arXiv subjects

P. Adelmann

Publications and source records attributed to P. Adelmann.

At least 19 recordsLinked to original sources

arXiv2018

Spectral Evidence for Emergent Order in Ba$_{1-x}$Na$_x$Fe$_2$As$_2$

We report an angle-resolved photoemission spectroscopy study of the iron-based superconductor family, Ba$_{1-x}$Na$_x$Fe$_2$As$_2$. This system harbors the recently discovered double-Q magnetic order appearing in a reentrant C$_4$ phase deep within the underdoped regime of the phase diagram that is otherwise dominated by the coupled nematic phase and collinear antiferromagnetic order. From a detailed temperature-dependence study, we identify the electronic response to the nematic phase in an orbital-dependent band shift that strictly follows the rotational symmetry of the lattice and disappears when the system restores C$_4$ symmetry in the low temperature phase. In addition, we report the observation of a distinct electronic reconstruction that cannot be explained by the known electronic orders in the system.

cond-mat.supr-con↗

arXiv2016

Experimental evidence for the importance of Hund's exchange interaction for the incoherence of the charge carriers in iron-based superconductors

Angle-resolved photoemission spectroscopy (ARPES) is used to study the scattering rates of charge carriers from the hole pockets near Gamma in the iron-based high-Tc hole doped superconductors KxBa1-xFe2As2 x=0.4 and KxEu1-xFe2As2 x=0.55$ and the electron doped compound Ba(Fe1-xCox)2As2 x=0.075. The scattering rate for any given band is found to depend linearly on energy, indicating a non-Fermi liquid regime. The scattering rates in the hole-doped compound are considerably larger than those in the electron-doped compounds. In the hole-doped systems the scattering rate of the charge carriers of the inner hole pocket is about three times bigger than the binding energy indicating that the spectral weight is heavily incoherent. The strength of the scattering rates and the difference between electron and hole doped compounds signals the importance of Hund's exchange coupling for correlation effects in these iron-based high-Tc superconductors. The experimental results are in qualitative agreement with theoretical calculations in the framework of combined density functional dynamical mean-field theory.

cond-mat.supr-con↗

arXiv2016

Muon spin rotation and infrared spectroscopy study of magnetism and superconductivity in Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $

Using muon spin rotation and infrared spectroscopy we study the relation between magnetism and superconductivity in Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $ single crystals from the underdoped to the slightly overdoped regime. We find that the Fe magnetic moment is only moderately suppressed in most of the underdoped region where it decreases more slowly than the N\'{e}el-temperature, $ T^{\mathrm{N}} $. This applies for both the total Fe moment obtained from muon spin rotation and for the itinerant component that is deduced from the spectral weight of the spin-density-wave pair breaking peak in the infrared response. In the moderately underdoped region, superconducting and static magnetic orders co-exist on the nano-scale and compete for the same electronic states. The static magnetic moment disappears rather sharply near optimal doping, however, in the slightly overdoped region there is still an enhancement or slowing down of spin fluctuations in the superconducting state. Similar to the gap magnitude reported from specific heat measurements, the superconducting condensate density is nearly constant in the optimally- and slightly overdoped region, but exhibits a rather pronounced decrease on the underdoped side. Several of these observations are similar to the phenomenology in the electron doped counterpart Ba(Fe$ _{1-y} $Co$ _{y} $)$ _{2} $As$ _{2} $.

cond-mat.supr-con↗

arXiv2016

Superconductivity and fluctuation in Ba$_{1-p}$K$_p$Fe$_2$As$_2$ and Ba(Fe$_{1-n}$Co$_n$)$_2$As$_2$

We study the interplay of fluctuations and superconductivity in BaFe$_2$As$_2$ (Ba-122) compounds with Ba and Fe substituted by K ($p$ doping) and Co ($n$ doping), respectively. To this end we measured electronic Raman spectra as a function of polarisation and temperature. We observe gap excitations and fluctuations for all doping levels studied. The response from fluctuations is much stronger for Co substitution and, according to the selection rules and the temperature dependence, originates from the exchange of two critical spin fluctuations with characteristic wave vectors $(\pm\pi, 0)$ and $(0,\pm\pi)$. At 22\% K doping ($p=0.22$), we find the same selection rules and spectral shape for the fluctuations but the intensity is smaller by a factor of 5. Since there exists no nematic region above the orthorhombic spin-density-wave (SDW) phase the identification of the fluctuations via the temperature dependence is not possible. The gap excitations in the superconducting state indicate strongly anisotropic near-nodal gaps for Co substitution which make the observation of collective modes difficult. The variation with doping of the spectral weights of the $A_{1g}$ and $B_{1g}$ gap features does not support the influence of fluctuations on Cooper pairing. Therefore, the observation of Bardasis-Schrieffer modes inside the nearly clean gaps on the K-doped side remains the only experimental evidence for the relevance of fluctuations for pairing.

