arXiv subjects

S. L. Bud'ko

Publications and source records attributed to S. L. Bud'ko.

At least 73 records · Page 4Linked to original sources

arXiv2017

Influence of the multiband sign changing superconductivity on the vortex cores and on vortex pinning in the new stoichiometric high Tc CaKFe4As4

We study the superconducting density of states and vortex lattice of single crystals of CaKFe$_4$As$_4$ using a scanning tunneling microscope (STM). This material has a critical temperature of $T_c= 35\,$ K, which is one of the highest among stoichiometric iron based superconductors (FeBSC) and is comparable to $T_c$ found near optimal doping in other FeBSC. Using quasi-particle interference we identify the hole sheets around the zone center and find that two superconducting gaps open in these sheets. The scattering centers are small defects that can be localized in the surface topography and just produce quasiparticle interference, without suppressing the superconducting order parameter. This shows that sign inversion is not within hole bands, but between hole and the electron bands. Vortex core bound states show electron-hole asymmetric bound states due to proximity of the top of one of the hole bands to the Fermi level $E_F$. This places CaKFe$_4$As$_4$ in a similar situation as FeSe or related materials, with a superconducting gap $Δ$ just a few times smaller than $E_F$. On the other hand, we also identify locations showing strong suppression of the superconducting order parameter. Their size is of order of the vortex core size and vortices are pinned at these locations, leading to a disordered vortex lattice.

cond-mat.supr-con↗

arXiv2017

NMR evidence for static local nematicity and its cooperative interplay with low-energy magnetic fluctuations in FeSe under pressure

We present $^{77}$Se-NMR measurements on single-crystalline FeSe under pressures up to 2 GPa. Based on the observation of the splitting and broadening of the NMR spectrum due to structural twin domains, we discovered that static, local nematic ordering exists well above the bulk nematic ordering temperature, $T_{\rm s}$. The static, local nematic order and the low-energy stripe-type antiferromagnetic spin fluctuations, as revealed by NMR spin-lattice relaxation rate measurements, are both insensitive to pressure application. These NMR results provide clear evidence for the microscopic cooperation between magnetism and local nematicity in FeSe.

cond-mat.str-el↗

arXiv2017

Local nematic susceptibility in stressed BaFe$_2$As$_2$ from NMR electric field gradient measurements

The electric field gradient (EFG) tensor at the $^{75}$As site couples to the orbital occupations of the As p-orbitals and is a sensitive probe of local nematicity in BaFe$_2$As$_2$. We use nuclear magnetic resonance to measure the nuclear quadrupolar splittings and find that the EFG asymmetry responds linearly to the presence of a strain field in the paramagnetic phase. We extract the nematic susceptibility from the slope of this linear response as a function of temperature and find that it diverges near the structural transition in agreement with other measures of the bulk nematic susceptibility. Our work establishes an alternative method to extract the nematic susceptibility which, in contrast to transport methods, can be extended inside the superconducting state.

cond-mat.str-el↗

arXiv2017

Magnetic fluctuations and superconducting properties of CaKFe4As4 studied by 75As NMR

We report $^{75}$As nuclear magnetic resonance (NMR) studies on a new iron-based superconductor CaKFe$_4$As$_4$ with $T_{\rm c}$ = 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 $ν_{\rm Q}$ of the As(1) and As(2) sites show an opposite temperature ($T$) dependence. Nearly $T$ independent behavior of the Knight shifts $K$ are observed in the normal state, and a sudden decrease in $K$ in the superconducting (SC) state clearly evidences 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 wavevector ${\bf q}$ = ($π$, 0) or (0, $π$) in the single-iron Brillouin zone notation are revealed by 1/$T_1T$ 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. In the SC state, 1/$T_1$ shows a rapid decrease below $T_{\rm c}$ without a Hebel-Slichter peak and decreases exponentially at low $T$, consistent with an $s^{\pm}$ nodeless two-gap superconductor.

