Engineering topics

Meier, W. R.

Publications and source records attributed to Meier, W. R..

At least 19 records

DOE OSTI2022

Superconductivity and phase diagrams of CaK(Fe 1- x Mn x ) 4 As 4 single crystals

In order to study the effects of Mn substitution on the superconducting and magnetic ground state of CaKFe 4 As 4 (T c = 35 K), members of the CaK (Fe 1–x Mn x ) 4 As 4 series have been synthesized by high-temperature solution growth in single-crystalline form and characterized by elemental analysis, thermodynamic, and transport measurements. These measurements show that the superconducting transition temperature decreases monotonically and is finally suppressed below 1.8 K as x is increased from 0 to 0.036. For x values greater than 0.016, signatures of a magnetic transition can be detected in both thermodynamic and transport measurements in which kinklike features allow for the determination of the transition temperature T* that increases as Mn substitution increases. Here, a temperature-composition (T–x) phase diagram is constructed, revealing a half-dome of superconductivity with the magnetic transition temperature T* appearing near 26 K for x ~ 0.017 and rising slowly up to 33 K for x ~ 0.036. In addition to the creation of the T–x phase diagram for CaK(Fe 1–x Mn x ) 4 As 4 , specific-heat data are used to track the jump in specific heat at T c ; the CaK(Fe 1–x Mn x ) 4 As 4 data do not follow the scaling of ΔC p with $T$$^{3}_{c}$ as many of the other Fe-based superconducting systems do. These data suggest that, as magnetic pair breaking is present, the jump in C p for a given T c is reduced. Elastoresistivity coefficients 2m 66 and m 11 – m 12 as a function of temperature are also measured. 2m 66 and m 11 – m 12 are qualitatively similar to CaK(Fe 1–x Ni x ) 4 As 4 . This may indicate that the magnetic order in Mn-substituted system may be still the same as CaK(Fe 1–x Mn x ) 4 As 4 . Superconductivity of CaK(Fe 1–x Mn x ) 4 As 4 is also studied as a function of magnetic field. A clear change in H' c2 (T)/T c , where H' c2 (T) is dH' c2 (T)/dT, at x ~ 0.015 is observed and probably is related to change of the Fermi surface due to magnetic order. Coherence lengths and the London penetration depths are also calculated based on H c1 and H c2 data. Coherence lengths as the function of x also show the changes near x = 0.015, again consistent with Fermi-surface changes associated with the magnetic ordering seen for higher-x values.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Geometry of the charge density wave in the kagome metal A V 3 Sb 5

Kagome lattice is a fertile platform for topological and intertwined electronic excitations. Recently, experimental evidence of an unconventional charge density wave (CDW) is observed in a Z 2 kagome metal AV 3 Sb 5 (A=K, Cs, Rb). This observation triggers wide interest in the interplay between frustrated crystal structure and Fermi surface instabilities. Here, we analyze the lattice effect and its impact on CDW in AV 3 Sb 5 . Based on published experimental data, we show that the 2×2×2 CDW breaks the sixfold rotational symmetry of the crystal due to the phase shift between kagome layers and can explain the twofold symmetric CDW peak intensity observed by scanning tunneling spectroscopy. The coupling between the lattice and electronic degrees of freedom yields a weak first-order structural transition without continuous change of lattice dynamics. Our result emphasizes the fundamental role of lattice geometry in proper understanding of unconventional electronic orders in AV 3 Sb 5 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Tuning the flat bands of the kagome metal CoSn with Fe, In, or Ni doping

We report CoSn is a Pauli paramagnet with relatively flat d bands centered about 100 meV below the Fermi energy, E F . Single crystals of CoSn lightly doped with Fe, In, or Ni are investigated using x-ray and neutron scattering, magnetic susceptibility and magnetization, AC susceptibility, specific heat, and resistivity measurements. Within the rigid-band approximation, hole doping with a few percent of Fe or In should move the flat bands closer to E F , whereas electron doping with Ni should move the flat bands further away from E F . We provide evidence that this indeed occurs. Fe and In doping drive CoSn toward magnetism, while Ni doping suppresses CoSn's already weak magnetic response. The resulting ground state is different for Fe versus In doping. For Fe-doped crystals, Co 1-x Fe x Sn, with 0.02< x <0.27, the magnetic and specific-heat data are consistent with the formation of a spin glass, with a glass temperature, T g , ranging from 1 K for x = 0.02 to 10 K for x = 0.27. Powder and single-crystal neutron diffraction found no evidence of long-range magnetic order below T g for samples with x ≈ 0.17. For In-doped crystals, CoSn 1-y In y , both the magnetic susceptibility and the Sommerfeld coefficient, γ, increase substantially relative to pure CoSn, but with no clear indication of a magnetic transition for 0.05< y <0.2. CoSn crystals doped with Ni (Co 0.93 Ni 0.07 Sn) have a significantly smaller magnetic susceptibility and γ than pure CoSn, consistent with flat bands further from E F .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Substantial reduction of the anisotropy in the critical current densities J c of Ni-doped CaKFe 4 As 4 single crystals by chemical and irradiation-induced disorder

