arXiv subjects

Rustem Khasanov

Publications and source records attributed to Rustem Khasanov.

At least 55 records · Page 3Linked to original sources

arXiv2021

Pressure-induced ferromagnetism in the topological semimetal EuCd$_2$As$_2$

The antiferromagnet and semimetal EuCd$_2$As$_2$ has recently attracted a lot of attention due to a wealth of topological phases arising from the interplay of topology and magnetism. In particular, the presence of a single pair of Weyl points is predicted for a ferromagnetic configuration of Eu spins along the $c$-axis in EuCd$_2$As$_2$. In the search for such phases, we investigate here the effects of hydrostatic pressure in EuCd$_2$As$_2$. For that, we present specific heat, transport and $μ$SR measurements under hydrostatic pressure up to $\sim\,2.5\,$GPa, combined with {\it ab initio} density functional theory (DFT) calculations. Experimentally, we establish that the ground state of EuCd$_2$As$_2$ changes from in-plane antiferromagnetic (AFM$_{ab}$) to ferromagnetic at a critical pressure of $\,\approx\,$2\,GPa, which is likely characterized by the moments dominantly lying within the $ab$ plane (FM$_{ab}$). The AFM$_{ab}$-FM$_{ab}$ transition at such a relatively low pressure is supported by our DFT calculations. Furthermore, our experimental and theoretical results indicate that EuCd$_2$As$_2$ moves closer to the sought-for FM$_c$ state (moments $\parallel$ $c$) with increasing pressure further. We predict that a pressure of $\approx$\,23\,GPa will stabilize the FM$_c$ state, if Eu remains in a 2+ valence state. Thus, our work establishes hydrostatic pressure as a key tuning parameter that (i) allows for a continuous tuning between magnetic ground states in a single sample of EuCd$_2$As$_2$ and (ii) enables the exploration of the interplay between magnetism and topology and thereby motivates a series of future experiments on this magnetic Weyl semimetal.

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arXiv2021

Unsplit superconducting and time reversal symmetry breaking transitions in Sr$_2$RuO$_4$ under hydrostatic pressure and disorder

There is considerable evidence that the superconducting state of Sr$_2$RuO$_4$ breaks time reversal symmetry. In the experiments showing time reversal symmetry breaking its onset temperature, $T_\text{TRSB}$, is generally found to match the critical temperature, $T_\text{c}$, within resolution. In combination with evidence for even parity, this result has led to consideration of a $d_{xz} \pm id_{yz}$ order parameter. The degeneracy of the two components of this order parameter is protected by symmetry, yielding $T_\text{TRSB} = T_\text{c}$, but it has a hard-to-explain horizontal line node at $k_z=0$. Therefore, $s \pm id$ and $d \pm ig$ order parameters are also under consideration. These avoid the horizontal line node, but require tuning to obtain $T_\text{TRSB} \approx T_\text{c}$. To obtain evidence distinguishing these two possible scenarios (of symmetry-protected versus accidental degeneracy), we employ zero-field muon spin rotation/relaxation to study pure Sr$_2$RuO$_4$ under hydrostatic pressure, and Sr$_{1.98}$La$_{0.02}$RuO$_4$ at zero pressure. Both hydrostatic pressure and La substitution alter $T_\text{c}$ without lifting the tetragonal lattice symmetry, so if the degeneracy is symmetry-protected $T_\text{TRSB}$ should track changes in $T_\text{c}$, while if it is accidental, these transition temperatures should generally separate. We observe $T_\text{TRSB}$ to track $T_\text{c}$, supporting the hypothesis of $d_{xz} \pm id_{yz}$ order.

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arXiv2020

Formation of short-range magnetic order and avoided ferromagnetic quantum criticality in pressurized LaCrGe$_3$

LaCrGe$_3$ has attracted attention as a paradigm example of the avoidance of ferromagnetic (FM) quantum criticality in an itinerant magnet. By combining thermodynamic, transport, x-ray and neutron scattering as well as $μ$SR measurements, we refined the temperature-pressure phase diagram of LaCrGe$_3$. We provide thermodynamic evidence (i) for the first-order character of the FM transition when it is suppressed to low temperatures and (ii) for the formation of new phases at high pressures. From our microscopic data, we infer that short-range FM ordered clusters exist in these high-pressure phases. These results suggest that LaCrGe$_3$ is a rare example, which fills the gap between the two extreme limits of avoided FM quantum criticality in clean and strongly disordered metals.

