The physics of iron pnictides has been the subject of intense research for half a decade since the discovery of superconductivity in doped LaFeAsO in 2008. By now there exists a large number of different materials that are summarized under the term "pnictides'' with significant differences in their crystal structure, electronic properties, and their phase diagrams. This thesis is concerned with the investigation of the various phase transitions that are observed in the underdoped compounds of the pnictide subgroups RFeAsO, where R is a rare-earth element, and AFe_2As_2, where A is an alkaline-earth element. These compounds display two closely bound transitions from a tetragonal to an orthorhombic phase and from a paramagnetic to an antiferromagnetic metal. Both symmetry-broken phases are suppressed by doping or pressure and close to their disappearance superconductivity sets in. The superconducting state is stabilized until some optimal doping or pressure is reached and gets suppressed thereafter. The central goal of this thesis is to improve our understanding of the interplay between these three phases and to describe the various phase transitions. We start from an itinerant picture that explains the magnetism as a result of an excitonic instability and show how the other phases can be included into this picture. This approach is based on the the observation that the compounds we are interested in have a Fermi surface with multiple nested electron and hole pockets and that they have small to intermediate interaction strengths. The thesis starts with a study of the doping dependence of the antiferromagnetic phase transition in four different five-orbital models. We use the random-phase approximation to determine the transition temperature, the dominant ordering vector, and the contribution of the different orbitals to the ordering. This allows us to identify the more realistic models, which give results that are in good agreement with experimental observations. In addition to the frequently made assumption of orbital-independent interaction potentials we study the effect of a reduction of the interaction strengths that involve the d_{xy} orbital. We find that this tunes the system between two different nesting instabilities. A reduction of the interactions that involve the d_{xy} orbital also enhances the tendency towards incommensurate (IC) order. For a weak reduction this tendency is compensated by the presence of the orthorhombic phase. However, for a reduction of 30%, as it is suggested by constrained random-phase-approximation calculations, we always find large doping ranges, where a state with IC order has the highest transition temperature. We continue the investigation of the magnetic phase transition by studying the competition of different possible types of antiferromagnetic order that arises from the presence of two degenerate nesting instabilities with the ordering vectors (pi,0) and (0,pi). We derive a Ginzburg-Landau free energy from a microscopic two-band model and find that the presence of the experimentally observed stripe phase strongly depends on the number and size of the hole pockets in the system and on the doping. We show that within the picture of a purely magnetically driven nematic phase transition, which breaks the C_4 symmetry and induces the orthorhombic distortion, the nematic phase displays exactly the same dependence on the model parameters as the magnetic stripe phase. We propose that in addition to the purely magnetically driven nematic instability there is a ferro-orbital instability in the system that stabilizes the nematic transition and, thus, explains the experimentally observed robustness of the orthorhombic transition. We argue that including a ferro-orbital instability into the picture may also be necessary to reproduce the transition from simultaneous first-order transitions into an orthorhombic antiferromagnetic state to two separate second-order transitions, which is observed as a function of doping. Finally, a study of the superconducting phase transition inside the antiferromagnetic phase that is observed in some pnictide compounds is presented. We present an approach to calculate the fluctuation-mediated pairing interaction in the spin-density-wave phase of a multiband system, which is based on the random-phase approximation. This approach is applied to a minimal two-band model for the pnictides to study the effect of the various symmetry-allowed bare on-site interactions on the gap symmetry and structure. We find a competition between various even- and odd-parity states and over a limited parameter range a p_x-wave state is the dominant instability. The largest part of the parameter space is dominated by even parity states but the gap structure sensitively depends on the bare interactions. We propose that the experimentally observed transition from a nodeless to a nodal gap can be due to changes in the on-site interaction potentials.

In the present work, the magnetotransport measurement technique was developed and various materials, exhibiting resistances from 1 mOhm up to several tens of kOhm, were investigated in pulsed magnetic fields of up to 60 T. Phase diagrams of irreversibility and upper critical fields for pure and Zn-doped YBa2Cu3O_7-x high-temperature superconductors were measured. A high-field study of the electronic properties of the two semimetals LaBiPt and CeBiPt were presented. Magnetoresistance of La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 thin films were investigated.