cond-mat.supr-con↗

arXiv2016

Strong Correlations, Strong Coupling and s-wave Superconductivity in Hole-doped BaFe2As2 Single Crystals

We present a comprehensive study of the low-temperature heat capacity and thermal expansion of single crystals of the hole-doped Ba1-xKxFe2As2 series (0<x<1) and the end-members RbFe2As2 and CsFe2As2. A large increase of the Sommerfeld coefficient is observed with both decreasing band filling and isovalent substitution (K, Rb, Cs) revealing a strong enhancement of electron correlations and the possible proximity of these materials to a Mott insulator. This trend is well reproduced theoretically by our Density-Functional Theory + Slave-Spin (DFT+SS) calculations, confirming that 122-iron pnictides are effectively Hund metals, in which sizable Hund's coupling and orbital selectivity are the key ingredients for tuning correlations. We also find direct evidence for the existence of a coherence-incoherence crossover between a low-temperature heavy Fermi liquid and a highly incoherent high-temperature regime similar to heavy fermion systems. In the superconducting state, clear signatures of multiband superconductivity are observed with no evidence for nodes in the energy gaps, ruling out the existence of a doping-induced change of symmetry (from s to d-wave). We argue that the disappearance of the electron band in the range 0.4<x<1.0 is accompanied by a strong-to-weak coupling crossover and that this shallow band remains involved in the superconducting pairing, although its contribution to the normal state fades away. Differences between hole- and electron-doped BaFe2As2 series are emphasized and discussed in terms of strong pair breaking by potential scatterers beyond the Born limit.

cond-mat.supr-con↗

arXiv2016

Simultaneous Evidence for Pauli Paramagnetic Effects and Multiband Superconductivity in KFe$_2$As$_2$ by Small-Angle Neutron Scattering Studies of the Vortex Lattice

We study the intrinsic anisotropy of the superconducting state in KFe$_2$As$_2$, using small-angle neutron scattering to image the vortex lattice as the applied magnetic field is rotated towards the FeAs crystalline planes. The anisotropy is found to be strongly field dependent, indicating multiband superconductivity. Furthermore, the high field anisotropy significantly exceeds that of the upper critical field, providing further support for Pauli limiting in KFe$_2$As$_2$ for field applied in the basal plane. The effect of Pauli paramagnetism on the unpaired quasiparticles in the vortex cores is directly evident from the ratio of scattered intensities due to the longitudinal and transverse vortex lattice field modulation.

cond-mat.supr-con↗

arXiv2015

Synthesis and structure of tetragonal Bi12.5Nd1.5ReO24.5

The Bi12.5Nd1.5ReO24.5 tetragonal phase has been synthesized and lattice cell parameters have been determined. According to X-ray data the phase has I4/m sym-metry with lattice parameters a = 0.86742 (12) nm, c =1.7408 (3) nm.

cond-mat.mtrl-sci↗

arXiv2013

Lack of coupling between superconductivity and orthorhombic distortion in stoichiometric single-crystalline FeSe

The coupling between superconductivity and othorhombic distortion is studied in vapor-grown FeSe single crystals using high-resolution thermal-expansion measurements. In contrast to the Ba122-based (Ba122) superconductors, we find that superconductivity does not reduce the orthorhombicity below Tc. Instead we find that superconductivity couples strongly to the in-plane area, which explains the large hydrostatic pressure effects. We discuss our results in light of the spinnematic scenario and argue that FeSe has many features quite different from the typical Fe-based superconductors.

cond-mat.supr-con↗

arXiv2013

Evidence of Strong Correlations and Coherence-Incoherence Crossover in the Iron Pnictide Superconductor KFe2As2

