arXiv subjects

F. F. Balakirev

Publications and source records attributed to F. F. Balakirev.

At least 37 records · Page 2Linked to original sources

arXiv2017

Electronic in-plane symmetry breaking at field-tuned quantum criticality in CeRhIn5

Electronic nematics are exotic states of matter where electronic interactions break a rotational symmetry of the underlying lattice, in analogy to the directional alignment without translational order in nematic liquid crystals. Intriguingly such phases appear in the copper- and iron-based superconductors, and their role in establishing high-temperature superconductivity remains an open question. Nematicity may take an active part, cooperating or competing with superconductivity, or may appear accidentally in such systems. Here we present experimental evidence for a phase of nematic character in the heavy fermion superconductor CeRhIn5. We observe a field-induced breaking of the electronic tetragonal symmetry of in the vicinity of an antiferromagnetic (AFM) quantum phase transition at Hc~50T. This phase appears in out-of-plane fields of H*~28T and is characterized by substantial in-plane resistivity anisotropy. The anisotropy can be aligned by a small in-plane field component, with no apparent connection to the underlying crystal structure. Furthermore no anomalies are observed in the magnetic torque, suggesting the absence of metamagnetic transitions in this field range. These observations are indicative of an electronic nematic character of the high field state in CeRhIn5. The appearance of nematic behavior in a phenotypical heavy fermion superconductor highlights the interrelation of nematicity and unconventional superconductivity, suggesting nematicity to be a commonality in such materials.

cond-mat.str-el↗

arXiv2016

Upper Critical Field and Kondo Effects in Fe(Te0.9Se0.1) Thin Films by Pulsed Field Measurements

The transition temperatures of epitaxial films of Fe(Te0:9Se0:1) are remarkably insensitive to applied magnetic field, leading to predictions of upper critical fields Bc2(T = 0) in excess of 100 T. Using pulsed magnetic fields, we find Bc2(0) to be on the order of 45 T, similar to values in bulk material and still in excess of the paramagnetic limit. The same films show strong magnetoresistance in fields above Bc2(T), consistent with the observed Kondo minimum seen above Tc. Fits to the temperature dependence in the context of the WHH model, using the experimental value of the Maki parameter, require an effective spin-orbit relaxation parameter of order unity. We suggest that Kondo localization plays a similar role to spin-orbit pair breaking in making WHH fits to the data.

cond-mat.str-el↗

arXiv2016

Landau levels and shallow donor states in GaAs/AlGaAs multiple quantum wells at mega-gauss magnetic fields

Landau levels and shallow donor states in multiple GaAs/AlGaAs quantum wells (MQWs) are investigated by means of the cyclotron resonance at mega-gauss magnetic fields. Measurements of magneto-optical transitions were performed in pulsed fields up to 140 T and temperatures from 6 to 300 K. The $14\times14$ \textbf{P}$\cdot$\textbf{p} band model for GaAs is used to interpret free-electron transitions in a magnetic field. Temperature behavior of the observed resonant structure indicates, in addition to the free-electron Landau states, contributions of magneto-donor states in the GaAs wells and possibly in the AlGaAs barriers. The magneto-donor energies are calculated using a variational procedure suitable for high magnetic fields and accounting for conduction band nonparabolicity in GaAs. It is shown that the above states, including their spin splitting, allow one to interpret the observed magneto-optical transitions in MQWs in the middle infrared region. Our experimental and theoretical results at very high magnetic fields are consistent with the picture used previously for GaAs/AlGaAs MQWs at lower magnetic fields.

cond-mat.str-el↗

arXiv2016

Reduction of the low-temperature bulk gap in samarium hexaboride under high magnetic fields

SmB$_6$ exhibits a small (15-20 meV) bandgap at low temperatures due to hybridized $d$ and $f$ electrons, a tiny (3 meV) transport activation energy $(E_{A})$ above 4 K, and surface states accessible to transport below 2 K. We study its magnetoresistance in 60-T pulsed fields between 1.5 K and 4 K. The response of the nearly $T$-independent surface states (which show no Shubnikov-de Haas oscillations) is distinct from that of the activated bulk. $E_{A}$ shrinks by 50% under fields up to 60 T. Data up to 93 T suggest that this trend continues beyond 100 T, in contrast with previous explanations. It rules out emerging theories to explain observed exotic magnetic quantum oscillations.

