arXiv subjects

F. Balakirev

Publications and source records attributed to F. Balakirev.

7 recordsLinked to original sources

arXiv2020

Spin-valley locking, bulk quantum Hall effect and chiral surface state in a noncentrosymmetric Dirac semimetal BaMnSb$_2$

Spin-valley locking in the band structure of monolayers of MoS$_2$ and other group-VI dichalcogenides has attracted enormous interest, since it offers potential for valleytronic and optoelectronic applications. Such an exotic electronic state has sparsely been seen in bulk materials. Here, we report spin-valley locking in a bulk Dirac semimetal BaMnSb$_2$. We find valley and spin are inherently coupled for both valence and conduction bands in this material. This is revealed by comprehensive studies using first principle calculations, tight-binding and effective model analyses, angle-resolved photoemission spectroscopy and quantum transport measurements. Moreover, this material also exhibits a stacked quantum Hall effect. The spin-valley degeneracy extracted from the plateau height of quantized Hall resistivity is close to 2. This result, together with the observed Landau level spin splitting, further confirms the spin-valley locking picture. In the extreme quantum limit, we have also observed a two-dimensional chiral metal at the side surface, which represents a novel topological quantum liquid. These findings establish BaMnSb$_2$ as a rare platform for exploring coupled spin and valley physics in bulk single crystals and accessing 3D interacting topological states.

cond-mat.mtrl-sci↗

arXiv2018

Superconductivity at 250 K in lanthanum hydride under high pressures

The discovery of superconductivity at 203 K in H3S brought attention back to conventional superconductors whose properties can be described by the Bardeen-Cooper-Schrieffer (BCS) and the Migdal-Eliashberg theories. These theories predict that high, and even room temperature superconductivity (RTSC) is possible in metals possessing certain favorable parameters such as lattice vibrations at high frequencies. However, these general theories do not suffice to predict real superconductors. New superconducting materials can be predicted now with the aid of first principles calculations based on Density Functional Theory (DFT). In particular, the calculations suggested a new family of hydrides possessing a clathrate structure, where the host atom (Ca, Y, La) is at the center of the cage formed by hydrogen atoms. For LaH10 and YH10 superconductivity, with critical temperatures Tc ranging between 240 and 320 K is predicted at megabar pressures. Here, we report superconductivity with a record Tc ~ 250 K within the Fm-3m structure of LaH10 at a pressure P ~ 170 GPa. We proved the existence of superconductivity at 250 K through the observation of zero-resistance, isotope effect, and the decrease of Tc under an external magnetic field, which suggests an upper critical magnetic field of 120 T at zero-temperature. The pressure dependence of the transition temperatures Tc (P) has a maximum of 250-252 K at the pressure of about 170 GPa. This leap, by ~ 50 K, from the previous Tc record of 203 K indicates the real possibility of achieving RTSC (that is at 273 K) in the near future at high pressures and the perspective of conventional superconductivity at ambient pressure.

cond-mat.supr-con↗

arXiv2018

Quantum limit transport and destruction of the Weyl nodes in TaAs

Weyl fermions are a new ingredient for correlated states of electronic matter. A key difficulty has been that real materials also contain non-Weyl quasiparticles, and disentangling the experimental signatures has proven challenging. We use magnetic fields up to 95 tesla to drive the Weyl semimetal TaAs far into its quantum limit (QL), where only the purely chiral 0th Landau levels (LLs) of the Weyl fermions are occupied. We find the electrical resistivity to be nearly independent of magnetic field up to 50 tesla: unusual for conventional metals but consistent with the chiral anomaly for Weyl fermions. Above 50 tesla we observe a two-order-of-magnitude increase in resistivity, indicating that a gap opens in the chiral LLs. Above 80 tesla we observe strong ultrasonic attenuation below 2 kelvin, suggesting a mesoscopically-textured state of matter. These results point the way to inducing new correlated states of matter in the QL of Weyl semimetals.

cond-mat.str-el↗

arXiv2011

Significant enhancement of upper critical fields by doping and strain in Fe-based superconductors

We report measurements of Hc2(T) up to 85 Tesla on Ba1-xKxAs2Fe2 single crystals and FeSe1-xTex films tuned by doping and strain. We observed an Hc2 enhancement by nearly 25 T at 30 K for the optimally-doped Ba1-xKxAs2Fe2 as compared to the previous results and extraordinarily high slopes dHc2/dT = 250-500 T/K near Tc in FeSe1-xTex indicating an almost complete suppression of the orbital pair-breaking. Theoretical analysis of Hc2(T) in FeSe1-xTex and the optimally doped Ba1-xKxAs2Fe2 predicts an inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov state for H//ab and T < 3-10 K, and shows that Hc2 in multiband Fe based superconductor can be enhanced by doping and strain much more effectively than by the conventional way of increasing disorder.

cond-mat.supr-con↗

arXiv2009

Magnetically-induced electric polarization in an organo-metallic magnet

The coupling between magnetic order and ferroelectricity has been under intense investigation in a wide range of transition-metal oxides. The strongest coupling is obtained in so-called magnetically-induced multiferroics where ferroelectricity arises directly from magnetic order that breaks inversion symmetry. However, it has been difficult to find non-oxide based materials in which these effects occur. Here we present a study of copper dimethyl sulfoxide dichloride (CDC), an organo-metallic quantum magnet containing $S = 1/2$ Cu spins, in which electric polarization arises from non-collinear magnetic order. We show that the electric polarization can be switched in a stunning hysteretic fashion. Because the magnetic order in CDC is mediated by large organic molecules, our study shows that magnetoelectric interactions can exist in this important class of materials, opening the road to designing magnetoelectrics and multiferroics using large molecules as building blocks. Further, we demonstrate that CDC undergoes a magnetoelectric quantum phase transition where both ferroelectric and magnetic order emerge simultaneously as a function of magnetic field at very low temperatures.

cond-mat.mtrl-sci↗

arXiv2008

Disorder, Metal-Insulator crossover and Phase diagram in high-Tc cuprates

We have studied the influence of disorder induced by electron irradiation on the normal state resistivities $ρ(T)$ of optimally and underdoped YBa2CuOx single crystals, using pulsed magnetic fields up to 60T to completely restore the normal state. We evidence that point defect disorder induces low T upturns of rho(T) which saturate in some cases at low T in large applied fields as would be expected for a Kondo-like magnetic response. Moreover the magnitude of the upturns is related to the residual resistivity, that is to the concentration of defects and/or their nanoscale morphology. These upturns are found quantitatively identical to those reported in lower Tc cuprates, which establishes the importance of disorder in these supposedly pure compounds. We therefore propose a realistic phase diagram of the cuprates, including disorder, in which the superconducting state might reach the antiferromagnetic phase in the clean limit.

cond-mat.supr-con↗

arXiv2003

The Origin of Weak Ferromagnetism in CaB6

We have combined the results of magnetization and Hall effect measurements to conclude that the ferromagnetic moments of lightly doped CaB6 samples display no systematic variation with electron doping level. Removal of the surface with acid etching substantially reduces the measured moment, although the Hall constant and resistivity are unaffected, indicating that the ferromagnetism largely resides on and near the sample surface. Electron microprobe experiments reveal that Fe and Ni are found at the edges of facets and growth steps, and on other surface features introduced during growth. Our results indicate that the weak ferromagnetism previously reported in undoped CaB6 is at least partly extrinsic.

cond-mat.str-el↗