Using resistivity, heat-capacity, thermal-expansion, and susceptibility measurements we study the normal-state behavior of KFe2As2. We find that both the Sommerfeld coefficient gamma = 103 mJ mol-1 K-2 and the Pauli susceptibility chi = 4x10-4 are strongly enhanced, which confirm the existence of heavy quasiparticles inferred from previous de Haas-van Alphen and ARPES experiments. We discuss this large enhancement using a Gutzwiller slave-boson mean-field calculation, which reveals the proximity of KFe2As2 to an orbital-selective Mott transition. The temperature dependence of the magnetic susceptibility and the thermal expansion provide strong experimental evidence for the existence of a coherence-incoherence crossover, similar to what is found in heavy fermion and ruthenate compounds, due to Hund's coupling between orbitals.

cond-mat.supr-con↗

arXiv2012

Magnetic properties of single-crystalline CeCuGa3

The magnetic behavior of single-crystalline CeCuGa3 has been investigated. The compound forms in a tetragonal BaAl4-type structure consisting of rare-earth planes separated by Cu-Ga layers. If the Cu-Ga site disorder is reduced, CeCuGa3 adopts the related, likewise tetragonal BaNiSn3-type structure, in which the Ce ion are surrounded by different Cu and Ga layers and the inversion symmetry is lost. In the literature conflicting reports about the magnetic order of CeCuGa3 have been published. Single crystals with the centrosymmetric structure variant exhibit ferromagnetic order below approx. 4 K with a strong planar anisotropy. The magnetic behavior above the transition temperature can be well understood by the crystal-field splitting of the 4f Hund's rule ground-state multiplet of the Ce ions.

cond-mat.str-el↗

arXiv2011

Complex magneto-elastic properties in the frustrated kagome-staircase compounds (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$

High resolution heat capacity and thermal expansion experiments performed on single crystalline kagome-staircase compounds (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$ are presented. The parent compounds Co$_3$V$_2$O$_8$ and Ni$_3$V$_2$O$_8$ undergo a complex sequence of first- and second-order magnetic phase transitions. The low-temperature ($T<40$ K) magnetic entropy evolves monotonously with the doping content $x$, from the full S=1 Ni$^{2+}$ magnetic entropy in Ni$_3$V$_2$O$_8$ to half of the S=3/2 Co$^{2+}$ magnetic entropy in Co$_3$V$_2$O$_8$. Thermal expansion coefficients $α_i$ ($i = a$, $b$ and $c$) show a strong anisotropy for all (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$ compounds. The low-temperature magnetic distortion indicates that Co-doping (Ni-doping) has similar effects to applying a uniaxial pressures along $a$ or $b$ ($c$). Linear Grüneisen parameters $Γ_i$ are extracted for the three main axes $i$ and exhibit a complex behavior with both temperature and doping. For each axis, $Γ_i$ and $α_i$ exhibit a sign change (at low temperature) at the critical concentration $x_c\simeq0.25$, at which the incommensurate magnetic propagation vector changes. Beyond our study, an understanding of the multiple and complex parameters (magnetic frustration, magnetic anisotropy, mixture of S=1 and S=3/2 ions, etc.) is now necessarily to bring light to the rich magneto-elastic properties of (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$.

cond-mat.str-el↗

arXiv2010

Doping evolution of superconducting gaps and electronic densities of states in Ba(Fe1-xCox)2As2 iron pnictides

An extensive calorimetric study of the normal- and superconducting-state properties of Ba(Fe1-xCox)2As2 is presented for 0 < x < 0.2. The normal-state Sommerfeld coefficient increases (decreases) with Co doping for x < 0.06 (x > 0.06), which illustrates the strong competition between magnetism and superconductivity to monopolize the Fermi surface in the underdoped region and the filling of the hole bands for overdoped Ba(Fe1-xCox)2As2. All superconducting samples exhibit a residual electronic density of states of unknown origin in the zero-temperature limit, which is minimal at optimal doping but increases to the normal-state value in the strongly under- and over-doped regions. The remaining specific heat in the superconducting state is well described using a two-band model with isotropic s-wave superconducting gaps.

cond-mat.supr-con↗

arXiv2010

Magnetisation distribution in the tetragonal phase of BaFe2As2

We have determined the spatial distribution of the magnetisation induced by a field of 9 T in the tetragonal phase of BaFe2As2 using polarised neutron diffraction. Magnetic structure factors derived from the polarisation dependence of the intensities of Bragg reflections were used to make a maximum entropy reconstruction of the distribution projected on the 110 plane. The reconstruction shows clearly that the magnetisation is confined to the region around the iron atoms and that there is no significant magnetisation associated with either the As or Ba atoms. The distribution of magnetisation around the Fe atom is significantly non-spherical with a shape which is extended in the <111> directions in the projection. These results show that the electrons which give rise to the paramagnetic susceptibility are confined to the Fe atoms their distribution suggests that they occupy 3d t_2g type orbitals with about 60% in those of xy symmetry.

