arXiv subjects

Nikolai D. Zhigadlo

Publications and source records attributed to Nikolai D. Zhigadlo.

At least 19 recordsLinked to original sources

arXiv2026

Anomalous field evolution of the mixed-state linewidth in the second superconducting dome of LaFeAsO$_{1-x}M_x$ ($M={\rm F,H}$)

We report a transverse-field muon-spin rotation/relaxation ($μ$SR) study of the internal-field distribution in the mixed state of LaFeAsO$_{0.89}$F$_{0.11}$ and LaFeAsO$_{0.75}$H$_{0.25}$, representative of the first (SC1) and second (SC2) superconducting domes of the LaFeAsO$_{1-x}M_x$ ($M={\rm F,H}$) family, respectively. Below the superconducting transition temperature $T_{\rm c}$, the linewidth of the internal-field distribution increases in both samples, indicating the formation of a vortex lattice. Above $T_{\rm c}$, the linewidth remains field dependent and increases approximately linearly with field, consistent with broadening of the powder spectrum caused by an anisotropic Knight shift. After subtraction of this normal-state contribution, the superconducting linewidth $σ_{\rm sc}$ exhibits qualitatively different field dependences in the two samples. At 4K, the SC1 ($x_{\rm F}=0.11$) sample shows the expected monotonic decrease with increasing field, whereas the SC2 ($x_{\rm H}=0.25$) sample develops a pronounced local maximum near 3T. A contour representation of $σ_{\rm sc}(T,H)$ further reveals a ridge of local maxima whose field position, $H_{σ,\max}(T)$, shifts to lower fields upon warming and disappears near $T_{\rm c}$. The anomalous field evolution observed in the SC2 sample is consistent with an additional field-induced contribution associated with enhanced Pauli-paramagnetic effects, highlighting the distinct electronic character of the two superconducting domes.

cond-mat.supr-con↗

arXiv2026

$μ$SR study of time-reversal symmetry constraints and bulk superfluid response in Li$_{0.95}$FeAs

We report zero-field (ZF) and transverse-field (TF) muon-spin rotation/relaxation ($μ$SR) measurements on superconducting Li$_{0.95}$FeAs ($T_{\rm c}\simeq16.0$ K) grown by a high-pressure self-flux method. The ZF-$μ$SR data show no detectable change of the electronic relaxation rate on cooling through $T_{\rm c}$, providing no evidence for time-reversal-symmetry breaking in the superconducting state. TF-$μ$SR measurements reveal a well-developed vortex response with strong flux pinning and a negligible nonsuperconducting contribution, confirming that superconductivity is a bulk property of the sample. From the second moment of the internal field distribution we determine a low-temperature in-plane magnetic penetration depth $λ_{ab}= 245(15)$ nm. The temperature dependence of the normalized superfluid density is well described by an effective two-gap model with $Δ_1 = 2.0(2)$ meV and $Δ_2 = 0.7(2)$ meV. A quantitative comparison with ARPES-based band weights shows that the $μ$SR response is dominated by the Fermi-surface sheets carrying the intermediate and small superconducting gaps, whereas the band hosting the largest gap contributes only about 3\% to the total superfluid density and is therefore not resolved in the present analysis. Taken together, these results establish Li$_{0.95}$FeAs as a bulk multigap superconductor without detectable time-reversal symmetry breaking and show how $μ$SR reconciles the gap scales reported by bulk and surface-sensitive probes in this multiband system.

cond-mat.supr-con↗

arXiv2023

A model for critical current effects in point-contact Andreev-reflection spectroscopy

It is well known that point-contact Andreev reflection spectroscopy provides reliable measurements of the energy gap(s) in a superconductor when the contact is in the ballistic or nearly-ballistic regime. However, especially when the mean free path of the material under study is small, obtaining ballistic contacts can be a major challenge. One of the signatures of a Maxwell contribution to the contact resistance is the presence of "dips" in the differential conductance, associated to the sudden appearance of a Maxwell term, in turn due to the attainment of the critical current of the material in the contact region. Here we show that, using a proper model for the $R(I)$ of the material under study, it is possible to fit the experimental curves (without the need of normalization) obtaining the correct values of the gap amplitudes even in the presence of such dips, as well as the temperature dependence of the critical current in the contact. We present a test of the procedure in the case of Andreev-reflection spectra in Mg$_{0.85}$Al$_{0.15}$B$_2$ single crystals.