Here, we demonstrate that the anisotropy in the critical current densities, J c , of CaK(Fe 1–x Ni x ) 4 As 4 (x = 0, 0.015, 0.025, and 0.030) single crystals reduces with increasing chemical and irradiation-induced disorder. The magnetic field dependences of J c are analyzed by performing magnetization measurements with H applied parallel and perpendicular to the crystallographic c-axis. The results show that undoped crystals display large anisotropies in Jc due to an enhancement of the vortex pinning with H applied parallel to the crystallographic ab-planes. This anisotropy reduces substantially as Ni addition increases. Moreover, we found that random disorder introduced by proton irradiation enhances mainly the vortex pinning for H parallel to the c-axis. Consequently, using adequate fluencies, the vortex pinning at low temperatures in both undoped and doped samples becomes close to isotropic. These results make the CaKFe 4 As 4 system promising for applications that require isotropic J c under magnetic fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

Magnetic detwinning and biquadratic magnetic interaction in EuFe 2 As 2 revealed by 153 Eu NMR

In the nematic state of iron-based superconductors, twin formation often obscures the intrinsic, anisotropic, in-plane physical properties. Relatively high in-plane external magnetic fields H ext greater than the typical laboratory-scale magnetic fields 10–15 T are usually required to completely detwin a sample. However, recently a very small in-plane H ext ~ 0.1 T was found to be sufficient for detwinning the nematic domains in EuFe 2 As 2 . To explain this behavior, a microscopic theory based on biquadratic magnetic interactions between the Eu and Fe spins has been proposed. In this paper, using 153 Eu nuclear magnetic resonance (NMR) measurements below the Eu 2+ ordering temperature, we show experimental evidence of the detwinning under small in-plane H ext . Our NMR study also reveals the evolution of the angles between the Eu and Fe spins during the detwinning process, which provides experimental evidence for the existence of biquadratic coupling in the system.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Analysis of the London penetration depth in Ni-doped CaKFe 4 As 4

We report combined experimental and theoretical analysis of superconductivity in CaK(Fe 1-x Ni x ) 4 As 4 (CaK1144) for $x = 0$, 0.017, and 0.034. To obtain the superfluid density $ρ = [1 + Δλ_L$($T$)/$λ_L(0)]^{-2}$, the temperature dependence of the London penetration depth $Δλ_L$ ($T$) was measured by using a tunnel-diode resonator (TDR) and the results agreed with the microwave coplanar resonator (MWR) with the small differences accounted for by considering a three orders of magnitude higher frequency of MWR. The absolute value of $λ_L (T«T_c) ≈ λ_L (0)$ was measured by using MWR, $λ_L (5 K) ≈ 170 ± 20$ nm, which agreed well with the NV centers in diamond optical magnetometry that gave $λ_L (5 K) ≈ 196 ± 12$ nm. The experimental results are analyzed within the Eliashberg theory, showing that the superconductivity of CaK1144 is well described by the nodeless $s_±$ order parameter and that upon Ni doping the interband interaction increases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Effect of Ni doping on vortex pinning in CaK ( F e 1 - x N i x ) 4 A s 4 single crystals