cond-mat.str-el↗

arXiv2015

Thermodynamic signature of a magnetic-field-driven phase transition within the superconducting state of an underdoped high-temperature superconductor

More than a quarter century after the discovery of the high temperature superconductor (HTS) YBa$_2$Cu$_3$O$_{6+δ}$ (YBCO) studies continue to uncover complexity in its phase diagram. In addition to HTS and the pseudogap there is growing evidence for multiple phases with boundaries which are functions of temperature ($T$), doping (p) and magnetic field. Here we report the low temperature electronic specific heat (C$_{elec}$) of YBCO6.47 (p=0.08) up to a magnetic field (H) of 34.5 teslas (T), a poorly understood region of the underdoped H-$T$-p phase space. We observe two regimes: below a characteristic magnetic field H'$\approx$10 T, C$_{elec}/T$ obeys an expected H$^{1/2}$ behavior, however, near H' there is a sharp inflection followed by a linear-in-H behavior. H' rests deep within the superconducting phase and the linear-in-H behavior is observed in the zero resistance regime. In the limit of zero temperature, C$_{elec}/T$ is proportional to the zero-energy electronic density of states. Thus this inflection is evidence of a magnetic-field-driven quantum phase transition.

cond-mat.supr-con↗

arXiv2015

Tunable excitonic insulator in quantum limit graphite

Half a century ago, Mott noted that tuning the carrier density of a semimetal towards zero produces an insulating state in which electrons and holes form bound pairs. It was later argued that such pairing persists even if a semiconducting gap opens in the underlying band structure, giving rise to what has become known as the strong coupling limit of an `excitonic insulator.' While these `weak' and `strong' coupling extremes were subsequently proposed to be manifestations of the same excitonic state of electronic matter, the predicted continuity of such a phase across a band gap opening has not been realized experimentally in any material. Here we show the quantum limit of graphite, by way of temperature and angle-resolved magnetoresistance measurements, to host such an excitonic insulator phase that evolves continuously between the weak and strong coupling limits. We find that the maximum transition temperature T_EI of the excitonic phase is coincident with a band gap opening in the underlying electronic structure at B_0= 46 +/- 1 T, which is evidenced above T_EI by a thermally broadened inflection point in the magnetoresistance. The overall asymmetry of the observed phase boundary around B_0 closely matches theoretical predictions of a magnetic field-tuned excitonic insulator phase in which the opening of a band gap marks a crossover from predominantly momentum-space pairing to real-space pairing.

cond-mat.mtrl-sci↗

arXiv2015

Fragile charge order in the non-superconducting ground state of the underdoped high temperature superconductors

The normal state in the hole underdoped copper oxide superconductors has proven to be a source of mystery for decades. The measurement of a small Fermi surface by quantum oscillations on suppression of superconductivity by high applied magnetic fields, together with complementary spectroscopic measurements in the hole underdoped copper oxide superconductors, point to a nodal electron pocket from charge order in YBa2Cu3O6+x. Here we report quantum oscillation measurements in the closely related stoichiometric material YBa2Cu4O8, which reveal similar Fermi surface properties to YBa2Cu3O6+x, despite an absence of charge order signatures in the same spectroscopic techniques such as x-ray diffraction that revealed signatures of charge order in YBa2Cu3O6+x. Fermi surface reconstruction in YBa2Cu4O8 is suggested to occur from magnetic field enhancement of charge order that is rendered fragile in zero magnetic fields because of its potential unconventional symmetry, and/or its occurrence as a subsidiary to more robust underlying electronic correlations.