cond-mat.supr-con↗

arXiv2017

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 (NV) 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, Pb and Nb, and unconventional, LuNi$_2$B$_2$C, Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$, Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$, and CaKFe$_4$As$_4$, superconductors, with diffraction - limited spatial resolution using variable - temperature confocal system. Magnetic induction profiles across the crystal edges were measured in zero-field-cooled (ZFC) and field-cooled (FC) 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 (PME) 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. Our measurements of the spatial distribution of magnetic induction provide insight into microscopic physics of the Meissner effect.

cond-mat.supr-con↗

arXiv2017

Hedgehog spin-vortex crystal stabilized in a hole-doped iron-based superconductor

Magnetism is widely considered to be a key ingredient of unconventional superconductivity. In contrast to cuprate high-temperature superconductors, antiferromagnetism in Fe-based superconductors (FeSCs) is characterized by a pair of magnetic propagation vectors. Consequently, three different types of magnetic order are possible. Of theses, only stripe-type spin-density wave (SSDW) and spin-charge-density wave (SCDW) orders have been observed. A realization of the proposed spin-vortex crystal (SVC) order is noticeably absent. We report a magnetic phase consistent with the hedgehog variation of SVC order in Ni- and Co-doped CaKFe4As4 based on thermodynamic, transport, structural and local magnetic probes combined with symmetry analysis. The exotic SVC phase is stabilized by the reduced symmetry of the CaKFe4As4 structure. Our results suggest that the possible magnetic ground states in FeSCs have very similar energies, providing an enlarged configuration space for magnetic fluctuations to promote high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2017

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 multi-component 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 inter-grown 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$_{4}$. This optimization process offers insight into the growth of quaternary compounds and a glimpse of the four-component phase diagram in the pseudo-ternary FeAs-CaFe$_{2}$As$_{2}$-KFe$_{2}$As$_{2}$ system.

cond-mat.mtrl-sci↗

arXiv2017

Electronic structure of RSb (R = Y, Ce, Gd, Dy, Ho, Tm, Lu) studied by angle-resolved photoemission spectroscopy

We use high resolution angle-resolved photoemission spectroscopy (ARPES) and electronic structure calculations to study the electronic properties of rare-earth monoantimonides RSb (R = Y, Ce, Gd, Dy, Ho, Tm, Lu). The experimentally measured Fermi surface (FS) of RSb consists of at least two concentric hole pockets at the $Γ$ point and two intersecting electron pockets at the $X$ point. These data agree relatively well with the electronic structure calculations. Detailed photon energy dependence measurements using both synchrotron and laser ARPES systems indicate that there is at least one Fermi surface sheet with strong three-dimensionality centered at the $Γ$ point. Due to the "lanthanide contraction", the unit cell of different rare-earth monoantimonides shrinks when changing rare-earth ion from CeSb to LuSb. This results in the differences in the chemical potentials in these compounds, which is demonstrated by both ARPES measurements and electronic structure calculations. Interestingly, in CeSb, the intersecting electron pockets at the $X$ point seem to be touching the valence bands, forming a four-fold degenerate Dirac-like feature. On the other hand, the remaining rare-earth monoantimonides show significant gaps between the upper and lower bands at the $X$ point. Furthermore, similar to the previously reported results of LaBi, a Dirac-like structure was observed at the $Γ$ point in YSb, CeSb, and GdSb, compounds showing relatively high magnetoresistance. This Dirac-like structure may contribute to the unusually large magnetoresistance in these compounds.

cond-mat.mtrl-sci↗

arXiv2017

Uniaxial strain control of spin-polarization in multicomponent nematic order of BaFe$_{2}$As$_{2}$