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arXiv2023

Emergence of superconductivity in single-crystalline LaFeAsO under simultaneous Sm and P substitution

We report on the high-pressure growth, structural characterization, and investigation of the electronic properties of single-crystalline LaFeAsO co-substituted by Sm and P, in both its normal- and superconducting states. Here, the appearance of superconductivity is attributed to the inner chemical pressure induced by the smaller-size isovalent substituents. X-ray structural refinements show that the partial substitution of La by Sm and As by P in the parent LaFeAsO compound leads to a contraction in both the conducting Fe2(As,P)2 layers and the interlayer spacing. The main parameters of the superconducting state, including the critical temperature, the lower- and upper critical fields, as well as the coherence length, the penetration depth, and their anisotropy, were determined from magnetometry measurements on a single-crystalline La0.87Sm0.13FeAs0.91P0.09O sample. The critical current density (jc), as resulting from loops of magnetization hysteresis in the self-generated magnetic field, is 2 x 106 A/cm2 at 2 K. Overall, our findings illustrate a rare and interesting case of how superconductivity can be induced by co-substitution in the 1111 family. Such approach delineates new possibilities in the creation of superconductors by design, thus stimulating the exploration of related systems under multi-chemical pressure conditions.

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arXiv2022

Spin-glass-like behaviour in SmFeAsO0.8F0.2

The iron-based oxypnictide superconductor SmFeAsO0.8F0.2 was synthesized under high-pressure and investigated by measuring dc magnetic susceptibility. The zero-field cooled (ZFC) magnetic susceptibility confirmed the bulk superconductivity of the sample with a critical temperature of Tc = 50 K and a significant jump in the magnetization at 4.3 K, usually attributed to the antiferromagnetic ordering of Sm3+ ions in this system. Since the occurrence of the jump depends on the cooling history, our data strongly suggest a spin-glass-like behaviour.

cond-mat.supr-con↗

arXiv2019

Self-consistent two-gap approach in studying multi-band superconductivity in NdFeAsO$_{0.65}$F$_{0.35}$

High-quality single crystals of NdFeAsO$_{0.65}$F$_{0.35}$ (the transition temperature $T_{\rm c} \simeq 30.6$~K) were studied in zero-field (ZF) and transverse-field (TF) muon-spin rotation/relaxation ($μ$SR) experiments. An upturn in muon-spin depolarization rate at $T\lesssim 3$~K was observed in ZF-$μ$SR measurements and it was associated with the onset of ordering of Nd electronic moments. Measurements of the magnetic field penetration depth ($λ$) were performed in the TF geometry. By applying the external magnetic field $B_{\rm ex}$ parallel to the crystallographic $c$-axis ($B_{\rm ex}\| c$) and parallel to the $ab$-plane ($B_{\rm ex}\| ab$), the temperature dependencies of the in-plane component ($λ_{ab}^{-2}$) and the combination of the in-plane and the out of plane components ($λ_{ab,c}^{-2}$) of the superfluid density were determined, respectively. The out-of-plane superfluid density component ($λ_{c}^{-2}$) was further obtained by combining the results of $B_{\rm ex} \| c$ and $B_{\rm ex} \| {ab}$ set of experiments. The temperature dependencies of $λ_{ab}^{-2}$, $λ_{ab,c}^{-2}$, and $λ_{c}^{-2}$ were analyzed within the framework of a self-consistent two-gap model despite of using the traditional $α$-model. Interband coupling was taken into account, instead of assuming it to be zero as it stated in the $α$-model. A relatively small value of the interband coupling constant $Λ_{12} \simeq 0.01$ was obtained, thus indicating that the energy bands in NdFeAsO$_{0.65}$F$_{0.35}$ are only weakly coupled. In spite of their small magnitude, the coupling between the bands leads to the single value of the superconducting transition temperature $T_{\rm c}$.

cond-mat.supr-con↗

arXiv2019

Spontaneous emission of color centers at 4eV in hexagonal boron nitride under hydrostatic pressure

The light emission properties of color centers emitting in 3.3-4 eV region are investigated for hydrostatic pressures ranging up to 5GPa at liquid helium temperature. The light emission energy decreases with pressure less sensitively than the bandgap. This behavior at variance from the shift of the bandgap is typical of deep traps. Interestingly, hydrostatic pressure reveals the existence of levels that vary differently under pressure (smaller increase of the emission wavelength compared to the rest of the levels in this energy region or even decrease of it) with pressure. This discovery enriches the physics of the color centers operating in the UV in hBN.