cond-mat.supr-con↗

arXiv2015

Normal-state nodal electronic structure in underdoped high-Tc copper oxides

An outstanding problem in the field of high-transition-temperature (high Tc) superconductivity is the identification of the normal state out of which superconductivity emerges in the mysterious underdoped regime. The normal state uncomplicated by thermal fluctuations is effectively accessed by the use of applied magnetic fields sufficiently strong to suppress long-range superconductivity at low temperatures. Proposals in which the normal ground state is characterised by small Fermi surface pockets that exist in the absence of symmetry breaking have been superseded by models based on the existence of a superlattice that breaks the translational symmetry of the underlying lattice. Recently, a charge superlattice model that positions a small electron-like Fermi pocket in the vicinity of the nodes (where the superconducting gap is minimum) has been proposed a replacement for the prevalent superlattice models that position the Fermi pocket in the vicinity of the pseudogap at the antinodes (where the superconducting gap is maximum). Although some ingredients of symmetry breaking have been recently revealed by crystallographic studies, their relevance to the electronic structure remains unresolved. Here we report angle-resolved quantum oscillation measurements in the underdoped copper oxide YBa2Cu3O6+x. These measurements reveal a normal ground state comprising electron-like Fermi surface pockets located in the vicinity of the superconducting gap minima (or nodes), and further point to an underlying superlattice structure of low frequency and long wavelength with features in common with the charge order identified recently by complementary spectroscopic techniques.

cond-mat.supr-con↗

arXiv2015

Anisotropy Reversal of the Upper Critical Field at Low Temperatures and Spin-Locked Superconductivity in K2Cr3As3

We report the first measurements of the anisotropic upper critical field $H_{c2}(T)$ for K$_{2}$Cr$_{3}$As$_{3}$ single crystals up to 60 T and $T > 0.6$ K. Our results show that the upper critical field parallel to the Cr chains, $H_{c2}^\parallel (T)$, exhibits a paramagnetically-limited behavior, whereas the shape of the $H_{c2}^\perp (T)$ curve (perpendicular to the Cr chains) has no evidence of paramagnetic effects. As a result, the curves $H_{c2}^\perp (T)$ and $H_{c2}^\parallel(T)$ cross at $T\approx 4$ K, so that the anisotropy parameter $γ_H(T)=H_{c2}^\perp/H_{c2}^\parallel (T)$ increases from $γ_H(T_c)\approx 0.35$ near $T_c$ to $γ_H(0)\approx 1.7$ at 0.6 K. This behavior of $H_{c2}^\|(T)$ is inconsistent with triplet superconductivity but suggests a form of singlet superconductivity with the electron spins locked onto the direction of Cr chains.

cond-mat.supr-con↗

arXiv2012

Robust magnetic order of Ce 4f-electrons coexisting with superconductivity in CeFeAsO$_{1-x}$F$_x$

The electrical resistance of CeFeAsO$_{1-x}$F$_x$ (x = 0.06 and 0.08) has been measured in a magnetic field up to 40T. At zero field, the sample with x = 0.06 shows a structural phase transition around T$_S$~100K, followed by a spin-density-wave (SDW) transition around T$_{SDW}$~30K. For x = 0.08, the structural phase transition is suppressed down to T$_S$~60K without a clear anomaly associated with the Fe-SDW transition, and superconductivity shows up at T$_C$~25K. At lower temperatures, both samples show a clear resistive peak around T$_N$~4K, which is associated with the antiferromagnetic (AFM) transition of Ce-4f electrons. Strikingly, zero resistance is recovered upon further lowering temperature below T$_N$ for x = 0.08. Moreover, we found that the AFM transition of Ce 4f-electrons at 4K hardly changes with applying a magnetic field up to 40T, even in the case of x = 0.08, where superconductivity has been partially suppressed at such a large field.

cond-mat.supr-con↗

arXiv2012

Upper critical field and thermally activated flux flow in single crystalline Tl$_{0.58}$Rb$_{0.42}$Fe$_{1.72}$Se$_2$

