arXiv subjects

Kyuil Cho

Publications and source records attributed to Kyuil Cho.

At least 19 recordsLinked to original sources

arXiv2024

Effect of Proton Irradiation in Thin-Film YBa$_2$Cu$_3$O$_{7-\delta}$ Superconductor

We investigated the effect of 0.6 MeV proton irradiation on the superconducting and normal state properties of thin-film $\text{YBa}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ superconductors. A thin-film YBCO superconductor ($\approx$ 567 nm thick) was subject to a series of proton irradiations with a total fluence of $7.6\times10^{16}$ $\text{p/cm}^2$. Upon irradiation, $T_c$ was drastically decreased from 89.3 K towards zero with a corresponding increase in its normal state resistivity above $T_c$. This increase in resistivity which indicates the increase of defects inside the thin-film sample can be converted to the dimensionless scattering rate. We found that the relation between $T_c$ and dimensionless scattering rate obtained during proton irradiation approximates the generalized d-wave Abrikosov-Gor'kov theory better than the previous results obtained from electron irradiations. This is an unexpected result since the electron irradiation is known to be most effective to suppress superconductivity over other heavier ion irradiations such as proton irradiation. It suggests that the type of defects created by proton irradiation evolves from cascade defects (in bulk single crystals) to point-like defects (in thin-film single crystals) as the thickness decreases.

cond-mat.supr-con↗

arXiv2024

Conventional s-wave superconductivity and hidden peak effect in single crystals of Mo$_8$Ga$_41$ superconductor

London and Campbell penetration depths were measured in single crystals of the endohedral gallide cluster superconductor, Mo$_{8}$Ga$_{41}$. The full temperature range superfluid density is consistent with the clean isotropic $s-$wave weak-coupling BCS theory without any signs of the second gap or strong coupling. The temperature dependence of the Campbell length is hysteretic between zero-field cooling (ZFC) and field-cooling (FC) protocols, indicating an anharmonic vortex pinning potential. The field dependence of the effective critical current density, $j_{c}\left(H\right)$, reveals an unusual result. While in the ZFC protocol, $j_{c}\left(H\right)$ is monotonically suppressed by the magnetic field, it exhibits a profound ``hidden'' peak effect in the FC protocol, that is, without a vortex density gradient. We suggest a possible novel mechanism for the formation of the peak effect, which involves both static and dynamic aspects.

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arXiv2023

Unconventional nodal superconductivity in miassite Rh$_{17}$S$_{15}$

Unconventional superconductivity has long been believed to arise from a lab-grown correlated electronic system. Here we report compelling evidence of unconventional nodal superconductivity in a mineral superconductor \rhs. We investigated the temperature-dependent London penetration depth $\Delta\lambda(T)$ and disorder evolution of the critical temperature $T_c$ and upper critical field $H_{c2}(T)$ in synthetic miassite \rhs. We found a power-law behavior of $\Delta\lambda(T)\sim T^n$ with $n\approx 1.1$ at low temperatures below $0.3T_c$ ($T_c$ = 5.4 K), which is consistent with the presence of lines of the node in the superconducting gap of \rhs. The nodal character of the superconducting state in \rhs~was supported by the observed pairbreaking effect in $T_c$ and $H_{c2}(T)$ in samples with the controlled disorder that was introduced by low-temperature electron irradiation. We propose a nodal sign-changing superconducting gap in the $A_{1g}$ irreducible representation, which preserves the cubic symmetry of the crystal and is in excellent agreement with the superfluid density, $\lambda^2(0)/\lambda^2(T)$.

cond-mat.supr-con↗

arXiv2023

Ion-selective scattering studied by the variable-energy electron irradiation of Ba$_{0.2}$K$_{0.8}$Fe$_2$As$_2$ superconductor

Low-temperature variable-energy electron irradiation was used to induce non-magnetic disorder in a single crystal of hole-doped iron-based superconductor, Ba$_{1-x}$K$_x$Fe$_2$As$_2$, $x=$0.80. To avoid systematic errors, the beam energy was adjusted non-consequently for five values between 1.0 and 2.5 MeV, whence sample resistance was measured in-situ at 22 K. For all energies, the resistivity raises linearly with the irradiation fluence suggesting the creation of uncorrelated dilute point-like disorder (confirmed by simulations). The rate of the resistivity increase peaks at energies below 1.5 MeV. Comparison with calculated partial cross-sections points to the predominant creation of defects in the iron sublattice. Simultaneously, superconducting $T_c$, measured separately between the irradiation runs, is monotonically suppressed as expected since it depends on the total scattering rate, hence total cross-section, which is a monotonically increasing function of energy. Our work confirms experimentally an often-made assumption of the dominant role of the iron sub-lattice in iron-based superconductors.