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arXiv2020

Anomalous gap ratio in anisotropic superconductors: aluminum under pressure

Pressure dependence of the thermodynamic critical field $B_{\mathrm{c}}$ in elemental aluminum was studied by means of the muon-spin rotation/relaxation technique. Pressure enhances the deviation of $B_{\mathrm{c}}(T)$ from the parabolic behavior, expected for a typical type-I superconductor, thus suggesting the weakening of the gap ratio $\langleα\rangle=\langle Δ\rangle/k_{\mathrm{B} }T_{\mathrm{c}}$ ($\langleΔ\rangle$ is the average value of the superconducting energy gap, $T_{\mathrm{c}}$ is the transition temperature and $k_{\mathrm{B}}$ is the Boltzmann constant). With the pressure increase from 0.0 to $\simeq1.6$ GPa, $\langleα\rangle$ decreases almost linearly from 1.73 to 1.67. Our results imply, therefore, that in elemental aluminum the gap ratio $\langleα\rangle$ is smaller than the weak-coupled BCS prediction $α_{\mathrm{BCS}}\simeq1.764$ and it is even further reduced under pressure.

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arXiv2020

Pressure-induced magnetism in iron-based superconductors $A$Fe$_2$As$_2$ ($A=$ K, Cs, Rb)

The magnetic properties of iron-based superconductors $A$Fe$_2$As$_2$ ($A=$K, Cs, and Rb), which are characterized by the V-shaped dependence of the critical temperature ($T_{\rm c}$) on pressure ($P$) were studied by means of the muon spin rotation/relaxation technique. In all three systems studied the magnetism was found to appear for pressures slightly below the critical one ($P_{\rm c}$), i.e. at pressure where $T_{\rm c}(P)$ changes the slope. Rather than competing, magnetism and superconductivity in $A$Fe$_2$As$_2$ are coexisting at $P\gtrsim P_{\rm c}$ pressure region. Our results support the scenario of a transition from one pairing state to another, with different symmetries on either side of $P_{\rm c}$.

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arXiv2020

Two-gap to Single-gap Superconducting Transition on a Honeycomb Lattice in Ca$_{1-x}$Sr$_{x}$AlSi

It is a well-established fact that the physical properties of compounds follow their crystal symmetries. This has especially pronounced implications on emergent collective quantum states in materials. Specifically, the effect of crystal symmetries on the properties of superconductors is widely appreciated, although the clarification of this relationship is a core effort of on-going research. Emergent phenomena on honeycomb lattices are of special interest, as they can give rise to spectacular phenomenology, as manifested by the recent discovery of correlated states in magic-angle graphene, or by the high-temperature superconductivity in MgB$_2$. Here, we report on the structural and microscopic superconducting properties of a class of ternary superconductors with Al/Si honeycomb layers, i.e. Ca$_{1-x}$Sr$_{x}$AlSi. We show that this solid solution is a remarkable model system with a highly tunable two-gap to single-gap superconducting system on a honeycomb lattice, where the superconductivity is enhanced by a subtle structural instability, i.e. the buckling of the Al/Si layers.

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arXiv2020

Magnetism and its coexistence with superconductivity in CaK(Fe$_{0.949}$Ni$_{0.051}$)$_4$As$_4$: muon spin rotation/relaxation studies

