arXiv subjects

J. T. Park

Publications and source records attributed to J. T. Park.

At least 37 records · Page 2Linked to original sources

arXiv2015

Consequences of critical interchain couplings and anisotropy on a Haldane chain

Effects of interchain couplings and anisotropy on a Haldane chain have been investigated by single crystal inelastic neutron scattering and density functional theory (DFT) calculations on the model compound SrNi$_2$V$_2$O$_8$. Significant effects on low energy excitation spectra are found where the Haldane gap ($Δ_0 \approx 0.41J$; where $J$ is the intrachain exchange interaction) is replaced by three energy minima at different antiferromagnetic zone centers due to the complex interchain couplings. Further, the triplet states are split into two branches by single-ion anisotropy. Quantitative information on the intrachain and interchain interactions as well as on the single-ion anisotropy are obtained from the analyses of the neutron scattering spectra by the random phase approximation (RPA) method. The presence of multiple competing interchain interactions is found from the analysis of the experimental spectra and is also confirmed by the DFT calculations. The interchain interactions are two orders of magnitude weaker than the nearest-neighbour intrachain interaction $J$ = 8.7~meV. The DFT calculations reveal that the dominant intrachain nearest-neighbor interaction occurs via nontrivial extended superexchange pathways Ni--O--V--O--Ni involving the empty $d$ orbital of V ions. The present single crystal study also allows us to correctly position SrNi$_2$V$_2$O$_8$ in the theoretical $D$-$J_{\perp}$ phase diagram [T. Sakai and M. Takahashi, Phys. Rev. B 42, 4537 (1990)] showing where it lies within the spin-liquid phase.

cond-mat.str-el↗

arXiv2015

Neutron spin resonance as a probe of superconducting gap anisotropy in partially detwinned electron underdoped NaFe$_{0.985}$Co$_{0.015}$As

We use inelastic neutron scattering (INS) to study the spin excitations in partially detwinned NaFe$_{0.985}$Co$_{0.015}$As which has coexisting static antiferromagnetic (AF) order and superconductivity ($T_c=15$ K, $T_N=30$ K). In previous INS work on a twinned sample, spin excitations form a dispersive sharp resonance near $E_{r1}=3.25$ meV and a broad dispersionless mode at $E_{r1}=6$ meV at the AF ordering wave vector ${\bf Q}_{\rm AF}={\bf Q}_1=(1,0)$ and its twinned domain ${\bf Q}_2=(0,1)$. For partially detwinned NaFe$_{0.985}$Co$_{0.015}$As with the static AF order mostly occurring at ${\bf Q}_{\rm AF}=(1,0)$, we still find a double resonance at both wave vectors with similar intensity. Since ${\bf Q}_1=(1,0)$ characterizes the explicit breaking of the spin rotational symmetry associated with the AF order, these results indicate that the double resonance cannot be due to the static and fluctuating AF orders, but originate from the superconducting gap anisotropy.

cond-mat.supr-con↗

arXiv2014

Superconducting properties and pseudogap from preformed Cooper pairs in the triclinic (CaFe$_{1-x}$Pt$_x$As)$_{10}$Pt$_3$As$_8$

Using a combination of muon-spin relaxation ($μ$SR), inelastic neutron scattering (INS) and nuclear magnetic resonance (NMR), we investigated the novel iron-based superconductor with a triclinic crystal structure (CaFe$_{1-x}$Pt$_x$As)$_{10}$Pt$_3$As$_8$ (T$_{\rm c}$ = 13 K), containing platinum-arsenide intermediary layers. The temperature dependence of the superfluid density obtained from the $μ$SR relaxation-rate measurements indicates the presence of two superconducting gaps, $Δ_\text{1}\ggΔ_\text{2}$. According to our INS measurements, commensurate spin fluctuations are centered at the ($π$, 0) wave vector, like in most other iron arsenides. Their intensity remains unchanged across T$_\text{c}$, indicating the absence of a spin resonance typical for many Fe-based superconductors. Instead, we observed a peak in the spin-excitation spectrum around $\hslashω_0=\,$7 meV at the same wave vector, which persists above T$_{\rm c}$ and is characterized by the ratio $\hslashω_0/k_\text{B}T_\text{c}\approx\,$6.2, which is significantly higher than typical values for the magnetic resonant modes in iron pnictides (~4.3). The temperature dependence of magnetic intensity at 7 meV revealed an anomaly around T* = 45 K related to the disappearance of this new mode. A suppression of the spin-lattice relaxation rate, $1/T_1T$, observed by NMR immediately below T* without any notable subsequent anomaly at T$_{\rm c}$, indicates that T* could mark the onset of a pseudogap in (CaFe$_{1-x}$Pt$_x$As)$_{10}$Pt$_3$As$_8$, which is likely associated with the emergence of preformed Cooper pairs.