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arXiv2022

Possible unconventional order parameter in single crystals of SrPt$_3$P superconductor

Anisotropic properties of single crystals of SrPt$_{3}$P were studied using London penetration depth and electrical resistivity measurements. The upper critical field, $H_{c2}(T)$, was determined from four-probe electrical resistivity measurements for three orthogonal directions of a magnetic field with respect to the crystal. The London penetration depth, $\lambda(T)$, was determined from the magnetic susceptibility of the Meissner-London state measured using a tunnel-diode resonator technique. Whereas $H_{c2}(T)$ and the normal-state $\rho(T)$ are practically identical for all three magnetic field orientations, the London penetration depth shows significant unidirectional anisotropy. The low-temperature $\lambda(T)$ is exponentially attenuated when a small excitation radiofrequency magnetic field, $H_{rf}$, is applied along the $c"-$direction, in which case screening currents flow in the $a"b"-$ plane, while for the other two orientations, $H_{rf}\parallel a"$ and $H_{rf}\parallel b"$, the London penetration depth shows a much weaker, $\lambda(T)\sim T^{2}$, variation. Such unusual and contrasting behavior of the two anisotropies, $\gamma_{H}\left(T\right)=H_{c2,ab}/H_{c2,c}=\xi_{ab}/\xi_{c}$ and $\gamma_{\lambda}\left(T\right)=\lambda_{c}/\lambda_{ab}$, imposes significant constraints on the possible order parameter. Although our measurements are insufficient to derive conclusively the superconducting gap anisotropy, qualitatively, order parameter with two point nodes and a modulation in the perpendicular direction is consistent with the experimental observations.

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arXiv2021

Multi-band $s_{++}$ superconductivity in $\textrm{V}_{3}\textrm{Si}$ determined from the response to a controlled disorder

The London penetration depth, $λ(T)$, was measured in a single crystal V$_{3}$Si. The superfluid density obtained from this measurement shows a distinct signature of two almost decoupled superconducting gaps. This alone is insufficient to distinguish between $s_{\pm}$ and $s_{++}$ pairing states, but it can be achieved by studying the effect of a controlled non-magnetic disorder on the superconducting transition temperature, $T_{c}$. For this purpose, the same $\text{V}_{3}\text{Si}$ crystal was sequentially irradiated by 2.5 MeV electrons three times, repeating the measurement between the irradiation runs. A total dose of 10 C/cm$^{2}$ ($6.24\times10^{19}$ electrons/$\textrm{cm}^{2}$) was accumulated, for which $T_{c}$ has changed from 16.4 K in a pristine state to 14.7 K (9.3 $\%$). This substantial suppression is impossible for a single isotropic gap, but also it is not large enough for a sign-changing $s_{\pm}$ pairing state. Our electronic band-structure calculations show how five bands crossing the Fermi energy can be naturally grouped to support two effective gaps, not dissimilar from the iron pnictides physics. We analyze the results using two-gap models for both, $λ(T)$ and $T_{c}$, which describe the data very well. Thus, the experimental results and theoretical analysis provide strong support for an $s_{++}$ superconductivity with two unequal gaps, $Δ_{1}\left(0\right)\approx2.53\;\textrm{meV}$ and $Δ_{2}\left(0\right)\approx1.42\;\textrm{meV}$, and a very weak inter-band coupling in $\text{V}_{3}\text{Si}$ superconductor.

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arXiv2021

Temperature dependent striction effect in a single crystalline Nd2Fe14B revealed using a novel high temperature resistivity measurement technique

We studied the temperature dependence of resistivity in a single crystalline Nd2Fe14B using a newly developed high temperature probe. This novel probe uses mechanical pin connectors instead of conducting glue/paste. From warming and cooling curves, the Curie temperature was consistently measured around Tc = 580 K. In addition, anomalous discrete jumps were found only in cooling curves between 400 and 500 K, but not shown in warming curves. More interestingly, when the jumps occurred during cooling, the resistivity was increased. This phenomenon can be understood in terms of temperature dependent striction effect induced by the re-orientation of magnetic domains well below the Curie temperature.

cond-mat.mtrl-sci↗

arXiv2021

Intermediate scattering potential strength in electron-irradiated $\text{YBa}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ from London penetration depth measurements