cond-mat.str-el↗

arXiv2009

Calorimetric Evidence of Multiband Superconductivity in Ba(Fe0.925Co0.075)2As2

We report on the determination of the electronic heat capacity of a slightly overdoped (x = 0.075) Ba(Fe1-xCox)2As2 single crystal with a Tc of 21.4 K. Our analysis of the temperature dependence of the superconducting-state specific heat provides strong evidence for a two-band s-wave order parameter with gap amplitudes 2D1(0)/kBTc=1.9 and 2D2(0)/kBTc=4.4. Our result is consistent with the recently predicted s+- order parameter [I. I. Mazin et al., Phys. Rev. Lett. 101, 057003 (2008)].

cond-mat.supr-con↗

arXiv2009

Resistivity of Mn$_{1-x}$Fe$_x$Si single crystals: Evidence for quantum critical behavior

Resistivity measurements have been made on Mn$_{1-x}$Fe$_x$Si single crystals between 2 and 300K for $x$ = 0, 0.05, 0.08, 0.12 and 0.15. Fe doping is found to depress the magnetic ordering temperature from 30K for $x$ = 0 to below 2K for $x$ = 0.15. Although Fe doping results in a large increase of the low-temperature residual resistivity, the temperature dependence of the resistivity above the magnetic transition remains practically unaffected by increasing Fe content. An analysis of the temperature derivative of the resistivity provides strong evidence for the existence of a non-Fermi-liquid ground state near $x$ = 0.15 and thus for a quantum critical point tuned by Fe content.

cond-mat.str-el↗

arXiv2008

Suppression of the ferromagnetic state in LaCoO3 films by rhombohedral distortion

Epitaxially strained LaCoO3 (LCO) thin films were grown with different film thickness, t, on (001) oriented (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates. After initial pseudomorphic growth the films start to relieve their strain partly by the formation of periodic nano-twins with twin planes predominantly along the <100> direction. Nano-twinning occurs already at the initial stage of growth, albeit in a more moderate way. Pseudomorphic grains, on the other hand, still grow up to a thickness of at least several tenths of nanometers. The twinning is attributed to the symmetry lowering of the epitaxially strained pseudo-tetragonal structure towards the relaxed rhombohedral structure of bulk LCO. However, the unit-cell volume of the pseudo-tetragonal structure is found to be nearly constant over a very large range of t. Only films with t > 130 nm show a significant relaxation of the lattice parameters towards values comparable to those of bulk LCO.

cond-mat.str-el↗

arXiv2008

Neutron diffraction on antiferromagnetic ordering in single-crystal BaFe2As2

Neutron diffraction experiments have been carried out on a Sn-flux grown BaFe2As2 single crystal, the parent compound of the A-122 family of FeAs-based high-Tc superconductors. A tetragonal to orthorhombic structural phase transition and a three dimensional long-range antiferromagnetic ordering of the iron moment, with a unique magnetic propagation wavevector k = (1, 0, 1), have been found to take place at ~90 K. The magnetic moments of iron are aligned along the long a axis in the low temperature orthorhombic phase (Fmmm with b<a<c). Our results thus demonstrate that the magnetic structure of BaFe2As2 single crystal is the same as those in other A-122 iron pnictides compounds. We argue that the tin incorporation in the lattice is responsible for a smaller orthorhombic splitting and lower Neel temperature T_N observed in the experiment.

cond-mat.supr-con↗

arXiv2008

$T_c$ Dependence of Energy Gap and Asymmetry of Coherence Peaks in NdBa$_2$Cu$_3$O$_{7-δ}$

We report here the results of scanning tunneling spectroscopic experiments performed on hole doped NdBa$_2$Cu$_3$O$_{7-δ}$ single crystals of $T_c$ values of 76 K, 93.5 K and 95.5 K. The energy gaps are observed to be increasing with decreasing $T_c$ values. The coherence peaks are asymmetric with the peaks at the filled states being larger than those at the empty ones. The asymmetry increases with decreasing $T_c$. The observed asymmetry and its $T_c$ dependence can be explained by considering the Mott insulating nature of the material at the undoped state.

cond-mat.supr-con↗