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arXiv2014

Competing exchange interactions on the verge of a metal-insulator transition in the two-dimensional spiral magnet Sr$_3$Fe$_2$O$_7$

We report a neutron scattering study of the magnetic order and dynamics of the bilayer perovskite Sr$_3$Fe$_2$O$_7$, which exhibits a temperature-driven metal-insulator transition at 340 K. We show that the Fe$^{4+}$ moments adopt incommensurate spiral order below $T_\text{N}=115$ K and provide a comprehensive description of the corresponding spin wave excitations. The observed magnetic order and excitation spectra can be well understood in terms of an effective spin Hamiltonian with interactions ranging up to third nearest-neighbor pairs. The results indicate that the helical magnetism in Sr$_3$Fe$_2$O$_7$ results from competition between ferromagnetic double-exchange and antiferromagnetic superexchange interactions whose strengths become comparable near the metal-insulator transition. They thus confirm a decades-old theoretical prediction and provide a firm experimental basis for models of magnetic correlations in strongly correlated metals.

cond-mat.str-el↗

arXiv2014

Nematic spin correlations in the tetragonal state of uniaxial strained BaFe2-xNixAs2

Understanding the microscopic origins of electronic phases in high-transition temperature (high-Tc) superconductors is important for elucidating the mechanism of superconductivity. In the paramagnetic tetragonal phase of BaFe2-xTxAs2 (where T is Co or Ni) iron pnictides, an in-plane resistivity anisotropy has been observed. Here we use inelastic neutron scattering to show that low-energy spin excitations in these materials change from four-fold symmetric to two-fold symmetric at temperatures corresponding to the onset of the in-plane resistivity anisotropy. Because resistivity and spin excitation anisotropies both vanish near optimal superconductivity, we conclude that they are likely intimately connected.

cond-mat.supr-con↗

arXiv2013

Enhanced low-energy magnetic excitations via suppression of the itinerancy in Fe0.98-zCuzTe0.5Se0.5

We have performed resistivity and inelastic neutron scattering measurements on three samples of Fe0.98-zCuzTe0.5Se0.5 with z = 0, 0.02, and 0.1. It is found that with increasing Cu doping the sample's resistivity deviates progressively from that of a metal. However, in contrast to expectations that replacing Fe with Cu would suppress the magnetic correlations, the low-energy (no larger than 12 meV) magnetic scattering is enhanced in strength, with greater spectral weight and longer dynamical spin-spin correlation lengths. Such enhancements can be a consequence of either enlarged local moments or a slowing down of the spin fluctuations. In either case, the localization of the conduction states induced by the Cu doping should play a critical role. Our results are not applicable to models that treat 3d transition metal dopants simply as effective electron donors.

cond-mat.supr-con↗

arXiv2013

Possible realization of an antiferromagnetic Griffiths phase in Ba[Fe(1-x)Mn(x)](2)As(2)