We study the correlation between chemical composition and vortex dynamics in Ni-doped CaK(Fe 1–x Ni x ) 4 As 4 (x = 0, 0.015, 0.025, 0.03, and 0.05) single crystals by performing measurements of the critical current densities J c and the flux creep rates S . The magnetic relaxation of all the crystals is well described by the collective creep theory. The samples display a glassy exponent μ within the predictions for vortex bundles in a weak pinning scenario and relatively small characteristic pinning energy ( U 0 < 100K). The undoped crystals display modest J c values at low temperatures and high magnetic fields applied along the c axis. J c ( T ) dependences at high fields display an unusual peak. The enhancement in J c ( T ) matches with an increase in U 0 and the appearance of a second peak in the magnetization. As Ni doping increases, whereas there is a monotonic decrease in T c there is a nonmonotonic change in J c . Initially J c increases, reaching a maximum value for x = 0.015, and then J c decreases for x ≥ 0.025. This change in J c ( x ) is coincident with the onset of antiferromagnetic order. The magnetic field dependence of J c ( H ) also manifests a change in behavior between these x values. The analysis of the vortex dynamics for small and intermediate magnetic fields shows a gradual evolution in the glassy exponent μ with Ni content, x . As a result, this implies that there is no appreciable change in the mechanism that determines the vortex relaxation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Measuring the Lower Critical Field of Superconductors Using Nitrogen-Vacancy Centers in Diamond Optical Magnetometry

The lower critical magnetic eld, H c1 , of superconductors is measured by using ensembles of NV-centers-in-diamond optical magnetometry. The technique is minimally invasive, and has sub- gauss eld sensitivity and submicrometer spatial resolution, which allow for accurate detection of the vector eld at which the vortices start penetrating the sample from the corners. Aided by the revised calculations of the effective demagnetization factors of actual cuboid - shaped samples, H c1 and the London penetration depth, λ, derived from H c1 can be obtained. We apply this method to three well-studied superconductors: optimally doped Ba(Fe 1-x Co x ) 2 As 2 , stoichiometric CaKFe 4 As 4 , and high-Tc cuprate YBa 2 Cu 3 O 7-δ . Our results are well compared with the values of λ obtained using other techniques, thus adding another non-destructive and sensitive method to measure these important parameters of superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

High- T c superconductivity in CaKFe 4 As 4 in absence of nematic fluctuations

Here, we employ polarization-resolved Raman spectroscopy to study the multiband stoichiometric T c =35 K superconductor CaKFe 4 As 4 . We do not detect Pomeranchuk-like electronic nematic fluctuations which were universally observed in the XY (B 2g ) symmetry Raman response for most families of the Fe-based superconductors. In the superconducting state we observe, consistent with a nodeless order parameter, a full spectral weight suppression at low energies and the emergence of a composite pair-breaking coherence feature at energies between 12 and 20 meV. We analyze the superconductivity-induced phonon self-energy effects and give an estimation for the electron-phonon coupling constant λ Γ =0.0015 which is insufficient to provide attraction for high- T c pairing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Hedgehog Spin-Vortex Crystal Antiferromagnetic Quantum Criticality in CaK ( Fe 1 - x Ni x ) 4 As 4 Revealed by NMR

Two ordering states, antiferromagnetism and nematicity, have been observed in most iron-based superconductors (SCs). In contrast to those SCs, the newly discovered SC CaK(Fe 1-x Ni x ) 4 As 4 exhibits an antiferromagnetic (AFM) state, called hedgehog spin-vortex crystal structure, without nematic order, providing the opportunity for the investigation into the relationship between spin fluctuations and SC without any effects of nematic fluctuations. Our 75 As nuclear magnetic resonance studies on CaK(Fe 1-x Ni x ) 4 As 4 (0≤ x ≤ 0.049) revealed that CaKFe 4 As 4 is located close to a hidden hedgehog SVC AFM quantum-critical point (QCP). The magnetic QCP without nematicity in CaK(Fe 1-x Ni x ) 4 As 4 highlights the close connection of spin fluctuations and superconductivity in iron-based SCs. The advantage of stoichiometric composition also makes CaKFe 4 As 4 an ideal platform for further detailed investigation of the relationship between magnetic QCP and superconductivity in iron-based SCs without disorder effects.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2018

Indication of subdominant d-wave interaction in superconducting CaKFe 4 As 4

We report inelastic light scattering results on the stoichiometric and fully ordered superconductor CaKFe 4 As 4 as a function of temperature and light polarization. In the energy range between 10 and 315 cm -1 (1.24 and 39.1 meV) we observe the particle-hole continuum above and below the superconducting transition temperature T c and seven of the eight Raman active phonons. The main focus is placed on the analysis of the electronic excitations. Below T c all three symmetries projected with in-plane polarizations display a redistribution of spectral weight characteristic for superconductivity. The energies of the pair-breaking peaks in A 1g and B 2g symmetry are in approximate agreement with the results from photoemission studies. In B 1g symmetry the difference between the normal and superconducting state is most pronounced, and the feature is shifted downwards with respect to those in A 1g and B 2g symmetry. The maximum peaking at 134 cm -1 (16.6 meV) has a substructure on the high-energy side. We interpret the peak at 134 cm -1 in terms of a collective Bardasis-Schrieffer (BS) mode and the substructure as a remainder of the pair-breaking feature on the electron bands. Finally, there is a very weak peak at 50 cm -1 (6.2 meV) which is tentatively assigned to another BS mode.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Antiferromagnetic order in CaK ( Fe 1 – x Ni x ) 4 As 4 and its interplay with superconductivity