Diese Arbeit präsentiert die Ergebnisse einer Rastertunnelmikroskopiestudie an dem stöchiometrischen Supraleiter Lithium-Eisenarsenid (LiFeAs). Topographie- sowie Spektroskopieuntersuchungen an defektfreien Bereichen der Oberfläche zeigen eine Variation der Atompositionen in Abhängigkeit von der Tunnelspannung. Weiterhin wurde die Temperaturabhängigkeit der supraleitenden Energielücke untersucht. Dabei konnte die Signatur einer bosonischen Mode und damit eine Kopplung von Quasiteilchen beobachtet werden. Neben der Untersuchung defektfreier Oberflächen wurden auch Defekte und deren Einfluss auf die supraleitenden Eigenschaften analysiert. Es wurde dabei festgestellt, dass Defekte die supraleitende Energielücke ortsabhängig verändern. Die Defekte lassen sich aufgrund ihrer Symmetrie einer möglichen Gitterposition zuordnen. Eine detaillierte spektroskopische Untersuchung verschiedener Defekte zeigt deren Einfluss auf die Zustandsdichte der supraleitenden Quasiteilchen. Dabei stellt sich heraus, dass As-Defekte die supraleitende Energielücke erheblich beeinflussen. Fe-Defekte zeigen hingegen nur einen geringen Effekt. Für die Bestimmung der Ginzburg-Landau-Kohärenzlänge wurden Messungen im Magnetfeld durchgeführt. Hierfür wird in dieser Arbeit eine geeignete Näherungsfunktion hergeleitet. Die Näherung der differentiellen Leitfähigkeit bei U=0 V in einem Flussschlauch erlaubt die Bestimmung einer Kohärenzlänge von 3,9 nm. Dies entspricht einem oberen kritischen Feld von 21 Tesla. Neben der Bestimmung der Ginzburg-Landau-Kohärenzlänge wird auch eine Analyse des Flussschlauch-Gitters durchgeführt. Dabei zeigt sich, dass der Flussschlauch-Gitterabstand dem eines tetragonalen Gitters entspricht. Allerdings zeigt sich für Magnetfelder größer als 6 Tesla eine zunehmende Unordnung des Flussschlauch-Gitters, was auf eine stärker werdende Flussschlauch-Flussschlauch-Wechselwirkung hindeutet. / This work presents scanning tunneling microscopy and spectroscopy investigations on the stoichiometric superconductor lithium iron arsenide (LiFeAs). To reveal the electronic properties, measurements on defect-free surfaces as well as near defects have been performed. The former shows a shift of atomic position with respect to the applied bias voltage. Furthermore, temperature dependent spectroscopic measurements indicate the coupling of quasiparticles in the vicinity of the superconducting coherence peaks. LiFeAs surfaces influenced by atomic defects show a spacial variation of the superconducting gap. The defects can be characterized by their symmetry and thus can be assigned to a position in the atomic lattice. Detailed spectroscopic investigations of defects reveal their influence on the quasiparticle density of states. In particular, Fe-defects show a small effect on the superconductivity while As-defects strongly disturb the superconducting gap. Measurements in magnetic field have been performed for the determination of the Ginzburg-Landau coherence length . For this purpose, a suitable fit-function has been developed in this work. This function allows to fit the differential conductance of a magnetic vortex at U=0 V. The fit results in a coherence length of 3,9 nm which corresponds to an upper critical field of 21 Tesla. Besides measurements on a single vortex, investigation on the vortex lattice have been performed. The vortex lattice constant follows thereby the predicted behavior of a trigonal vortex lattice. However, for magnetic fields larger than 6 Tesla an increasing lattice disorder sets in, presumably due to vortex-vortex-interactions.