We investigate magnetic ordering in metallic Ba[Fe(1-x)Mn(x)](2)As(2) and discuss the unusual magnetic phase, which was recently discovered for Mn concentrations x > 10%. We argue that it can be understood as a Griffiths-type phase that forms above the quantum critical point associated with the suppression of the stripe-antiferromagnetic spin-density-wave (SDW) order in BaFe2As2 by the randomly introduced localized Mn moments acting as strong magnetic impurities. While the SDW transition at x = 0, 2.5% and 5% remains equally sharp, in the x = 12% sample we observe an abrupt smearing of the antiferromagnetic transition in temperature and a considerable suppression of the spin gap in the magnetic excitation spectrum. According to our muon-spin-relaxation, nuclear magnetic resonance and neutron-scattering data, antiferromagnetically ordered rare regions start forming in the x = 12% sample significantly above the Néel temperature of the parent compound. Upon cooling, their volume grows continuously, leading to an increase in the magnetic Bragg intensity and to the gradual opening of a partial spin gap in the magnetic excitation spectrum. Using neutron Larmor diffraction, we also demonstrate that the magnetically ordered volume is characterized by a finite orthorhombic distortion, which could not be resolved in previous diffraction studies most probably due to its coexistence with the tetragonal phase and a microstrain-induced broadening of the Bragg reflections. We argue that Ba[Fe(1-x)Mn(x)](2)As(2) could represent an interesting model spin-glass system, in which localized magnetic moments are randomly embedded into a SDW metal with Fermi surface nesting.

cond-mat.supr-con↗

arXiv2012

Conformity of spin fluctuations in alkali-metal iron selenide superconductors inferred from the observation of a magnetic resonant mode in K(x)Fe(2-y)Se(2)

Spin excitations stemming from the metallic phase of the ferrochalcogenide superconductor K(0.77)Fe(1.85)Se(2) (T_c=32 K) were mapped out in the ab plane by means of the time-of-flight neutron spectroscopy. We observed a magnetic resonant mode at Q_res=(1/2 1/4), whose energy and in-plane shape are almost identical to those in the related compound Rb(0.8)Fe(1.6)Se(2). This lets us infer that there is a unique underlying electronic structure of the bulk superconducting phase K(x)Fe(2)Se(2), which is universal for all alkali-metal iron selenide superconductors and stands in contrast to the doping-tunable phase diagrams of the related iron pnictides. Furthermore, the spectral weight of the resonance on the absolute scale, normalized to the volume fraction of the superconducting phase, is several times larger than in optimally doped BaFe(2-x)Co(x)As(2). We also found no evidence for any additional low-energy branches of spin excitations away from Q_res. Our results provide new input for theoretical models of the spin dynamics in iron based superconductors.

cond-mat.supr-con↗

arXiv2012

Mn local moments prevent superconductivity in iron-pnictides Ba(Fe 1-x Mn x)2As2

75As nuclear magnetic resonance (NMR) experiments were performed on Ba(Fe1-xMnx)2As2 (xMn = 2.5%, 5% and 12%) single crystals. The Fe layer magnetic susceptibility far from Mn atoms is probed by the75As NMR line shift and is found similar to that of BaFe2As2, implying that Mn does not induce charge doping. A satellite line associated with the Mn nearest neighbours (n.n.) of 75As displays a Curie-Weiss shift which demonstrates that Mn carries a local magnetic moment. This is confirmed by the main line broadening typical of a RKKY-like Mn-induced staggered spin polarization. The Mn moment is due to the localization of the additional Mn hole. These findings explain why Mn does not induce superconductivity in the pnictides contrary to other dopants such as Co, Ni, Ru or K.

cond-mat.supr-con↗

arXiv2012

Similar zone-center gaps in the low-energy spin-wave spectra of NaFeAs and BaFe2As2

We report results of inelastic-neutron-scattering measurements of low-energy spin-wave excitations in two structurally distinct families of iron-pnictide parent compounds: Na(1-δ)FeAs and BaFe2As2. Despite their very different values of the ordered magnetic moment and Néel temperatures, T_N, in the antiferromagnetic state both compounds exhibit similar spin gaps of the order of 10 meV at the magnetic Brillouin-zone center. The gap opens sharply below T_N, with no signatures of a precursor gap at temperatures between the orthorhombic and magnetic phase transitions in Na(1-δ)FeAs. We also find a relatively weak dispersion of the spin-wave gap in BaFe2As2 along the out-of-plane momentum component, q_z. At the magnetic zone boundary (q_z = 0), spin excitations in the ordered state persist down to 20 meV, which implies a much smaller value of the effective out-of-plane exchange interaction, J_c, as compared to previous estimates based on fitting the high-energy spin-wave dispersion to a Heisenberg-type model.

