Magnetic-field-induced density of states inMgB2: Spin susceptibility measured by conduction-electron spin resonance

Abstract

The magnetic field dependence of the spin-susceptibility, χ s was measured in the superconducting state of high purity MgB2 fine powders below 1.3 T. χ s was determined from the intensity of the conduction electron spin resonance spectra at 3.8, 9.4, and 35 GHz. At the lowest magnetic fields (0.14 T), a gap opens in the density of states at the Fermi energy and, accordingly, χ s (T ) is small at low temperatures. Fields above 0.2 T (about 15 % of H c c2 , the minimum upper critical field), destroy the gap. The field induced χ s is much larger than expected from current superconductor models of MgB2. [4] estimate that about half of the density of states (DOS) is on the π band thus the two-gap model predicts that closing the gap at 1 T restores about half of the normal state DOS. Naively, one would expect that properties proportional to the DOS, like the specific heat coefficient or the spin-susceptibility should reach half of their normal state values at about 1 T, whereas the remaining half originating from the σ bands should be restored gradually with further increasing the field to H c2 .In this Letter, we report on the magnetic field dependence of the conduction electron spin-susceptibility, χ s , in the superconducting state of high purity MgB 2 powders. This is the first time χ s is measured using conduction electron spin resonance below T c in a superconductor. We observe an unusually strong increase of χ s with the magnetic field, H, which cannot be reconciled with current models easily. The DOS induced by fields well below H c2 is larger than 50 %, the value expected from closing the gap on the π band FS only.We studied samples from several batches. Sample 1 is made from 99.99 % purity natural isotopic mixture of 10 B and 11 B amorphous boron, Samples 2 and 3 are made from crystalline, isotopically pure 11 B. Chemical analysis, the high normal state conductivities and narrow CESR lines at T c attest the high purity of the samples. Details of sample characterization are discussed in Ref. [14]. Sample 2 was made from the batch used in the CESR work of Ref. [11]. Fine powders with grain sizes less than 1 µm were selected from the starting materials to reduce the inhomogeneity of microwave excitation. The aggregates of small grains of the starting materials were first thoroughly hand-crushed in a borocarbide mortar. The resulting powders were suspended in isopropanol and mixed in a 1:1 weight ratio with pure SnO 2 fine powder. The larger particles were eliminated by sedimentation for long times or in a centifuge and the small grains were extracted from the suspension by filtering. Mixing with SnO 2 is important to separate the MgB 2 particles to avoid eddy-currents. SEM microscopy has confirmed that this procedure results in MgB 2 grains smaller than 0.5 µm size. SQUID magnetometry of the original batch and the final fine powders confirmed that superconducting properties were not affected by this procedure. The powders were finally cast into epoxy for the ESR experiments. We detail CESR experiments at 3.8, ...

Defect structure of ultrafine MgB2 nanoparticles

“…So far, very limited works have been presented for investigating the defect structure by the aid of EPR. Most of the works [15][16][17][18][19][20][21] have been dealt with the superconducting critical transition temperature or spin susceptibility at low temperatures. Thus, the defect structure of the pure MgB 2 at room temperature remained unclear.…”

Electron spin dynamics of the superconductorCaC6probed by ESR

“…The magnetic field inhomogeneity arising from the presence of vortices is averaged out because within the lifetime the conduction electrons travel long distances compared to the distance between vortices. The resonance field in the superconducting state in case of MgB 2 is shifted to higher magnetic fields due to diamagnetic shielding currents [7]. The diamagnetic shift is decreasing in increasing magnetic field (see Fig.…”

“…They observed a sudden decrease in intensity below T c and an increase in T 1 . CESR on the powder of the two-gapped superconductor, MgB 2 , have also been studied [7]. The measured intensity decrease below T c but the two-gap nature of MgB 2 makes proper analysis difficult.…”

“…in Ref. [7]), at H 0 = 8.1 T (∼225 GHz) it is only ∼3 G [31]. Comparing the ratio of H 0 and the upper critical field of MgB 2 in the ab-plane, H 0 /H ab c2 ∼0.5 (H ab c2 ∼16 T [32]) with that in the present work H 0 /H ab c2 ∼0.3 and taking into account that the diamagnetic shift scales with the upper critical field its absence in CaC 6 is reasonable.…”

ESR spectrometer with a loop-gap resonator for cw and time resolved studies in a superconducting magnet

“…The spectra were measured by the currently described spectrometer (a and b) at S and X-band, respectively, as well as on a Bruker ESP300 commercial spectrometer at X-band (c). In the fine powder of the MgB 2 superconductor the conducting particles were separated by a powder of the non-magnetic SnO 2 insulator [10,26]. Measurement in the superconducting magnet requires a magnetic field calibrating g-factor standard.…”