Temperature-dependent London penetration depth, $\lambda(T)$, of a high quality optimally-doped $\text{YBa}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ single crystal was measured using tunnel-diode resonator. Controlled artificial disorder was induced at low-temperature of 20~K by 2.5 MeV electron irradiation at accumulating large doses of $3.8\times10^{19}$ and $5.3\times10^{19}$ electrons per $\textrm{cm}^{2}$. The irradiation caused significant suppression of the superconductor's critical temperature, $T_{c}$, from 94.6 K to 90.0 K, and then to 78.7 K, respectively. The low-temperature behavior of $\lambda\left(T\right)$ evolves from a $T-$linear in pristine state to a $T^{2}-$behavior after the irradiation, expected for a line-nodal $d-$wave superconductor. However, the original theory that explained such behavior had assumed a unitary limit of the scattering potential, whereas usually in normal metals and semiconductors, Born scattering is sufficient to describe the experiment. To estimate the scattering potential strength, we calculated the normalized superfluid density, $\rho_{s}\left(t=T/T_{c}\right)=\lambda^{2}\left(0\right)/\lambda^{2}\left(t\right)$, varying the amount and the strength of non-magnetic scattering using a self-consistent $t-$matrix theory. Fitting the obtained curves to a power-law, $\rho_{s}=1-Rt^{n}$, and to a polynomial, $\rho_{s}=1-At-Bt^{2}$, and comparing the coefficients $n$ in one set, and $A$ and $B$ in another with the experimental values, we estimate the phase shift to be around 70$^{\circ}$ and 65$^{\circ}$, respectively. We correlate this result with the evolution of the density of states with non-magnetic disorder.

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arXiv2021

Effect of controlled artificial disorder on the magnetic properties of EuFe$_2$(As$_{1-x}$P$_{x }$)$_2$ ferromagnetic superconductor

Static (DC) and dynamic (AC, at 14 MHz and 8 GHz) magnetic susceptibilities of single crystals of a ferromagnetic superconductor, $\textrm{EuFe}_{2}(\textrm{As}_{1-x}\textrm{P}_{x})_{2}$ (x = 0.23), were measured in pristine state and after different doses of 2.5 MeV electron or 3.5 MeV proton irradiation. The superconducting transition temperature, $T_{c}(H)$, shows an extraordinarily large decrease. It starts at $T_{c}(H=0)\approx24\:\textrm{K}$ in the pristine sample for both AC and DC measurements, but moves to almost half of that value after moderate irradiation dose. Our results suggest that in $\textrm{EuFe}_{2}(\textrm{As}_{1-x}\textrm{P}_{x})_{2}$ superconductivity is affected by local-moment ferromagnetism mostly via the spontaneous internal magnetic fields induced by the FM subsystem. Another mechanism is revealed upon irradiation where magnetic defects created in ordered $\text{Eu}^{2+}$ lattice act as efficient pairbreakers leading to a significant $T_{c}$ reduction upon irradiation compared to other 122 compounds. On the other hand, the exchange interactions seem to be weakly screened by the superconducting phase leading to a modest increase of $T_{m}$ (less than 1 K) after the irradiation drives $T_{c}$ to below $T_{m}$. The results suggest that FM and SC phases coexist microscopically in the same volume.

cond-mat.supr-con↗

arXiv2020

Effect of controlled point-like disorder induced by 2.5 MeV electron irradiation on nematic resistivity anisotropy of hole-doped (Ba,K)Fe$_2$As$_2$

In-plane anisotropy of electrical resistivity was studied in samples of the hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ in the composition range $0.21 \leq x \leq 0.26$ where anisotropy changes sign. Low-temperature ($\sim$20~K) irradiation with relativistic 2.5 MeV electrons was used to control the level of disorder and residual resistivity of the samples. Modification of the stress-detwinning technique enabled measurements of the same samples before and after irradiation, leading to conclusion of anisotropic character of predominantly inelastic scattering processes. Our main finding is that the resistivity anisotropy is of the same sign irrespective of residual resistivity, and remains the same in the orthorhombic $C_2$ phase above the re-entrant tetragonal transition. Unusual $T$-linear dependence of the anisotropy $Δρ\equiv ρ_a(T)-ρ_b(T)$ is found in pristine samples with $x=$0.213 and $x=$0.219, without similar signatures in either $ρ_a(T)$ or $ρ_b(T)$. We show that this feature can be reproduced by a phenomenological model of R.~M.~Fernandes {\it et al.} Phys. Rev. Lett. {\bf 107},217002 (2011). We speculate that onset of fluctuations of nematic order on approaching the instability towards the re-entrant tetragonal phase contributes to this unusual dependence.