cond-mat.supr-con↗

arXiv2012

Raman Scattering Study of the Lattice Dynamics of Superconducting LiFeAs

We report an investigation of the lattice dynamical properties of LiFeAs using inelastic light scattering. Five out of the six expected phonon modes are observed. The temperature evolution of their frequencies and linewidths is in good agreement with an anharmonic-decay model. We find no evidence for substantial electron-phonon coupling, and no superconductivity-induced phonon anomalies.

cond-mat.supr-con↗

arXiv2011

Reciprocal-space structure and dispersion of the magnetic resonant mode in the superconducting phase of Rb(x)Fe(2-y)Se2 single crystals

Inelastic neutron scattering is employed to study the reciprocal-space structure and dispersion of magnetic excitations in the normal and superconducting states of single-crystalline Rb0.8Fe1.6Se2. We show that the recently discovered magnetic resonant mode in this compound has a quasi-two-dimensional character, similar to overdoped iron-pnictide superconductors. Moreover, it has a rich in-plane structure that is dominated by four elliptical peaks, symmetrically surrounding the Brillouin zone corner, without sqrt(5) x sqrt(5) reconstruction. We also present evidence for the dispersion of the resonance peak, as its position in momentum space depends on energy. Comparison of our findings with the results of band structure calculations provides strong support for the itinerant origin of the observed signal. It can be traced back to the nesting of electron-like Fermi pockets in the doped metallic phase of the sample in the absence of iron-vacancy ordering.

cond-mat.supr-con↗

arXiv2011

Magnetic Resonant Mode in the Low-Energy Spin-Excitation Spectrum of Superconducting Rb2Fe4Se5 Single Crystals

We have studied the low-energy spin-excitation spectrum of the single-crystalline Rb2Fe4Se5 superconductor (Tc = 32 K) by means of inelastic neutron scattering. In the superconducting state, we observe a magnetic resonant mode centered at an energy of 14 meV and at the (0.5 0.25 0.5) wave vector (unfolded Fe-sublattice notation), which differs from the ones characterizing magnetic resonant modes in other iron-based superconductors. Our finding suggests that the 245-iron-selenides are unconventional superconductors with a sign-changing order parameter, in which bulk superconductivity coexists with the sqrt(5) x sqrt(5) magnetic superstructure. The estimated ratios of the resonance energy to Tc and the superconducting gap indicate moderate pairing strength in this compound, similar to that in optimally doped 1111- and 122-pnictides.

cond-mat.supr-con↗

arXiv2011

Crossover from weak to strong pairing in unconventional superconductors

Superconductors are classified by their pairing mechanism and the coupling strength, measured as the ratio of the energy gap to the critical temperature, Tc. We present an extensive comparison of the gap ratios among many single- and multiband superconductors from simple metals to high-Tc cuprates and iron pnictides. Contrary to the recently suggested universality of this ratio in Fe-based superconductors, we find that the coupling in pnictides ranges from weak, near the BCS limit, to strong, as in cuprates, bridging the gap between these two extremes. Moreover, for Fe- and Cu-based materials, our analysis reveals a universal correlation between the gap ratio and Tc, which is not found in conventional superconductors and therefore supports a common unconventional pairing mechanism in both families. An important consequence of this result for ferropnictides is that the separation in energy between the excitonic spin-resonance mode and the particle-hole continuum, which determines the resonance damping, no longer appears independent of Tc.

cond-mat.supr-con↗

arXiv2011

Evidence for conventional superconductivity in SrPd2Ge2

Electronic structure of SrPd2Ge2 single crystals is studied by angle-resolved photoemission spectroscopy (ARPES), scanning tunneling spectroscopy (STS) and band-structure calculations within the local-density approximation (LDA). The STS measurements show single s-wave superconducting energy gap Δ(0) = 0.5 meV. Photon-energy dependence of the observed Fermi surface reveals a strongly three-dimensional character of the corresponding electronic bands. By comparing the experimentally measured and calculated Fermi velocities a renormalization factor of 0.95 is obtained, which is much smaller than typical values reported in Fe-based superconductors. We ascribe such an unusually low band renormalization to the different orbital character of the conduction electrons and using ARPES and STS data argue that SrPd2Ge2 is likely to be a conventional superconductor, which makes it clearly distinct from isostructural iron pnictide superconductors of the "122" family.