The magnetic order in CaK(Fe 1–x Ni x ) 4 As 4 (1144) single crystals (x = 0.051 and 0.033) has been studied by neutron diffraction. We observe magnetic Bragg peaks associated with the same propagation vectors as found for the collinear stripe antiferromagnetic (AFM) order in the related BaFe 2 As 2 (122) compound. The AFM state in 1144 preserves tetragonal symmetry and only a commensurate, noncollinear structure with a hedgehog spin-vortex crystal (SVC) arrangement in the Fe plane and simple AFM stacking along the c direction is consistent with our observations. The SVC order is promoted by the reduced symmetry in the FeAs layer in the 1144 structure. The long-range SVC order coexists with superconductivity, however, similar to the doped 122 compounds, the ordered magnetic moment is gradually suppressed with the developing superconducting order parameter. Lastly, this supports the notion that both collinear and noncollinear magnetism and superconductivity are competing for the same electrons coupled by Fermi surface nesting in iron arsenide superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Robust s ± pairing in CaK ( Fe 1 – x Ni x ) 4 As 4 ( x = 0 and 0.05) from the response to electron irradiation

Here, controlled pointlike disorder introduced by 2.5-MeV electron irradiation was used to probe the superconducting state of single crystals of CaK(Fe 1–x Ni x ) 4 As 4 superconductor at x = 0 and 0.05 doping levels. Both compositions show an increase of the residual resistivity and a decrease of the superconducting transition temperature, T c , at the rate of dT c /dρ(T c ) ≈ 0.19 K/(μΩcm) for x = 0 and 0.38 K/(μΩcm) for x = 0.05, respectively. In the Ni-doped compound (x = 0.05), the coexisting spin-vortex crystal (SVC) magnetic phase is suppressed at the rate of dT N /dρ(T N ) ≈ 0.16 K/(μΩcm). The low-temperature variation of London penetration depth is well approximated by the power-law function, Δλ(T) = AT n , with n ≈ 2.5 for x = 0 and n ≈ 1.9 for x = 0.05 in the pristine state. Detailed analysis of λ(T) and T c evolution with disorder is consistent with two effective nodeless energy gaps in the density of states due to robust s± pairing. Overall the behavior of CaK(Fe 1–x Ni x ) 4 As 4 at x = 0 is similar to a slightly overdoped Ba 1–y K y Fe 2 As 2 at y ≈ 0.5, and at x = 0.05 to an underdoped composition at y ≈ 0.2.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Spatially-resolved study of the Meissner effect in superconductors using NV-centers-in-diamond optical magnetometry

Non-invasive magnetic field sensing using optically-detected magnetic resonance of nitrogen-vacancy centers in diamond was used to study spatial distribution of the magnetic induction upon penetration and expulsion of weak magnetic fields in several representative superconductors. Vector magnetic fields were measured on the surface of conventional, elemental Pb and Nb, and compound LuNi 2 B 2 C and unconventional iron-based superconductors Ba 1-x K x Fe 2 As 2 (x = 0.34 optimal hole doping), Ba(Fe 1-x Co x )2As2 (x = 0.07 optimal electron doping), and stoichiometric CaKFe 4 As 4 , using variable-temperature confocal system with diffraction-limited spatial resolution. Magnetic induction profiles across the crystal edges were measured in zero-field-cooled and field-cooled conditions. While all superconductors show nearly perfect screening of magnetic fields applied after cooling to temperatures well below the superconducting transition, T c , a range of very different behaviors was observed for Meissner expulsion upon cooling in static magnetic field from above T c . Substantial conventional Meissner expulsion is found in LuNi 2 B 2 C, paramagnetic Meissner effect is found in Nb, and virtually no expulsion is observed in iron-based superconductors. In all cases, good correlation with macroscopic measurements of total magnetic moment is found.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

NMR Study of the New Magnetic Superconductor CaK(Fe 0:951 Ni0:049) 4 As 4 : Microscopic Coexistence of Hedgehog Spin-vortex Crystal and Superconductivity