cond-mat.supr-con↗

arXiv2020

Electron irradiation effects on superconductivity in PdTe$_2$: an application of a generalized Anderson theorem

Low temperature ($\sim$ 20~K) electron irradiation with 2.5 MeV relativistic electrons was used to study the effect of controlled non-magnetic disorder on the normal and superconducting properties of the type-II Dirac semimetal PdTe$_2$. We report measurements of longitudinal and Hall resistivity, thermal conductivity and London penetration depth using tunnel-diode resonator technique for various irradiation doses. The normal state electrical resistivity follows Matthiessen rule with an increase of the residual resistivity at a rate of $\sim$0.77$ μΩ$cm/$(\textrm{C}/\textrm{cm}^2)$. London penetration depth and thermal conductivity results show that the superconducting state remains fully gapped. The superconducting transition temperature is suppressed at a non-zero rate that is about sixteen times slower than described by the Abrikosov-Gor'kov dependence, applicable to magnetic impurity scattering in isotropic, single-band $s$-wave superconductors. To gain information about the gap structure and symmetry of the pairing state, we perform a detailed analysis of these experimental results based on insight from a generalized Anderson theorem for multi-band superconductors. This imposes quantitative constraints on the gap anisotropies for each of the possible pairing candidate states. We conclude that the most likely pairing candidate is an unconventional $A_{1g}^{+-}$ state. While we cannot exclude the conventional $A_{1g}^{++}$ and the triplet $A_{1u}$, we demonstrate that these states require additional assumptions about the orbital structure of the disorder potential to be consistent with our experimental results, e.g., a ratio of inter- to intra-band scattering for the singlet state significantly larger than one. Due to the generality of our theoretical framework, we think that it will also be useful for irradiation studies in other spin-orbit-coupled multi-orbital systems.

cond-mat.supr-con↗

arXiv2018

Dependence of the absolute value of the penetration depth in $\mathrm{\mathbf{{(Ba_{1-x}K_x)Fe_2As_2}}}$ on doping

We report magnetic force microscopy (MFM) measurements on the iron-based superconductor $\mathrm{Ba_{1-x}K_xFe_2As_2}$. By measuring locally the Meissner repulsion with the magnetic MFM tip, we determine the absolute value of the in-plane magnetic penetration depth ($λ_{ab}$) in underdoped, optimally-doped, and overdoped samples. The results suggest an abrupt increase of $λ_{ab}$ as doping is increased from $x_\mathrm{opt}$, which is potentially related to the presence of a quantum critical point. The response of superconducting vortices to magnetic forces exerted by the MFM tip for $x=0.19$ and $0.58$ is compatible with previously observed structural symmetries at those doping levels.

cond-mat.supr-con↗

arXiv2018

Using controlled disorder to probe the interplay between charge order and superconductivity in NbSe2

The interplay between superconductivity and charge density waves (CDW) in $H$-NbSe2 is not fully understood despite decades of study. Artificially introduced disorder can tip the delicate balance between two competing forms of long-range order, and reveal the underlying interactions that give rise to them. Here we introduce disorders by electron irradiation and measure in-plane resistivity, Hall resistivity, X-ray scattering, and London penetration depth. With increasing disorder, $T_{\textrm{c}}$ varies nonmonotonically, whereas $T_{\textrm{CDW}}$ monotonically decreases and becomes unresolvable above a critical irradiation dose where $T_{\textrm{c}}$ drops sharply. Our results imply that CDW order initially competes with superconductivity, but eventually assists it. We argue that at the transition where the long-range CDW order disappears, the cooperation with superconductivity is dramatically suppressed. X-ray scattering and Hall resistivity measurements reveal that the short-range CDW survives above the transition. Superconductivity persists to much higher dose levels, consistent with fully gapped superconductivity and moderate interband pairing.

cond-mat.supr-con↗

arXiv2018

Nodeless superconductivity in type-II Dirac semimetal PdTe2: low-temperature London penetration depth and symmetry analysis