The iron-based high temperature superconductors exhibit a rich phase diagram reflecting a complex interplay between spin, lattice, and orbital degrees of freedom [1-4]. The nematic state observed in many of these compounds epitomizes this complexity, by entangling a real-space anisotropy in the spin fluctuation spectrum with ferro-orbital order and an orthorhombic lattice distortion [5-7]. A more subtle and much less explored facet of the interplay between these degrees of freedom arises from the sizable spin-orbit coupling present in these systems, which translates anisotropies in real space into anisotropies in spin space. Here, we present a new technique enabling nuclear magnetic resonance under precise tunable strain control, which reveals that upon application of a tetragonal symmetry-breaking strain field, the magnetic fluctuation spectrum in the paramagnetic phase of BaFe$_{2}$As$_{2}$ also acquires an anisotropic response in spin-space. Our results unveil a hitherto uncharted internal spin structure of the nematic order parameter, indicating that similar to liquid crystals, electronic nematic materials may offer a novel route to magneto-mechanical control.

cond-mat.str-el↗

arXiv2017

Optical properties of $A$Fe$_\mathbf{2}$As$_\mathbf{2}$ ($A=\,$Ca, Sr, and Ba) single crystals

The detailed optical properties have been determined for the iron-based materials $A$Fe$_2$As$_2$, where $A=\,$Ca, Sr, and Ba, for light polarized in the iron-arsenic ($a-b$) planes over a wide frequency range, above and below the magnetic and structural transitions at $T_N =$ 172, 195, and 138 K, respectively. The real and imaginary parts of the complex conductivity are fit simultaneously using two Drude terms in combination with a series of oscillators. Above $T_N$, the free-carrier response consists of a weak, narrow Drude term, and a strong, broad Drude term, both of which show only a weak temperature dependence. Below $T_N$ there is a slight decrease of the plasma frequency but a dramatic drop in the scattering rate for the narrow Drude term, and for the broad Drude term there is a significant decrease in the plasma frequency, while the decrease in the scattering rate, albeit significant, is not as severe. The small values observed for the scattering rates for the narrow Drude term for $T\ll{T_N}$ may be related to the Dirac cone-like dispersion of the electronic bands. Below $T_N$ new features emerge in the optical conductivity that are associated with the reconstruction Fermi surface and the gapping of bands at $Δ_1 \simeq$ 45 $-$ 80 meV, and $Δ_2 \simeq$ 110 $-$ 210 meV. The reduction in the spectral weight associated with the free carriers is captured by the gap structure, specifically, the spectral weight from the narrow Drude term appears to be transferred into the low-energy gap feature, while the missing weight from the broad term shifts to the high-energy gap.

cond-mat.str-el↗

arXiv2017

Three-dimensionality of the bulk electronic structure in WTe2

We use temperature- and field-dependent resistivity measurements [Shubnikov--de Haas (SdH) quantum oscillations] and ultrahigh resolution, tunable, vacuum ultraviolet (VUV) laser-based angle-resolved photoemission spectroscopy (ARPES) to study the three-dimensionality (3D) of the bulk electronic structure in WTe2, a type-II Weyl semimetal. The bulk Fermi surface (FS) consists of two pairs of electron pockets and two pairs of hole pockets along the X-Gamma-X direction as detected by using an incident photon energy of 6.7 eV, which is consistent with the previously reported data. However, if using an incident photon energy of 6.36 eV, another pair of tiny electron pockets is detected on both sides of the Gamma point, which is in agreement with the small quantum oscillation frequency peak observed in the magnetoresistance. Therefore, the bulk, 3D FS consists of three pairs of electron pockets and two pairs of hole pockets in total. With the ability of fine tuning the incident photon energy, we demonstrate the strong three-dimensionality of the bulk electronic structure in WTe2. The combination of resistivity and ARPES measurements reveal the complete, and consistent, picture of the bulk electronic structure of this material.