The magnetic response of CaK(Fe$_{0.949}$Ni$_{0.051}$)$_4$As$_4$ was investigated by means of the muon-spin rotation/relaxation. The long-range commensurate magnetic order sets in below the Néel temperature $T_{\rm N}= 50.0(5)$~K. The density-functional theory calculations have identified three possible muon stopping sites. The experimental data were found to be consistent with only one type of magnetic structure, namely, the long-range magnetic spin-vortex-crystal order with the hedgehog motif within the $ab-$plane and the antiferromagnetic stacking along the $c-$direction. The value of the ordered magnetic moment at $T\approx3$ K was estimated to be $m_{\rm Fe}=0.38(11)$ $μ_{\rm B}$ ($μ_{\rm B}$ is the Bohr magneton). A microscopic coexistence of magnetic and superconducting phases accompanied by a reduction of the magnetic order parameter below the superconducting transition temperature $T_{\rm c}\simeq 9$ K is observed. Comparison with 11, 122, and 1144 families of Fe-based pnictides points to existence of correlation between the reduction of the magnetic order parameter at $T\rightarrow 0$ and the ratio of the transition temperatures $T_{\rm c}/T_{\rm N}$. Such correlations were found to be described by Machida's model for coexistence of itinerant spin-density wave magnetism and superconductivity [Machida, J. Phys. Soc. Jpn. 50, 2195 (1981) and Bud'ko et al., Phys. Rev. B 98, 144520 (2018)].

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arXiv2020

The single- vs. two-gap scenario: the specific heat and the thermodynamic critical field of BeAu superconductor

The puzzling situation where some thermodynamic quantities require a single-gap description, while others need a more complex gap scenario, is discussed. Our approach reveals that in some cases, the conclusions based on measurements of only one thermodynamic quantity may lead to conflicting results. As an example, temperature evolutions of the electronic specific heat [$C_{\rm e}(T)$] and the thermodynamic critical field [$B_{\rm c}(T)$] of BeAu superconductor were reconsidered within the framework of the single-gap and the two-gap scenarios. The analysis shows that the single-gap approach describes the temperature dependencies of $C_{\rm e}(T)$ and the superfluid density $ρ_{\rm s}(T)$ satisfactorily. However it fails in the case of $B_{\rm c}(T)$. The self-consistent two-gap model, in contrast, is able to describe all of the mentioned thermodynamic quantities by using a similar set of parameters. Our results reveal that a proper description of the thermodynamic quantities, related to the superconducting pairing mechanism, requires the use of similar model containing the same set of parameters.

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arXiv2020

Multiple-gap response of type-I noncentrosymmetric BeAu superconductor

Precise measurements of the thermodynamic critical field ($B_{\rm c}$) in type-I noncentrosymmetric superconductor BeAu were performed by means of the muon-spin rotation/relaxation technique. The temperature evolution of $B_{\rm c}$ can not be described within the single gap scenario and it requires the presence of at least two different types of the superconducting order parameters. The self-consistent two-gap approach, adapted for analysis of $B_{\rm c}(T)$ behavior, suggests the presence of two superconducing energy gaps with the gap to $T_{\rm c}$ ratios $2Δ/k_{\rm B}T_{\rm c}\simeq4.52$ and $\simeq2.37$ for the big and the small gap, respectively. This implies that the superconductivity in BeAu is unconventional and that the supercarrier pairing occurs at various energy bands.

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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 $T_{\rm c}$.

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arXiv2019

Robust block magnetism in the spin ladder compound BaFe$_2$Se$_3$ under hydrostatic pressure

The majority of the iron-based superconductors (FeSCs) exhibit a two-dimensional square lattice structure. Recent reports of pressure-induced superconductivity in the spin-ladder system, BaFe$_2$X$_3$ (X =S,Se), introduce a quasi-one-dimensional prototype and an insulating parent compound to the FeSCs. Here we report X-ray, neutron diffraction and muon spin relaxation experiments on BaFe$_2$Se$_3$ under hydrostatic pressure to investigate its magnetic and structural properties across the pressure-temperature phase diagram. A structural phase transition was identified at a pressure of 3.7(3) GPa. Neutron diffraction measurements at 6.8(3) GPa and 120 K show that the block magnetism persists even at these high pressures. A steady increase and then fast drop of the magnetic transition temperature $T\rm_N$ and greatly reduced moment above the pressure $P_s$ indicate potentially rich and competing phases close to the superconducting phase in this ladder system.