Coexistence of a new-type antiferromagnetic (AFM) state, the so-called hedgehog spin-vortex crystal (SVC), and superconductivity (SC) is evidenced by 75 As nuclear magnetic resonance study on single-crystalline CaK(Fe 0:951 Ni0:049) 4 As 4 . The hedgehog SVC order is clearly demonstrated by the direct observation of the internal magnetic induction along the c axis at the As1 site (close to K) and a zero net internal magnetic induction at the As2 site (close to Ca) below an AFM ordering temperature T N ~ 52 K. The nuclear spin-lattice relaxation rate 1/T 1 shows a distinct decrease below T c ~ 10 K, providing also unambiguous evidence for the microscopic coexistence. Furthermore, based on the analysis of the 1/T 1 data, the hedgehog SVC-type spin correlations are found to be enhanced below T ~ 150 K in the paramagnetic state. Furthermore, these results indicate the hedgehog SVC-type spin correlations play an important role for the appearance of SC in the new magnetic superconductor.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Magnetic fluctuations and superconducting properties of CaKFe 4 As 4 studied by As 75 NMR

In this paper, we report 75 As nuclear magnetic resonance (NMR) studies on a new iron-based superconductor, CaKFe 4 As 4 , with T c = 35 K. 75 As NMR spectra show two distinct lines corresponding to the As(1) and As(2) sites close to the K and Ca layers, respectively, revealing that K and Ca layers are well ordered without site inversions. We found that nuclear quadrupole frequencies ν Q of the As(1) and As(2) sites show an opposite temperature T dependence. Nearly T independent behavior of the Knight shifts K is observed in the normal state, and a sudden decrease in K in the superconducting (SC) state suggests spin-singlet Cooper pairs. 75 As spin-lattice relaxation rates 1/T 1 show a power-law T dependence with different exponents for the two As sites. The isotropic antiferromagnetic spin fluctuations characterized by the wave vector q = (π, 0) or (0, π) in the single-iron Brillouin zone notation are revealed by 1/T 1 T and K measurements. Such magnetic fluctuations are necessary to explain the observed temperature dependence of the 75 As quadrupole frequencies, as evidenced by our first-principles calculations. Finally, in the SC state, 1/T 1 shows a rapid decrease below T c without a Hebel-Slichter peak and decreases exponentially at low T, consistent with an s ± nodeless two-gap superconductor.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2017

Optimization of the crystal growth of the superconductor CaKFe 4 As 4 from solution in the FeAs - CaFe 2 As 2 - KFe 2 As 2 system

Measurements of the anisotropic properties of single crystals play a crucial role in probing the physics of new materials. Determining a growth protocol that yields suitable high-quality single crystals can be particularly challenging for multicomponent compounds. Here we present a case study of how we refined a procedure to grow single crystals of CaKFe 4 As 4 from a high temperature, quaternary liquid solution rich in iron and arsenic (“FeAs self-flux”). Temperature dependent resistance and magnetization measurements are emphasized, in addition to the x-ray diffraction, to detect intergrown CaKFe 4 As 4 , CaFe 2 As 2 , and KFe 2 As 2 within what appear to be single crystals. Guided by the rules of phase equilibria and these data, we adjusted growth parameters to suppress formation of the impurity phases. The resulting optimized procedure yielded phase-pure single crystals of CaKFe 4 As 4 . In conclusion, this optimization process offers insight into the growth of quaternary compounds and a glimpse of the four-component phase diagram in the pseudoternary FeAs–CaFe 2 As 2 –KFe 2 As 2 system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Nodeless multiband superconductivity in stoichiometric single-crystalline CaKFe 4 As 4

Measurements of the London penetration depth Δλ(T) and tunneling conductance in single crystals of the recently discovered stoichiometric iron-based superconductor CaKFe 4 As 4 (CaK1144) show nodeless, two-effective-gap superconductivity with a larger gap of about 6–10 meV and a smaller gap of about 1–4 meV. Having a critical temperature T c,onset ≈ 35.8 K, this material behaves similar to slightly overdoped (Ba 1–x K x )Fe 2 As 2 (e.g., x = 0.54,T c ≈ 34 K), a known multigap s ± superconductor. Here, we conclude that the superconducting behavior of stoichiometric CaK1144 demonstrates that two-gap s± superconductivity is an essential property of high-temperature superconductivity in iron-based superconductors, independent of the degree of substitutional disorder.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