cond-mat.mtrl-sci↗

arXiv2016

Effect of biaxial strain on the phase transitions of Ca(Fe1-xCox)2As2

We study the effect of applied strain as a physical control parameter for the phase transitions of Ca(Fe1-xCox)2As2 using resistivity, magnetization, x-ray diffraction and 57Fe Mössbauer spectroscopy. Biaxial strain, namely compression of the basal plane of the tetragonal unit cell, is created through firm bonding of samples to a rigid substrate, via differential thermal expansion. This strain is shown to induce a magneto-structural phase transition in originally paramagnetic samples; and superconductivity in previously non-superconducting ones. The magneto-structural transition is gradual as a consequence of using strain instead of pressure or stress as a tuning parameter.

cond-mat.supr-con↗

arXiv2016

A de Haas van Alphen study of the role of 4f electrons in antiferromagnetic CeZn11 as compared to its non-magnetic analogue LaZn11

We present a de Haas-van Alphen study of the Fermi surface of the low temperature antiferromagnet CeZn11 and its non-magnetic analogue LaZn11, measured by torque magnetometry up to fields of 33T and at temperatures down to 320 mK. Both systems possess similar de Haas-van Alphen frequencies, with three clear sets of features - ranging from 50T to 4kT - corresponding to three bands of a complex Fermi surface, with an expected fourth band also seen weakly in CeZn11. The effective masses of the charge carriers are very light (<1 me) in LaZn11 but a factor of 2 - 4 larger in CeZn11, indicative of stronger electronic correlations. We perform detailed density functional theory (DFT) calculations for CeZn11 and find that only DFT+U calculations with U=1.5 eV, which localize the 4f states, provide a good match to the measured de Haas-van Alphen frequencies, once the presence of magnetic breakdown orbits is also considered. Our study suggests that the Fermi surface of CeZn11 is very close to that of LaZn11 being dominated by Zn 3d, as the Ce 4f states are localised and have little influence on its electronic structure, however, they are responsible for its magnetic order and contribute to enhance electronic correlations.

cond-mat.str-el↗

arXiv2016

Coexistence of antiferromagnetic and ferromagnetic spin correlations in Ca(Fe1-xCox)2As2 revealed by 75As nuclear magnetic resonance

Recent nuclear magnetic resonance (NMR) measurements revealed the coexistence of stripe-type antiferromagnetic (AFM) and ferromagnetic (FM) spin correlations in both the hole- and electron-doped BaFe$_2$As$_2$ families of iron-pnictide superconductors by a Korringa ratio analysis. Motivated by the NMR work, we investigate the possible existence of FM fluctuations in another iron pnictide superconducting family, Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$. We re-analyzed our previously reported data in terms of the Korringa ratio and found clear evidence for the coexistence of stripe-type AFM and FM spin correlations in the electron-doped CaFe$_2$As$_2$ system. These NMR data indicate that FM fluctuations exist in general in iron-pnictide superconducting families and thus must be included to capture the phenomenology of the iron pnictides.

cond-mat.supr-con↗

arXiv2016

NMR study of nematic spin fluctuations in a detwinned single crystal of underdoped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$

We report the experimental details of how mechanical detwinning can be implemented in tandem with high sensitivity nuclear magnetic resonance measurements and use this setup to measure the in-plane anisotropy of the spin-lattice relaxation rate in underdoped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ with $x=0.048$. The anisotropy reaches a maximum of 30\% at $T_{N}$, and the recovery data reveal that the glassy behavior of the spin fluctuations present in the twinned state persist in the fully detwinned crystal. A theoretical model is presented to describe the spin-lattice relaxation rate in terms of anisotropic nematic spin fluctuations.

cond-mat.str-el↗

arXiv2016

Asymmetric mass acquisition in LaBi - a new topological semimetal candidate

We use our high resolution He-lamp based, tunable laser-based ARPES measurements and density functional theory calculations to study the electronic properties of LaBi, a binary system that was proposed to be a member of a new family of topological semimetals. Both bulk and surface bands are present in the spectra. The dispersion of the surface state is highly unusual. It resembles a Dirac cone, but upon closer inspection we can clearly detect an energy gap. The bottom band follows roughly a parabolic dispersion. The dispersion of the top band remains very linear, "V" shape like, with the tip approaching very closely to the extrapolated location of Dirac point. Such asymmetric mass acquisition is highly unusual and opens a possibility of a new topological phenomena that has yet to be understood.