In this thesis the superconducting and magnetic phases of LiOH(Fe,Co)(Se,S), CuFeAs/CuFeSb, and LaFeP_1-xAs_xO - belonging to the 11, 111 and 1111 structural classes of iron-based arsenides and chalcogenides - are investigated by means of 57Fe Mössbauer spectroscopy and muon spin rotation/relaxation (μSR). Of major importance in this study is the application of high magnetic fields in Mössbauer spectroscopy to distinguish and characterize ferro- (FM) and antiferromagnetic (AFM) order. A user-friendly Mössbauer data analysis program was developed to provide suitable model functions not only for high field spectra, but relaxation spectra or parameter distributions in general. In LaFeP_1-xAs_xO the reconstruction of the Fermi surface is described by the vanishing of the Γ hole pocket with decreasing x. The continuous change of the orbital character and the covalency of the d-electrons is shown by Mössbauer spectroscopy. A novel antiferromagnetic phase with small magnetic moments of ~ 0.1 μ_B state is characterized. The superconducting order parameter is proven to continuously change from a nodal to a fully gapped s-wave like Fermi surface in the superconducting regime as a function of x, partially investigated on (O,F) substituted samples. LiOHFeSe is one of the novel intercalated FeSe compounds, showing strongly increased T_C = 43 K mainly due to increased interlayer spacing and resulting two-dimensionality of the Fermi surface. The primary interest of the samples of this thesis is the simultaneously observed ferromagnetism and superconductivity. The local probe techniques prove that superconducting sample volume gets replaced by ferromagnetic volume. Ferromagnetism arises from magnetic order with T_C = 10 K of secondary iron in the interlayer. The tendency of this system to show (Li,Fe) disorder is preserved upon (Se,S) substitution. However, superconductivity gets suppressed. The results of Mössbauer spectroscopy indicate that the systems tends to a secondary structural phase, where the local iron environment observed in pure FeS is absent. Moreover, two interlayer positions of the iron are identified. The absence of enhanced superconducting T_C in LiOHFeS thus is related to a structural instability. Also, in CuFeAs the role of secondary iron at the Cu position turns out to be decisive for the observed magnetic behaviour. As in LiOHFeSe, it orders ferromagnetically at T_C ~ 11 K and superimposes with the magnetic instability of the main iron site. It is shown that a small charge doping of 0.1e/Fe, which is expected from (Cu,Fe) disorder, is sufficient to switch the system between a paramagnetic and an AFM ground state. Both magnetic orders are indistinguishable, because the magnetic order parameters are strongly coupled. This coupling was observed in the structurally identical CuFeSb, where the magnetic order parameters of both iron sites scale perfectly. The magnetically unstable CuFeAs and the ferromagnetic CuFeSb can be classified according to the theory of As height driven magnetism, predicting a change from paramagnetism to AFM and finally FM with increasing As height.:1 Acronyms and Symbols 2 Introduction 3 Iron-based arsenides and chalcogenides 3.1 Structural properties 3.2 Electronic properties 3.2.1 Magnetism 3.2.2 Superconductivity 3.2.3 Nematic phase 3.3 Investigated samples 4 Moessfit - a free Mössbauer fitting program 4.1 Aspects of program design 4.2 Errors 4.2.1 Uncorrelated 4.2.2 Hesse 4.2.3 MonteCarlo 4.2.4 Minos 4.3 Fitting algorithm 4.4 Maximum entropy method (MEM) 4.5 Kolmogorov-Smirnov confidence 5 Mössbauer spectroscopy 5.1 Mössbauer effect 5.2 Relativistic Doppler effect 5.3 Full static Hamiltonian 5.3.1 Quadrupole interaction 5.3.2 Isomer shift. 