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arXiv2019

Structural phases of elemental Gallium: universal relations in type-I superconductors

The temperature dependent measurements of the thermodynamic critical field and the specific heat for the pressure stabilized Ga-II phase of elemental Gallium are presented. The discussion of these and other Ga phases data in the context of elemental and binary phonon-mediated type-I superconductors allowed to establish simple scaling relations between BCS quantities such as $B_{\rm c}(0)/T_{\rm c}\sqrt{γ_{\rm e}}$ and the specific heat jump at $T_{\rm c}$ versus the coupling strength $2Δ/k_{\rm B} T_{\rm c}$ [$Δ$ and $B_{\rm c}(0)$ are the zero-temperature values of the superconducting energy gap and the thermodynamic critical field, respectively, $T_{\rm c}$ is the transition temperature, and $γ_{\rm e}$ is the electronic specific heat]. The scaling relations can be analytically expressed by taking into account strong-coupling corrections to BCS theory. Such correlations can naturally explain the linear relation between $B_{\rm c}(0)$ and $T_{\rm c}$, which holds for type-I superconducting materials.

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arXiv2019

Muon spin rotation study of type-I superconductivity: elemental $β-$Sn

The application of the muon-spin rotation/relaxation ($μ$SR) technique for studying type-I superconductivity is discussed. In the intermediate state, i.e. when a type-I superconducting sample with non-zero demagnetization factor $N$ is separated into normal state and Meissner state (superconducting) domains, the $μ$SR technique allows to determine with very high precision the value of the thermodynamic critical field $B_{\rm c}$, as well as the volume of the sample in the normal and the superconducting state. Due to the microscopic nature of $μ$SR technique, the $B_{\rm c}$ values are determined directly via measurements of the internal field inside the normal state domains. No assumptions or introduction of any type of measurement criteria are needed. Experiments performed on a 'classical' type-I superconductor, a cylindrically shaped $β-$Sn sample, allowed to reconstruct the full $B-T$ phase diagram. The zero-temperature value of the thermodynamic critical field $B_{\rm c}(0)=30.578(6)$ mT and the transition temperature $T_{\rm c}=3.717(3)$ K were determined and found to be in a good agreement with the literature data. An experimentally obtained demagnetization factor is in very good agreement with theoretical calculations of the demagnetization factor of a finite cylinder. The analysis of $B_{\rm c}(T)$ dependence within the framework of the phenomenological $α-$model allow to obtain the value of the superconducting energy gap $Δ=0.59(1)$ meV, of the electronic specific heat $γ_e=1.781(3)$ ${\rm mJ}/{\rm mol}\; {\rm K}^2$ and of the jump in the heat capacity ${ΔC(T_c)}/{γT_{\rm c}}=1.55(2)$.

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arXiv2019

On the superconducting nature of the Bi-II phase of elemental Bismuth

The superconductivity in the Bi-II phase of elemental Bismuth (transition temperature $T_{\rm c}\simeq3.92$ K at pressure $p\simeq 2.80$ GPa) was studied experimentally by means of the muon-spin rotation as well as theoretically by using the Eliashberg theory in combination with Density Functional Theory calculations. Experiments reveal that Bi-II is a type-I superconductor with a zero temperature value of the thermodynamic critical field $B_{\rm c}(0)\simeq31.97$~mT. The Eliashberg theory approach provides a good agreement with the experimental $T_{\rm c}$ and the temperature evolution of $B_{\rm c}$. The estimated value for the retardation (coupling) parameter $k_{\rm B}T_{\rm c}/ω_{\rm ln} \approx 0.07$ ($ω_{\rm ln}$ is the logarithmically averaged phonon frequency) suggests that Bi-II is an intermediately-coupled superconductor.