cond-mat.mtrl-sci↗

arXiv2016

Super-heavy fermion material as metallic refrigerant for adiabatic demagnetization cooling

Low-temperature refrigeration is of crucial importance in fundamental research of condensed matter physics, as the investigations of fascinating quantum phenomena, such as superconductivity, superfluidity and quantum criticality, often require refrigeration down to very low temperatures. Currently, cryogenic refrigerators with $^3$He gas are widely used for cooling below 1 Kelvin. However, usage of the gas is being increasingly difficult due to the current world-wide shortage. Therefore, it is important to consider alternative methods of refrigeration. Here, we show that a new type of refrigerant, super-heavy electron metal, YbCo$_2$Zn$_{20}$, can be used for adiabatic demagnetization refrigeration, which does not require 3He gas. A number of advantages includes much better metallic thermal conductivity compared to the conventional insulating refrigerants. We also demonstrate that the cooling performance is optimized in Yb$_{1-x}$Sc$_x$Co$_2$Zn$_{20}$ by partial Sc substitution with $x\sim$0.19. The substitution induces chemical pressure which drives the materials close to a zero-field quantum critical point. This leads to an additional enhancement of the magnetocaloric effect in low fields and low temperatures enabling final temperatures well below 100 mK. Such performance has up to now been restricted to insulators. Since nearly a century the same principle of using local magnetic moments has been applied for adiabatic demagnetization cooling. This study opens new possibilities of using itinerant magnetic moments for the cryogen-free refrigeration.

cond-mat.str-el↗

arXiv2016

Anisotropic thermodynamic and transport properties of single crystalline CaKFe$_{4}$As$_{4}$

Single crystalline, single phase CaKFe$_{4}$As$_{4}$ has been grown out of a high temperature, quaternary melt. Temperature dependent measurements of x-ray diffraction, anisotropic electrical resistivity, elastoresistivity, thermoelectric power, Hall effect, magnetization and specific heat, combined with field dependent measurements of electrical resistivity and field and pressure dependent measurements of magnetization indicate that CaKFe$_{4}$As$_{4}$ is an ordered, stoichiometric, Fe-based superconductor with a superconducting critical temperature, $T_c$ = 35.0 $\pm$ 0.2 K. Other than superconductivity, there is no indication of any other phase transition for 1.8 K $\leq T \leq$ 300 K. All of these thermodynamic and transport data reveal striking similarities to that found for optimally- or slightly over-doped (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$, suggesting that stoichiometric CaKFe$_4$As$_4$ is intrinsically close to what is referred to as "optimal-doped" on a generalized, Fe-based superconductor, phase diagram. The anisotropic superconducting upper critical field, $H_{c\text{2}}(T)$, of CaKFe$_{4}$As$_{4}$ was determined up to 630 kOe. The anisotropy parameter $γ(T)=H_{c\text{2}}^{\perp}/H_{c\text{2}}^{\|}$, for $H$ applied perpendicular and parallel to the c-axis, decreases from $\simeq 2.5$ at $T_c$ to $\simeq 1.5$ at 25 K which can be explained by interplay of paramagnetic pairbreaking and orbital effects. The slopes of $dH_{c\text{2}}^{\|}/dT\simeq-44$ kOe/K and $dH_{c\text{2}}^{\perp}/dT \simeq-109$ kOe/K at $T_c$ yield an electron mass anisotropy of $m_{\perp}/m_{\|}\simeq 1/6$ and short Ginzburg-Landau coherence lengths $ξ_{\|}(0)\simeq 5.8 \textÅ$ and $ξ_{\perp}(0)\simeq 14.3 \textÅ$. The value of $H_{c\text{2}}^{\perp}(0)$ can be extrapolated to $\simeq 920$ kOe, well above the BCS paramagnetic limit.

cond-mat.supr-con↗