The upper critical field $μ_0H_{c2}(T_c)$ of Tl$_{0.58}$Rb$_{0.42}$Fe$_{1.72}$Se$_2$ single crystals has been determined by means of measuring the electrical resistivity in both a pulsed magnetic field ($\sim$60T) and a DC magnetic field ($\sim$14T). It is found that $H_{c2}$ linearly increases with decreasing temperature for $\textbf{H}$$\parallel$$c$, reaching $μ_0H_{c2}^{\textbf{H}\parallel c}(0\textrm{K})\simeq60$ T. On the other hand, a larger $μ_0H_{c2}(0\textrm{K})$ with a strong convex curvature is observed for $\textbf{H}$$\perp$$c$ ($μ_0H_{c2}^{\textbf{H}\perp c}$(18K)$\simeq$60T). This compound shows a moderate anisotropy of the upper critical field around $T_c$, but decreases with decreasing temperature. Analysis of the upper critical field based on the Werthamer-Helfand-Hohenberg (WHH) method indicates that $μ_0H_{c2}(0\textrm{K})$ is orbitally limited for $\textbf{H}$$\parallel$$c$, but the effect of spin paramagnetism may play an important role on the pair breaking for $\textbf{H}$$\perp$$c$. All these experimental observations remarkably resemble those of the iron pnictide superconductors, suggesting a unified scenario for the iron-based superconductors. Moreover, the superconducting transition is significantly broadened upon applying a magnetic field, indicating strong thermal fluctuation effects in the superconducting state of Tl$_{0.58}$Rb$_{0.42}$Fe$_{1.72}$Se$_2$. The derived thermal activation energy for vortex motion is compatible with those of the 1111-type iron pnictides.

cond-mat.supr-con↗

arXiv2011

Direct observation of multiple spin zeroes in the underdoped high temperature superconductor YBa2Cu3O6+x

We report the direct observation of multiple `spin zeroes' in angle-dependent magnetic quantum oscillations measured up to 85T in YBa2Cu3O6+x, at which the amplitude falls to a deep minimum accompanied by a phase inversion of the measured quantum oscillations, enabling the product of the effective mass and effective g-factor m*g* to be tightly constrained. We find an evolution of the location of the spin zeros with applied magnetic field, and suggest that this effect and the absence of a spin zero at low angles can be produced by more than one Fermi surface component, and an effective g-factor with a subtle anisotropy between in-plane and out-of-plane crystalline directions.

cond-mat.supr-con↗

arXiv2011

Localized and itinerant dichotomy of electrons in Ba(Fe,Co)2As2

Variant approaches, either based on the Fermi surface nesting or started from the proximity to a Mott-insulator, were proposed to elucidate the physics in iron pnictides, but no consensus has been reached. A fundamental problem concerns the nature of their 3d electrons. Here we report the magnetoresistivity (ρ_xx) and the Hall resistivity (ρ_xy) of Ba(Fe1-xCox)2As2 (x=0 and 0.05) in a magnetic field of up to 55T. The magnetic transition is extremely robust against magnetic field, giving strong evidence that the magnetic ordering is formed by local moments. The magnetic state is featured with a huge magnetoresistance and a distinguished Hall resistivity, ρ_xy(H), which shows a pronounced parabolic field dependence, while the paramagnetic state shows little magnetoresistance and follows a simple linear magnetic field dependence on the Hall resistivity. Analyses of our data, based on a two-carrier model, demonstrate that the electron carriers in the magnetic state rapidly increase upon applying a magnetic field, partially compensating the loss of electron carriers at T_M. We argue that the 3d-electrons in Ba(Fe1-xCox)2As2 are divided into those who are close to forming localized moments controlling the magnetic transition and the others giving rise to complex transport properties through their interaction with the former.

cond-mat.supr-con↗

arXiv2011

{Rearrangement of the antiferromagnetic ordering at high magnetic fields in SmFeAsO and SmFeAsO$_{0.9}$F$_{0.1}$ single crystals

The low-temperature antiferromagnetic state of the Sm-ions in both nonsuperconducting SmFeAsO and superconducting SmFeAsO$_{0.9}$F$_{0.1}$ single crystals was studied by magnetic torque, magnetization, and magnetoresistance measurements in magnetic fields up to 60~T and temperatures down to 0.6~K. We uncover in both compounds a distinct rearrangement of the antiferromagnetically ordered Sm-moments near $35-40$~T. This is seen in both, static and pulsed magnetic fields, as a sharp change in the sign of the magnetic torque, which is sensitive to the magnetic anisotropy and hence to the magnetic moment in the $ab$-plane, ({\it i.e.} the FeAs-layers), and as a jump in the magnetization for magnetic fields perpendicular to the conducting planes. This rearrangement of magnetic ordering in $35-40$~T is essentially temperature independent and points towards a canted or a partially polarized magnetic state in high magnetic fields. However, the observed value for the saturation moment above this rearrangement, suggests that the complete suppression of the antiferromagnetism related to the Sm-moments would require fields in excess of 60~T. Such a large field value is particularly remarkable when compared to the relatively small Néel temperature $T_{\rm N}\simeq5$~K, suggesting very anisotropic magnetic exchange couplings. At the transition, magnetoresistivity measurements show a crossover from positive to negative field-dependence, indicating that the charge carriers in the FeAs planes are sensitive to the magnetic configuration of the rare-earth elements. This is indicates a finite magnetic/electronic coupling between the SmO and the FeAs layers which are likely to mediate the exchange interactions leading to the long range antiferromagnetic order of the Sm ions.