5.3.3 Zeeman splitting 5.3.4 Combined interaction 5.3.5 Transition probabilities 5.3.6 The magic angle 5.4 Transmission integral 5.4.1 Absorption area 5.4.2 Ideal thickness 5.4.3 Line width and line shape 5.4.4 Levelling 5.5 Applied field measurements of powder samples 5.5.1 Paramagnet, axial symmetric EFG in transverse field geometry 6 5.5.2 Uniaxial antiferromagnet, axial symmetric EFG in transverse field geometry 6 5.5.3 Paramagnet, axial symmetric EFG in longitudinal field geometry 6 5.5.4 Uniaxial ferromagnet, axial symmetric EFG in transverse field geometry 6 5.5.5 Polarised photons 5.5.6 Total absorption cross section 5.5.7 Polarised sources 5.6 Blume line shape model 6 μSR 6.1 Muon decay and detection 6.2 Magnetic order and dynamic relaxation 6.2.1 Magnetic order 6.2.2 Time dependent field distributions 6.2.3 Aspects of μSR in iron-based arsenides and chalcogenides 6.2.4 Weak transverse field (WTF) 6.3 Superconductivity - transverse field (TF) experiments 7 Intercalated FeSe 7.1 Bulk properties: XRD, susceptibility, resistivity 7.2 Structural characterization 7.3 LiOHFeSe - Mössbauer spectroscopy 7.3.1 Applied transverse field 7.4 LiOHFeSe - μSR 7.4.1 Zero field (ZF) 7.4.2 Pinning experiment 7.4.3 Transverse field (TF) 7.5 Mössbauer investigation of LiOHFe_1-yCo_ySe_1-xS_x. 7.6 Discussion 8 LaFeO(As,P) 8.1 Preliminary measurements and electronic structure calculations 8.2 Mössbauer spectroscopy 8.3 μSR 8.3.1 Magnetic characterization 8.3.2 Spin dynamics 8.3.3 Superconductivity 8.4 Discussion 9 CuFeAs and CuFeSb 9.1 Preliminary results of CuFeAs and CuFeSb 9.2 CuFeAs: Mössbauer spectroscopy 9.2.1 Zero field (ZF) 9.2.2 Longitudinal field (LF) 9.2.3 Transverse field (TF) 9.3 CuFeAs: μSR 9.3.1 Zero field (ZF) 9.3.2 Weak transverse field (WTF) 9.4 Further investigations on CuFeAs 9.4.1 Neutron scattering 9.4.2 Theoretical calculation 9.4.3 Local element analysis with EDX/WDX 9.5 CuFeSb: Mössbauer spectroscopy 9.5.1 Zero Field (ZF) 9.5.2 Transverse field (TF) 9.6 Discussion 10 Conclusion 11 Appendix 11.1 Derivation of the quadrupole interaction and isomer shift 11.2 Matrix form of the static nuclear Hamiltonian 11.3 Mössbauer line intensities 11.4 Blume line shape model 11.4.1 Special case: two states with diagonal Hamiltonians 11.5 Moessfit models 11.5.1 FeSe_1-xS_x(Li_1-zFe_zOH) ZF, standard 11.5.2 FeSe_1-xS_x(Li_1-zFe_zOH) ZF, 4 fractions 11.5.3 FeSe_1-xS_x(Li_1-zFe_zOH) Pinning 11.5.4 FeSe_1-xS_x(Li_1-zFe_zOH) TF 11.5.5 FeSe_1-xS_x(Li_1-zFe_zOH) CS-Vzz-MEM 11.5.6 LaFeP_1-xAs_x+ ferrocene, ZF 11.5.7 LaFeP_1-xAs_x+ ferrocene, LF 11.5.8 LaFeP_1-xAs_x+ iron foil, ZF 11.5.9 LaFeAsO ZF 11.5.10 LaFeAsO TF 11.5.11 CuFeAs + ferrocen, ZF 11.5.12 CuFeAs + ferrocen, ZF, high statistics 11.5.13 CuFeAs + ferrocen, LF 11.5.14 CuFeAs + ferrocen, TF 11.5.15 CuFeSb ZF 11.5.16 CuFeSb TF

The discovery of superconductivity in iron-based pnictides in 2008 gave rise to a high advance in the research of high-temperature superconductors. But up to now there is no generally admitted theory of the non-BCS mechanism of these superconductors. The electron and hole doped Ba122 (BaFe2As2) compounds investigated in this thesis are supposed to be suitable model systems for studying the electronic behavior in order to shed light on the superconducting mechanisms. The 3d-transiton metal doped Ba122 compounds are investigated using the X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and X-ray magnetic circular dichroism (XMCD), while the completely hole doped K122 is observed using XPS. The experimental measurements are complemented by theoretical calculations. A further new class of superconductors is represented by the electride 12CaO*7Al2O3: Here superconductivity can be realized by electrons accommodated in the crystallographic sub-nanometer-sized cavities, while the mother compound is a wide band gap insulator. Electronic structure investigations, represented by XPS, XAS and resonant X-ray photoelectron spectroscopy (ResPES), carried out in this work, should help to illuminate this unconventional superconductivity and resolve a debate of competing models for explaining the existence of superconductivity in this compound.

No description available.

Nuclear Magnetic Resonance (NMR) has been a fundamental player in the studies of superconducting materials for many decades. This local probe technique allows for the study of the static electronic properties as well as the low energy excitations of the electrons in the normal and the superconducting state. On that account it has also been widely applied to Fe-based superconductors from the very beginning of their discovery in February 2008. This dissertation comprises some of these very first NMR results, reflecting the unconventional nature of superconductivity and its strong link to magnetism in the investigated compounds LaO(1-x)F(x)FeAs and LiFeAs.