cond-mat.mtrl-sci↗

arXiv2019

Crystal growth and characterization of the antiperovskite superconductor MgC1-xNi3-y

By varying the parameters controlling the growth of crystals, including the thermodynamic variables, such as temperature, pressure, and reagent composition and the kinetic factors, namely reaction time and cooling rate, we found the most appropriate conditions for the reproducible growth of the nonstoichiometric antiperovskite superconductors MgC1-xNi3-y. Bulk single crystals of MgC1-xNi3-y were grown by a self-flux method at 3 GPa and 1700 C using a mixture of Mg, C, and Ni powders in a molar ratio 1:1.25:3. The as-grown black colored crystals, mechanically extracted from solidified lump, exhibit various irregular three dimensional shapes, with flat surfaces and maximum dimensions up to ~ (1-1.2) x (0.8-1.0) x (0.4-0.6) mm3. Single-crystal x-ray diffraction refinement confirmed the high structural perfection of the grown crystals (Space group Pm-3m, No 221, Z = 1, a = 3.7913(1) Å, and V = 54.5(1) Å3), but also the presence of deficiencies on the C and Ni sites. Temperature-dependent magnetization measurements showed a single-phase behaviour with a critical temperature (Tc) ranging between 6.3 and 6.8 K due to the slightly different C and Ni stoichiometries of MgC1-xNi3-y crystals. The growth of relatively large crystals reported here could provide a helpful guidance for further syntheses of various 3d-based antiperovskite intermetallics under high pressure.

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arXiv2019

Enhanced Optoelectronic Response in Bilayer Lateral Heterostructures of Transition Metal Dichalcogenides

Two-dimensional lateral heterojunctions are basic components for low-power and flexible optoelectronics. In contrast to monolayers, devices based on few-layer lateral heterostructures could offer superior performance due to their lower susceptibility to environmental conditions. Here, we report the controlled synthesis of multi-junction bilayer lateral heterostructures based on MoS2-WS2 and MoSe2-WSe2, where the hetero-junctions are created via sequential lateral edge-epitaxy that happens simultaneously in both the first and the second layer. With respect to their monolayer counterparts, bilayer lateral heterostructures yield nearly one order of magnitude higher rectification currents. They also display a clear photovoltaic response, with short circuit currents ~103 times larger than those extracted from the monolayers, in addition to room-temperature electroluminescence. The superior performance of bilayer heterostructures significantly expands the functionalities of 2D crystals.

cond-mat.mtrl-sci↗

arXiv2018

Observation of high-Tc superconductivity in inhomogeneous combinatorial ceramics

A single-sample synthesis concept based on multi-element ceramic samples can produce a variety of local products. When applied to cuprate superconductors (SC), statistical modelling predicts the occurrence of possible compounds in a concentration range of about 50 ppm. In samples with such low concentrations, determining which compositions are superconducting is a challenging task and requires local probes or separation techniques. Here, we report results from samples with seven components: BaO2, CaCO3, SrCO3, La2O3, PbCO3, ZrO2 and CuO oxides and carbonates, starting from different grain sizes. The reacted ceramics show different phases, particular grain growth, as well as variations in homogeneity and superconducting properties. High-Tc superconductivity up to 118 K was found. Powder x-ray diffraction (XRD) in combination with energy-dispersive spectroscopy (EDS), scanning transmission electron microscopy (STEM) can assign Pb1223 and (Sr,Ca,Ba)0.7-1.0CuO2 phases in inhomogeneous samples milled with 10 mm ball sizes. Rather uniform samples featuring strong grain growth were obtained with 3 mm ball sizes, resulting in Tc =70 K superconductivity of the La(Ba,Ca)2Cu3Ox based phase. Scanning SQUID microscopy (SSM) establishes locally formed superconducting areas at a level of a few microns in inhomogeneous superconducting particles captured by a magnetic separation technique. The present results demonstrate a new synthetic approach for attaining high-Tc superconductivity in compounds without Bi, Tl, Hg, or the need for high-pressure synthesis.

cond-mat.supr-con↗

arXiv2018

Topological Type-II Dirac Fermions Approaching the Fermi Level in a Transition Metal Dichalcogenide NiTe2