cond-mat.supr-con↗

arXiv2011

Upper critical field of the 122-type iron pnictide superconductors

The upper critical fields ($H_{c2}$) of the single crystals $\rm(Sr,Na)Fe_2As_2$ and $\rm Ba_{0.55}K_{0.45}Fe_2As_2$ were determined by means of measuring the electrical resistivity, $ ρ_{xx}(μ_0H)$, using the facilities of pulsed magnetic field at Los Alamos. In general, these compounds possess a very large upper critical field ($H_{c2}(0)$) with a weak anisotropic effect. The detailed curvature of $H_{c2}(T_c)$ may depend on the magnetic field orientation and the sample compositions. We argue that such a difference mainly results from the multi-band effect, which might be modified via doping.

cond-mat.supr-con↗

arXiv2011

The magnetoresistance and Hall effect in CeFeAsO: a high magnetic field study

The longitudinal electrical resistivity and the transverse Hall resistivity of CeFeAsO are simultaneously measured up to a magnetic field of 45T using the facilities of pulsed magnetic field at Los Alamos. Distinct behaviour is observed in both the magnetoresistance Rxx(μ0H) and the Hall resistance Rxy(μ0H) while crossing the structural phase transition at Ts \approx 150K. At temperatures above Ts, little magnetoresistance is observed and the Hall resistivity follows linear field dependence. Upon cooling down the system below Ts, large magnetoresistance develops and the Hall resistivity deviates from the linear field dependence. Furthermore, we found that the transition at Ts is extremely robust against the external magnetic field. We argue that the magnetic state in CeFeAsO is unlikely a conventional type of spin-density-wave (SDW).

cond-mat.supr-con↗

arXiv2011

The upper critical field and its anisotropy in LiFeAs

The upper critical field $μ_0H_{c2}(T_c)$ of LiFeAs single crystals has been determined by measuring the electrical resistivity using the facilities of pulsed magnetic field at Los Alamos. We found that $μ_0H_{c2}(T_c)$ of LiFeAs shows a moderate anisotropy among the layered iron-based superconductors; its anisotropic parameter $γ$ monotonically decreases with decreasing temperature and approaches $γ\simeq 1.5$ as $T\rightarrow 0$. The upper critical field reaches 15T ($H\parallel c$) and 24.2T ($H\parallel ab$) at $T=$1.4K, which value is much smaller than other iron-based high $T_c$ superconductors. The temperature dependence of $μ_0H_{c2}(T_c)$ can be described by the Werthamer-Helfand-Hohenberg (WHH) method, showing orbitally and (likely) spin-paramagnetically limited upper critical field for $H\parallel c$ and $H\parallel ab$, respectively.

cond-mat.supr-con↗

arXiv2010

Nearly isotropic upper critical fields in a SrFe$_{1.85}$Co$_{0.15}$As$_{2}$ single crystal

We study temperature dependent upper critical field $H_{\rm c2}$ of a SrFe$_{1.85}$Co$_{0.15}$As$_{2}$ single crystal (\textit{T$_c$}=20.2 K) along \textit{ab}-plane and \textit{c}-axis through resistivity measurements up to 50 T. For the both crystalline directions, $H_{\rm c2}$ becomes nearly isotropic at zero temperature limit, reaching $\sim$ 48 T. The temperature dependence of the $H_{\rm c2}$ curves is explained by interplay between orbital and Pauli limiting behaviors combined with the two band effects.

cond-mat.supr-con↗