Superconducting gap structure was probed in type-II Dirac semimetal PdTe$_2$ by measuring the London penetration depth using tunnel diode resonator technique. At low temperatures, the data for two samples are well described by weak coupling exponential fit yielding $λ(T=0)=230$~nm as the only fit parameter at a fixed $Δ(0)/T_c\approx 1.76$, and the calculated superfluid density is consistent with a fully gapped superconducting state characterized by a single gap scale. Electrical resistivity measurements for in-plane and inter-plane current directions find very low and nearly temperature-independent normal- state anisotropy. The temperature dependence of resistivity is typical for conventional phonon scattering in metals. We compare these experimental results with expectations from a detailed theoretical symmetry analysis and reduce the number of possible superconducting pairing states in PdTe$_2$ to only three nodeless candidates: a regular, topologically trivial, $s$-wave pairing, and two distinct odd-parity triplet states that both can be topologically non-trivial depending on the microscopic interactions driving the superconducting instability.

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arXiv2018

Using electron irradiation to probe iron - based superconductors

High energy electron irradiation is an efficient way to create vacancy-interstitial Frenkel pairs in crystal lattice, thereby inducing controlled non-magnetic point - like scattering centers. In combination with London penetration depth and resistivity measurements, the irradiation was particularly useful as a phase - sensitive probe of the superconducting order parameter in iron - based superconductors lending strongest support to sign - changing $s_{\pm}$ pairing. Here we review the key results on the effect of electron irradiation in iron-based superconductors.

cond-mat.supr-con↗

arXiv2016

Energy Gap Evolution Across the Superconductivity Dome in Single Crystals of (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$

The mechanism of unconventional superconductivity in iron-based superconductors (IBSs) is one of the most intriguing questions in current materials research. Among non-oxide IBSs, (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ has been intensively studied because of its high superconducting transition temperature and fascinating evolution of the superconducting gap structure from being fully isotropic at optimal doping ($x\approx$0.4) to becoming nodal at $x > $0.8. Although this marked evolution was identified in several independent experiments, there are no details of the gap evolution to date because of the lack of high-quality single crystals covering the entire K-doping range of the superconducting dome. We conducted a systematic study of the London penetration depth, $λ(T)$, across the full phase diagram for different concentrations of point-like defects introduced by 2.5 MeV electron irradiation. Fitting the low-temperature variation with the power law, $Δλ\sim T^{n}$, we find that the exponent $n$ is the highest and $T_c$ suppression rate with disorder is the smallest at optimal doping, and they evolve with doping being away from optimal, which is consistent with increasing gap anisotropy, including an abrupt change around $x\simeq 0.8$, indicating the onset of nodal behavior. Our analysis using a self-consistent $t$-matrix approach suggests the ubiquitous and robust nature of s$_{\pm}$ pairing in IBSs and argues against a previously suggested transition to a $d-$wave state near $x=1$ in this system.

cond-mat.supr-con↗

arXiv2016

Nodeless multiband superconductivity in stoichiometric single crystalline CaKFe$_4$As$_4$

Measurements of the London penetration depth and tunneling conductance in single crystals of the recently discovered stoicheometric, iron - based superconductor, CaKFe$_4$As$_4$ (CaK1144) show nodeless, two effective gap superconductivity with a larger gap of about 6-9 meV and a smaller gap of about 1-4 meV. Having a critical temperature, $T_{c,onset}\approx$35.8 K, this material behaves similar to slightly overdoped Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ (e.g. $x=$0.54, $T_c \approx$ 34 K)---a known multigap $s_{\pm}$ superconductor. We conclude that the superconducting behavior of stoichiometric CaK1144 demonstrates that two-gap $s_{\pm}$ superconductivity is an essential property of high temperature superconductivity in iron - based superconductors, independent of the degree of substitutional disorder.

cond-mat.supr-con↗

arXiv2016

Structural and magnetic phase transitions in Ca$_{0.73}$La$_{0.27}$FeAs$_2$ with electron overdoped FeAs layers

We report a study of the Ca$_{0.73}$La$_{0.27}$FeAs$_2$ single crystals. We unravel a monoclinic to triclinic phase transition at 58 K, and a paramagnetic to stripe antiferromagnetic (AFM) phase transition at 54 K, below which spins order 45$^\circ$ away from the stripe direction. Furthermore, we demonstrate this material is substantially structurally untwinned at ambient pressure with the formation of spin rotation walls (S-walls). Finally, in addition to the central-hole and corner-electron Fermi pockets usually appearing in Fe pnictide superconductors, angle-resolved photoemission (ARPES) measurements resolve a Fermiology where an extra electron pocket of mainly As chain character exists at the Brillouin zone edge.

cond-mat.supr-con↗