Dominant In-Plane Symmetric Elastoresistance in CsFe2As2

KFe2As2 single crystals are studied using specific-heat, high-resolution thermal-expansion, magnetization, and magnetostriction measurements. The magnetization and magnetostriction data provide clear evidence for strong Pauli limiting effects of the upper critical field for magnetic fields parallel to the FeAs planes, suggesting that KFe2As2 may be a good candidate to search for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. Using standard thermodynamic relations, the uniaxial pressure derivatives of the critical temperature (Tc), the normal-state Sommerfeld coefficient (γn), the normal-state susceptibility (χ), and the thermodynamic critical field (Hc) are calculated from our data. We find that the close relationship between doping and pressure as found in other Fe-based systems does not hold for KFe2As2.

Physical Review B, 2008

Plate-like single crystals of SrFe 2 As 2 as large as 3×3×0.5 mm 3 have been grown out of Sn flux. The SrFe 2 As 2 single crystals show a structural phase transition from a high temperature tetragonal phase to a low temperature orthorhombic phase at T o = 198 K, and do not show any sign of superconductivity down to 1.8 K. The structural transition is accompanied by an anomaly in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In an intermediate temperature range from 198 K to 160 K, single crystal X-ray diffraction suggests a coexistence of the high-temperature tetragonal and the low-temperature orthorhombic phases.

Physical Review Letters, 2012

We report neutron scattering experiments probing the influence of uniaxial strain on both the magnetic and structural order parameters in the parent iron pnictide compound, BaFe2As2. Our data show that modest strain fields along the in-plane orthorhombic b-axis can affect significant changes in phase behavior simultaneous to the removal of structural twinning effects. As a result, we demonstrate in BaFe2As2 samples detwinned via uniaxial strain that the in-plane C4 symmetry is broken by both the structural lattice distortion and long-range spin ordering at temperatures far above the nominal (strain-free), phase transition temperatures. Surprising changes in the magnetic order parameter of this system under relatively small strain fields also suggest the inherent presence of magnetic domains fluctuating above the strain-free ordering temperature in this material.

Nanomaterials

We utilize first principles calculations to investigate the mechanical properties and strain-dependent electronic band structure of the hexagonal phase of two dimensional (2D) HfS2. We apply three different deformation modes within −10% to 30% range of two uniaxial (D1, D2) and one biaxial (D3) strains along x, y, and x-y directions, respectively. The harmonic regions are identified in each deformation mode. The ultimate stress for D1, D2, and D3 deformations is obtained as 0.037, 0.038 and 0.044 (eV/Ang3), respectively. Additionally, the ultimate strain for D1, D2, and D3 deformation is obtained as 17.2, 17.51, and 21.17 (eV/Ang3), respectively. In the next step, we determine the second-, third-, and fourth-order elastic constants and the electronic properties of both unstrained and strained HfS2 monolayers are investigated. Our findings reveal that the unstrained HfS2 monolayer is a semiconductor with an indirect bandgap of 1.12 eV. We then tune the bandgap of HfS2 with strain eng...

2010

The parent compounds of iron-arsenide superconductors, AFe2As2 (A=Ca, Sr, Ba), undergo a tetragonal to orthorhombic structural transition at a temperature TTO in the range 135 to 205 K depending on the alkaline earth element. Below TTO the free standing crystals split into equally populated structural domains, which mask intrinsic, in-plane, anisotropic properties of the materials. Here we demonstrate a way of mechanically detwinning CaFe2As2 and BaFe2As2. The detwinning is nearly complete, as demonstrated by polarized light imaging and synchrotron X-ray measurements, and reversible, with twin pattern restored after strain release. Electrical resistivity measurements in the twinned and detwinned states show that resistivity, ρ, decreases along the orthorhombic ao-axis but increases along the orthorhombic bo-axis in both compounds. Immediately below TTO the ratio ρ bo /ρao = 1.2 and 1.5 for Ca and Ba compounds, respectively. Contrary to CaFe2As2, BaFe2As2 reveals an anisotropy in the nominally tetragonal phase, suggesting that either fluctuations play a larger role above TTO in BaFe2As2 than in CaFe2As2, or that there is a higher temperature crossover or phase transition.