Type-II Dirac/Weyl semimetals are characterized by strongly tilted Dirac cones such that the Dirac/Weyl node emerges at the boundary of electron and hole pockets as a new state of quantum matter, distinct from the standard Dirac/Weyl points with a point-like Fermi surface which are referred to as type-I nodes. The type-II Dirac fermions were recently predicted by theory and have since been confirmed in experiments in the PtSe2-class of transition metal dichal-cogenides. However, the Dirac nodes observed in PtSe2, PdTe2 and PtTe2 candidates are quite far away from the Fermi level, making the signature of topological fermions obscure as the physical properties are still dominated by the non-Dirac quasiparticles. Here we report the synthesis of a new type-II Dirac semimetal NiTe2 in which a pair of type-II Dirac nodes are located very close to the Fermi level. The quantum oscillations in this material reveal a nontrivial Berry's phase associated with these Dirac fermions. Our first principles calculations further unveil a topological Dirac cone in its surface states. Therefore, NiTe2 may not only represent an improved system to formulate the theoretical understanding of the exotic consequences of type-II Dirac fermions, it also facilitates possible applications based on these topological carriers.

cond-mat.str-el↗

arXiv2018

Observation of Topological Surface State in High Temperature Superconductor MgB2

The hunt for the benchmark topological superconductor (TSc) has been an extremely active research subject in condensed matter research, with quite a few candidates identified or proposed. However, low transition temperatures (Tc) and/or strong sensitivity to disorder and dopant levels in known TSc candidates have greatly hampered progress in this field. Here, we use Angle-resolved Photoemission Spectroscopy (ARPES) to show the presence of Dirac Nodal Lines (DNLs) and the corresponding topological surface states (TSS's) on the [010] faces of the Tc=39K s-wave BCS superconductor MgB2. Not only is this nearly triple the current record of superconducting Tc among all candidate TSc's, but the nature of these DNL states should make them highly tolerant against disorder and inadvertent doping variations. This makes MgB2 a promising high temperature platform for the study of topological superconductivity.

cond-mat.supr-con↗

arXiv2017

High-pressure growth and characterization of bulk MnAs single crystals

Bulk single crystals of manganese arsenide (MnAs) were grown from melt at 1 GPa and 1100 °C by using a cubic-anvil, high-pressure, and high-temperature technique. The as-grown black colored crystals extracted from solidified lump exhibit a plate-like morphology, with flat surfaces and maximum dimensions up to ~ 3 x 2 x 0.5 mm3. The hexagonal crystal structure at room temperature was confirmed by X-ray diffraction [B81, space group P63/mmc, No 194, Z = 2, a = 3.7173(4) Å, b = 3.7173(4) Å, c = 5.7054(8) Å, and V = 68.277(16) Å3]. Temperature-dependent magnetization measurements reveal the occurrence of a first-order ferro- to paramagnetic transition at Tc = 318.5 K accompanied by a hysteresis of ~ 9 K. The successful growth of relatively large crystals reported here might be extended to various substituted analogues of MnAs, thus opening new possibilities for further exploration of this interesting system.

cond-mat.mtrl-sci↗

arXiv2017

Vacancies, disorder-induced smearing of the electronic structure, and its implications for the superconductivity of anti-perovskite MgC$_{0.93}$Ni$_{2.85}$

The anti-perovskite superconductor MgC$_{0.93}$Ni$_{2.85}$ was studied using high-resolution x-ray Compton scattering combined with electronic structure calculations. Compton scattering measurements were used to determine experimentally a Fermi surface that showed good agreement with that of our supercell calculations, establishing the presence of the predicted hole and electron Fermi surface sheets. Our calculations indicate that the Fermi surface is smeared by the disorder due to the presence of vacancies on the C and Ni sites, but does not drastically change shape. The 20\% reduction in the Fermi level density-of-states would lead to a significant ($\sim 70\%$) suppression of the superconducting $T_c$ for pair-forming electron-phonon coupling. However, we ascribe the observed much smaller $T_c$ reduction at our composition (compared to the stoichiometric compound) to the suppression of pair-breaking spin fluctuations.

cond-mat.supr-con↗

arXiv2017

Growth of bulk single-crystal MnP helimagnet and its structural and NMR characterization