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arXiv2019

Superconductivity of Bi-III phase of elemental Bismuth: insights from Muon-Spin Rotation and Density Functional Theory

Using muon-spin rotation the pressure-induced superconductivity in the Bi-III phase of elemental Bismuth (transition temperature $T_{\rm c}\simeq7.05$ K) was investigated. The Ginzburg-Landau parameter $κ=λ/ξ=30(6)$ ($λ$ is the magnetic penetration depth, $ξ$ is the coherence length) was estimated which is the highest among single element superconductors. The temperature dependence of the superconducting energy gap [$Δ(T)$] reconstructed from $λ^{-2}(T)$ deviates from the weak-coupled BCS prediction. The coupling strength $2Δ/k_{\rm B}T_{\rm c}\simeq 4.34$ was estimated thus implying that Bi-III stays within the strong coupling regime. The Density Functional Theory calculations suggest that superconductivity in Bi-III could be described within the Eliashberg approach with the characteristic phonon frequency $ω_{\rm ln}\simeq 5.5$ meV. An alternative pairing mechanism to the electron-phonon coupling involves the possibility of Cooper pairing induced by the Fermi surface nesting.

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arXiv2019

Anisotropy induced vortex lattice rearrangement in CaKFe$_4$As$_4$

The magnetic penetration depth anisotropy $γ_λ=λ_{c}/λ_{ab}$ ($λ_{ab}$ and $λ_{c}$ are the in-plane and the out-of-plane components of the magnetic penetration depth) in a CaKFe$_4$As$_4$ single crystal sample (the critical temperature $T_{\rm c}\simeq 35$ K) was studied by means of muon-spin rotation ($μ$SR). $γ_λ$ is almost temperature independent for $T\lesssim 20$ K ($γ_λ\simeq 1.9$) and it reaches $\simeq 3.0$ by approaching $T_{\rm c}$. The change of $γ_λ$ induces the corresponding rearrangement of the flux line lattice (FLL), which is clearly detected via enhanced distortions of the FLL $μ$SR response. Comparison of $γ_λ$ with the anisotropy of the upper critical field ($γ_{H_{\rm c2}}$) studied in Phys. Rev B {\bf 94}, 064501 (2016), reveals that $γ_λ$ is systematically higher than $γ_{H_{\rm c2}}$ at low-temperatures and approaches $γ_{H_{\rm c2}}$ for $T \rightarrow T_{\rm c}$. The anisotropic properties of $λ$ are explained by the multi-gap nature of superconductivity in CaKFe$_4$As$_4$ and are caused by anisotropic contributions of various bands to the in-plane and the out-of-plane components of the superfluid density.

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arXiv2018

Evidence of nodal gap structure in the basal plane of the FeSe superconductor

Identifying the symmetry of the wave function describing the Cooper pairs is pivotal in understanding the origin of high-temperature superconductivity in iron-based superconductors. Despite nearly a decade of intense investigation, the answer to this question remains elusive. Here we use the muon spin rotation/relaxation (muSR) technique to investigate the underlying symmetry of the pairing state of the FeSe superconductor, the basic building block of all iron-chalcogenide superconductors. Contrary to earlier muSR studies on powders and crystals, we show that while the superconducting gap is most probably anisotropic but nodeless along the crystallographic c-axis, it is nodal in the ab-plane, as indicated by the linear increase of the superfluid density at low temperature. We further show that the superconducting properties of FeSe display a less pronounced anisotropy than expected.

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arXiv2018

Magnetic tricritical point and nematicity in FeSe under pressure

Magnetism induced by external pressure ($p$) was studied in a FeSe crystal sample by means of muon-spin rotation. The magnetic transition changes from second-order to first-order for pressures exceeding the critical value $p_{\rm c}\simeq2.4-2.5$ GPa. The magnetic ordering temperature ($T_{\rm N}$) and the value of the magnetic moment per Fe site ($m_{\rm Fe}$) increase continuously with increasing pressure, reaching $T_{\rm N}\simeq50$~K and $m_{\rm Fe}\simeq0.25$ $μ_{\rm B}$ at $p\simeq2.6$ GPa, respectively. No pronounced features at both $T_{\rm N}(p)$ and $m_{\rm Fe}(p)$ are detected at $p\simeq p_{\rm c}$, thus suggesting that the stripe-type magnetic order in FeSe remains unchanged above and below the critical pressure $p_{\rm c}$. A phenomenological model for the $(p,T)$ phase diagram of FeSe reveals that these observations are consistent with a scenario where the nematic transitions of FeSe at low and high pressures are driven by different mechanisms.

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