cond-mat.supr-con↗

arXiv2010

Symmetry of spin excitation spectra in the tetragonal paramagnetic and superconducting phases of 122-ferropnictides

We study the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron scattering (INS) measurements on BaFe1.85Co0.15As2 and BaFe1.91Ni0.09As2 samples that exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum lacks the 42/m screw symmetry around the (1/2 1/2 L) axis that is implied by the I4/mmm space group. This is manifest both in the in-plane anisotropy of the normal- and SC-state spin dynamics and in the out-of-plane dispersion of the spin-resonance mode. We show that this effect originates from the higher symmetry of the magnetic Fe sublattice with respect to the crystal itself, hence the INS signal inherits the symmetry of the unfolded Brillouin zone (BZ) of the Fe sublattice. The in-plane anisotropy is temperature-independent and can be qualitatively reproduced in normal-state density-functional-theory calculations without invoking a symmetry-broken ("nematic") ground state that was previously proposed as an explanation for this effect. Below the SC transition, the energy of the magnetic resonant mode Er, as well as its intensity and the SC spin gap inherit the normal-state intensity modulation along the out-of-plane direction L with a period twice larger than expected from the body-centered-tetragonal BZ symmetry. The amplitude of this modulation decreases at higher doping, providing an analogy to the splitting between even and odd resonant modes in bilayer cuprates. Combining our and previous data, we show that at odd L a universal linear relationship Er=4.3*kB*Tc holds for all studied Fe-based superconductors, independent of their carrier type. Its validity down to the lowest doping levels is consistent with weaker electron correlations in ferropnictides as compared to the underdoped cuprates.

cond-mat.supr-con↗

arXiv2010

Weak superconducting pairing and a single isotropic energy gap in stoichiometric LiFeAs

We report superconducting (SC) properties of stoichiometric LiFeAs (Tc = 17 K) studied by small-angle neutron scattering (SANS) and angle-resolved photoemission (ARPES). Although the vortex lattice exhibits no long-range order, well-defined SANS rocking curves indicate better ordering than in chemically doped 122-compounds. The London penetration depth of 210 nm, determined from the magnetic field dependence of the form factor, is compared to that calculated from the ARPES band structure with no adjustable parameters. Its temperature dependence is best described by a single isotropic SC gap of 3.0 meV, which agrees with the ARPES value of 3.1 meV and corresponds to the ratio 2Delta/kTc = 4.1, approaching the weak-coupling limit predicted by the BCS theory. This classifies LiFeAs as a weakly coupled single-gap superconductor, similar to conventional metals.

cond-mat.supr-con↗

arXiv2009

Symmetry and disorder of the vitreous vortex lattice in an overdoped BaFe_{2-x}Co_xAs_2 superconductor: Indication for strong single-vortex pinning

The disordered flux line lattice in single crystals of the slightly overdoped aFe_{2-x}Co_xAs_2 (x = 0.19, Tc = 23 K) superconductor is studied by magnetization measurements, small-angle neutron scattering (SANS), and magnetic force microscopy (MFM). In the whole range of magnetic fields up to 9 T, vortex pinning precludes the formation of an ordered Abrikosov lattice. Instead, a vitreous vortex phase (vortex glass) with a short-range hexagonal order is observed. Statistical processing of MFM datasets lets us directly measure its radial and angular distribution functions and extract the radial correlation length ζ. In contrast to predictions of the collective pinning model, no increase in the correlated volume with the applied field is observed. Instead, we find that ζdecreases as 1.3*R1 ~ H^(-1/2) over four decades of the applied magnetic field, where R1 is the radius of the first coordination shell of the vortex lattice. Such universal scaling of ζimplies that the vortex pinning in iron arsenides remains strong even in the absence of static magnetism. This result is consistent with all the real- and reciprocal-space vortex-lattice measurements in overdoped as-grown aFe_{2-x}Co_xAs_2 published to date and is thus sample-independent. The failure of the collective pinning model suggests that the vortices remain in the single-vortex pinning limit even in high magnetic fields up to 9 T.

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