Bulk single crystals of manganese phosphide (MnP) were grown from melt at 1 GPa and 1200 C by using a cubic-anvil, high-pressure, and high-temperature technique. The obtained black colored crystals exhibit a plate-like morphology, with flat surfaces and maximum dimensions up to 4 x 2 x 0.5 mm3. The orthorhombic crystal structure was confirmed by X-ray diffraction [Pnma, 62, Z = 4, a = 5.2510(4) Å, b = 3.1670(3) Å, c = 5.90098 (4) Å and V = 98.279(14) Å3]. Temperature-dependent magnetization measurements reveal the occurrence of two successive transitions: a paramagnetic to ferromagnetic transition at Tc = 290.5 K and the development of a double helimagnetic order at Ts = 44.5 K. Zero-field 31P NMR measurements in the FM and in the screw-spin AFM state show prominent features, which are compared with previous experimental data and theoretical calculations. The relatively large crystals obtained here open up new possibilities for further explorations of this interesting material.

cond-mat.supr-con↗

arXiv2016

High pressure crystal growth of the antiperovskite centrosymmetric superconductor SrPt3P

Bulk single crystals of SrPt3P have been grown for the first time by a self-flux method at 2 GPa and 1500 °C using the cubic-anvil, high-pressure and high-temperature technique. The grown black crystals were found to have either a plate-like or pillar-like morphology with maximum dimensions of ~ 1 - 0.6 - 0.3 mm3. According to a goniometric study the crystals are elongated in the ab-plane with the c-axis perpendicular to the plane. The crystal structure was confirmed by X-ray diffraction (P4/nmm, # 129, Z = 2, a = b = 5.7927(2) Å, c = 5.3729(2) Å and V = 180.290(11) Å3). Temperature dependent susceptibility measurements showed a single-phase behaviour and a superconducting transition temperature of 8.6 K. This value for single crystals is slightly higher than that previously reported for polycrystalline SrPt3P. The sharpness of the susceptibility drop (ΔTc = 0.07 K) supports a high homogeneity of obtained crystals.

cond-mat.supr-con↗

arXiv2016

Non-adiabatic effects in the phonon dispersion of Mg 1--x Al x B 2

Superconducting MgB$\_2$ shows an E$\_{2g}$ zone center phonon, as measured by Raman spectroscopy, that is very broad in energy and temperature dependent. The Raman shift and lifetime show large differences with the values elsewhere in the Brillouin Zone measured by Inelastic X-ray Scattering (IXS), where its dispersion can be accounted for by standard harmonic phonon theory, adding only a moderate electron-phonon coupling. Here we show that the effects rapidly disappear when electron-phonon coupling is switched off by Al substitution on the Mg sites. Moreover, using IXS with very high wave-vector resolution in MgB$\_2$, we can follow the dispersion connecting the Raman and the IXS signal, in agreement with a theory using only electron-phonon coupling but without strong anharmonic terms. The observation is important in order to understand the effects of electron-phonon coupling on zone center phonons modes in MgB$\_2$, but also in all metals characterized by a small Fermi velocity in a particular direction, typical for layered compounds.

cond-mat.supr-con↗

arXiv2014

Quantum oscillations of the superconductor LaRu$_2$P$_2$ : comparable mass enhancement $λ\approx 1$ in Ru and Fe phosphides

We have studied the angular dependent de Haas-van Alphen oscillations of LaRu$_2$P$_2$ using magnetic torque in pulsed magnetic fields up to 60T. The observed oscillation frequencies are in excellent agreement with the geometry of the calculated Fermi surface. The temperature dependence of the oscillation amplitudes reveals effective masses m*($α$)=0.71 and m*($β$)=0.99 m$_e$, which are enhanced over the calculated band mass by $λ^{cyc}$ of 0.8. We find a similar enhancement $λ^γ \approx 1$ in comparing the measured electronic specific heat ($γ= 11.5$ mJ/mol K$^2$) with the total DOS from band structure calculations. Remarkably, very similar mass enhancements have been reported in other pnictides LaFe$_2$P$_2$, LaFePO ($T_c \approx 4K$), and LaRuPO, independent of whether they are superconducting or not. This is contrary to the common perceptions that the normal state quasi-particle renormalizations reflect the strength of the superconducting paring mechanism and leads to new questions about pairing in isostructural and isoelectronic Ru- and Fe-pnictide superconductors.

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