The present work reports on thermal low-temperature properties of rare earth transition metal borocarbides such as specific heat, thermal conductivity and thermopower. The influence of structural disorder, caused by stoichiometric variations and substitutions of the rare earth element or the transition metal, on the thermal and superconducting low-temperature properties is investigated. The structural disorder results in the reduction of the superconducting transition temperature Tc, of the Sommerfeld value gamma, of the upper critical magnetic field Hc2(0), of the negative curvature of the H-dependence of the T-linear specific heat contribution gamma(H), and in the reduction of the positive curvature of Hc2(T) near Tc. But isoelectronic rare earth substitutions do not result in the transition from clean to dirty limit. Due to Pt-substitutions similar reductions of thermal and superconducting properties are observed. The behaviour of Hc2(0) and the concentration dependence of the positive curvature of Hc2(T) near Tc point to the transition from clean to quasi-dirty limit in the case of Pt-substitutions. / In der vorliegenden Arbeit werden Untersuchungen zu thermischen Tieftemperatureigenschaften, wie der spezifischen Wärmekapazität, der Wärmeleitfähigkeit und der Thermokraft, an supraleitenden Seltenerd-Übergangsmetall-Borkarbiden vorgestellt. Es wurde der Einfluß von gezielt hervorgerufener Unordnung im kristallographischen Aufbau, die durch isoelektronische Substitutionen des Seltenerd-Elements und des Übergangsmetalls sowie durch Söchiometrievariationen erzeugt wurde, auf die thermischen und supraleitenden Tieftemperatureigenschaften untersucht. Folge der strukturellen Unordnung ist eine Reduzierung der charakteristischen Eigenschaften, wie der Sprungtemperatur der Supraleitung Tc, der Sommerfeldkonstanten gamma, des oberen kritischen Magnetfelds Hc2(0), der negativen Krümmung in der Feldabhängigkeit des T-linearen Beitrags zur spezifischen Wärme gamma(H) sowie eine Reduzierung der positiven Krümmung in der Temperaturabhängigkeit von Hc2(T). Isoelektronische Substitutionen auf dem Seltenerd-Platz führen aber nicht zum Erreichen des dirty limit. Eine Reduzierung der relevanten supraleitenden und thermischen Eigenschaften durch Pt-Beimengungen wird ähnlich wie im Falle der Lu-Substitution festgestellt. Die Konzentrationsabhängigkeit von Hc2(0) sowie die Krümmung von Hc2(T) weisen hier auf einen Übergang vom clean limit zum quasi-dirty limit durch die Pt-Substitution hin.

Die für technische Anwendungen attraktivsten Eigenschaften der Hochtemperatur-Supraleiter beruhen auf der Wechselwirkung der Materialien mit einem externen Magnetfeld. Für hochleistungsfähige Supraleiteranwendungen werden großvolumige Funktionselemente mit eindomänigem Magnetisierungsverhalten benötigt. Zur Vergrößerung der magnetischen Domäne schmelztexturierter YBa2Cu3O7-δ-Kompaktsupraleiter wurde anhand der Multi-Seeding-Technik, die Rekristallisation unter Verwendung mehrerer SmBa2Cu3O7-δ-Keimpräparate untersucht. Als Schlüsseltechnologie zur Herstellung anwendungsorientierter Supraleitererzeugnisse wurde zu Beginn der Arbeit die Keimkristallherstellung optimiert. Gemäß den Zielvorgaben stehen zukünftig quasi-einkristalline Keimkörper definierter Form und Orientierung in hoher Stückzahl zur Verfügung. Die Supraleiter betreffend ist es gelungen, für den wechselseitigen Abstand und die Ausrichtung der Keimpräparate ein Optimum zu finden sowie tolerierbare Winkel der Verschwenkung angrenzender Kristalle zu ermitteln. Es wurde festgestellt, dass das Auftreten isolierender Korngrenzeneinschlüsse mit den magnetischen Materialeigenschaften korreliert und vom Keimabstand und der Korngrenzenorientierung abhängig ist. Mit Einsatz von bis zu 16 Keimkristallen gelang es, eindomänige Supraleiterhalbzeuge mit Höchstmaßen von (79 x 39 x 20) mm3 und Remanenzflussdichten von bis zu 1,3 T zu erzeugen. Im Chargenprozess konnten abschließend Multi-Seeding-Funktionselemente mit anwendungskonformen geometrischen und magnetischen Materialeigenschaften zum Aufbau eines hochdynamischen Supraleitermotors reproduzierbar gefertigt werden. In Leistungstests wurde für die Dynamik des Motors mit 200.000 rpm/s ein Rekordwert erzielt – nun können die Motoren in der Anwendung erprobt werden.

