tunneling junctions composed of the heavy-fermion superconductor …

The influence of the dynamics of ionic multiplets onto electronic transport properties of heavy-fermion systems: a semi-phenomenological approach

We present calculations of the electronic transport properties of heavy-fermion systems within a semi-phenomenological approach to the dynamical mean field theory. In this approach the dynamics of the Hund's rules 4f (5f )-ionic multiplet split in a crystalline environment is taken into account. Within the scope of this calculation we use the linear response theory to reproduce qualitative features of the temperature-dependent resistivity and hall conductivity, the magneto-resistivity and the thermoelectric power typical for heavy-fermion systems. The model calculations are directly compared with experimental results on CeCu2Si2.

Re-entrance phase formation of CeSb thin films

Abstract We report the epitaxial growth of (1 0 0)- and (1 1 1)-oriented CeSb thin films on Al 2 O 3 (1 1 2 0) and (0 0 0 1) substrates by means of molecular beam epitaxy. Depending on the overall Sb-to-Ce flux ratio and the molecular state of Sb x , we observe a re-entrance behavior in the phase formation and orientation of CeSb on Al 2 O 3 (1 1 2 0) . This behavior is shown to exhibit, in some respect, similarities to III–V compound growth, but also more complexities due to the instability of the rare earth component against oxidation. A geometric orientation selection model is suggested which reproduces the observed re-entrance behavior as well as the overall CeSb growth rate.

Scaling properties of magnetic domain walls in Pt/Co/Pt trilayers on MgO (111)

Abstract We present a scaling analysis of the time evolution of domain walls in ultrathin magnetic films that are subject to different forms of uncorrelated and correlated disorder caused by the microstructure of the underlying template. The study is performed on ultrafine modulated Pt/Co/Pt trilayers grown on as-supplied and structured MgO (1 1 1) substrates employing polar Kerr microscopy for the imaging of the magnetic domains.