Physical Review B

Strain is a powerful experimental tool to explore new electronic states and understand unconventional superconductivity. Here, we investigate the effect of uniaxial strain on the nematic and superconducting phase of single crystal FeSe using magnetotransport measurements. We find that the resistivity response to the strain is strongly temperature dependent and it correlates with the sign change in the Hall coefficient being driven by scattering, coupling with the lattice and multiband phenomena. Band structure calculations suggest that under strain the electron pockets develop a large in-plane anisotropy as compared with the hole pocket. Magnetotransport studies at low temperatures indicate that the mobility of the dominant carriers increases with tensile strain. Close to the critical temperature, all resistivity curves at constant strain cross in a single point, indicating a universal critical exponent linked to a strain-induced phase transition. Our results indicate that the superconducting state is enhanced under compressive strain and suppressed under tensile strain, in agreement with the trends observed in FeSe thin films and overdoped pnictides, whereas the nematic phase seems to be affected in the opposite way by the uniaxial strain. By comparing the enhanced superconductivity under strain of different systems, our results suggest that strain on its own cannot account for the enhanced high Tc superconductivity of FeSe systems.

Physical review, 2023

In recent years, the application of mechanical stress has become a widespread experimental method to tune the electronic and optical properties of two-dimensional (2D) materials. In this work, we investigate the impact of uniaxial tensile strain along zigzag and armchair directions on the excitonic properties of graphene-like C 3 N, a single-layer indirect-gap material with relevant mechanical and optical properties. To do that, we develop a tightbinding Bethe-Salpeter equation framework based on a Wannier-function description of the frontier bands of the system, and use it to compute both dark and bright excitons of C 3 N for different applied strain configurations. Then, we use this model approach to classify excitons of pristine and strained C 3 N according to the crystal symmetry and to explain the appearance of bright excitons with intense optical anisotropy in strained C 3 N, even at small strains. Finally, the effect of strain on the exciton dispersion at small center-of-mass momenta is discussed, with special focus on the implications for 2D linear-nonanalytic dispersions.

Journal of Physics and Chemistry of Solids, 2011

Physical Review B, 2008

Single crystalline, single phase CaKFe4As4 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 CaKFe4As4 is an ordered, stoichiometric, Fe-based superconductor with a superconducting critical temperature, Tc = 35.0 ± 0.2 K. Other than superconductivity, there is no indication of any other phase transition for 1.8 K ≤ T ≤ 300 K. All of these thermodynamic and transport data reveal striking similarities to that found for optimally-or slightly over-doped (Ba1−xKx)Fe2As2, suggesting that stoichiometric CaKFe4As4 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, Hc2(T), of CaKFe4As4 was determined up to 630 kOe. The anisotropy parameter γ(T) = H ⊥ c2 /H c2 , for H applied perpendicular and parallel to the c-axis, decreases from 2.5 at Tc to 1.5 at 25 K which can be explained by interplay of paramagnetic pairbreaking and orbital effects. The slopes of dH c2 /dT −44 kOe/K and dH ⊥ c2 /dT −109 kOe/K at Tc yield an electron mass anisotropy of m ⊥ /m 1/6 and short Ginzburg-Landau coherence lengths ξ (0) 5.8Å and ξ ⊥ (0) 14.3Å. The value of H ⊥ c2 (0) can be extrapolated to 920 kOe, well above the BCS paramagnetic limit.

2010

Obtaining the electronic structure of the newly discovered iron-based superconductors is the key to understanding the mechanism of their high-temperature superconductivity. We used angle-resolved photoemission spectroscopy (ARPES) to make direct measurements of the electronic structure and Fermi surface (FS) of the untwinned uniaxial state of CaFe 2 As 2 , the parent compound of iron-based superconductors. We observed unequal dispersions and FS geometries along the orthogonal Fe-Fe bond directions. More importantly, unidirectional straight and flat FS segments are observed near the zone center, which indicates the existence of a unidirectional nematic charge density wave order, strengthening the case for a quantum electronic liquid crystalline "nematic" phase. Further, the doping dependence extrapolates to a possible quantum critical point of the disappearance of this order in the heavily overdoped regime of these materials.