The physics of iron pnictides has been the subject of intense research for half a decade since the discovery of superconductivity in doped LaFeAsO in 2008. By now there exists a large number of different materials that are summarized under the term "pnictides'' with significant differences in their crystal structure, electronic properties, and their phase diagrams. This thesis is concerned with the investigation of the various phase transitions that are observed in the underdoped compounds of the pnictide subgroups RFeAsO, where R is a rare-earth element, and AFe_2As_2, where A is an alkaline-earth element. These compounds display two closely bound transitions from a tetragonal to an orthorhombic phase and from a paramagnetic to an antiferromagnetic metal. Both symmetry-broken phases are suppressed by doping or pressure and close to their disappearance superconductivity sets in. The superconducting state is stabilized until some optimal doping or pressure is reached and gets suppressed thereafter. The central goal of this thesis is to improve our understanding of the interplay between these three phases and to describe the various phase transitions. We start from an itinerant picture that explains the magnetism as a result of an excitonic instability and show how the other phases can be included into this picture. This approach is based on the the observation that the compounds we are interested in have a Fermi surface with multiple nested electron and hole pockets and that they have small to intermediate interaction strengths. The thesis starts with a study of the doping dependence of the antiferromagnetic phase transition in four different five-orbital models. We use the random-phase approximation to determine the transition temperature, the dominant ordering vector, and the contribution of the different orbitals to the ordering. This allows us to identify the more realistic models, which give results that are in good agreement with experimental observations. In addition to the frequently made assumption of orbital-independent interaction potentials we study the effect of a reduction of the interaction strengths that involve the d_{xy} orbital. We find that this tunes the system between two different nesting instabilities. A reduction of the interactions that involve the d_{xy} orbital also enhances the tendency towards incommensurate (IC) order. For a weak reduction this tendency is compensated by the presence of the orthorhombic phase. However, for a reduction of 30%, as it is suggested by constrained random-phase-approximation calculations, we always find large doping ranges, where a state with IC order has the highest transition temperature. We continue the investigation of the magnetic phase transition by studying the competition of different possible types of antiferromagnetic order that arises from the presence of two degenerate nesting instabilities with the ordering vectors (pi,0) and (0,pi). We derive a Ginzburg-Landau free energy from a microscopic two-band model and find that the presence of the experimentally observed stripe phase strongly depends on the number and size of the hole pockets in the system and on the doping. We show that within the picture of a purely magnetically driven nematic phase transition, which breaks the C_4 symmetry and induces the orthorhombic distortion, the nematic phase displays exactly the same dependence on the model parameters as the magnetic stripe phase. We propose that in addition to the purely magnetically driven nematic instability there is a ferro-orbital instability in the system that stabilizes the nematic transition and, thus, explains the experimentally observed robustness of the orthorhombic transition. We argue that including a ferro-orbital instability into the picture may also be necessary to reproduce the transition from simultaneous first-order transitions into an orthorhombic antiferromagnetic state to two separate second-order transitions, which is observed as a function of doping. Finally, a study of the superconducting phase transition inside the antiferromagnetic phase that is observed in some pnictide compounds is presented. We present an approach to calculate the fluctuation-mediated pairing interaction in the spin-density-wave phase of a multiband system, which is based on the random-phase approximation. This approach is applied to a minimal two-band model for the pnictides to study the effect of the various symmetry-allowed bare on-site interactions on the gap symmetry and structure. We find a competition between various even- and odd-parity states and over a limited parameter range a p_x-wave state is the dominant instability. The largest part of the parameter space is dominated by even parity states but the gap structure sensitively depends on the bare interactions. We propose that the experimentally observed transition from a nodeless to a nodal gap can be due to changes in the on-site interaction potentials.