arXiv subjects

Christian Helm

Publications and source records attributed to Christian Helm.

3 recordsLinked to original sources

arXiv2001

Josephson Plasma Resonance in Tl$_2$Ba$_2$CaCu$_2$O$_8$ in a Magnetic Field Measured using THz Spectroscopy

We report the first measurements of the $c$-axis Josephson Plasma Resonance (JPR) in Tl$_2$Ba$_2$CaCu$_2$O$_8$ as a function of temperature with and without a $c$-axis magnetic field using terahertz time-domain spectroscopy in transmission. The JPR is sensitive to the alignment of pancake vortices along the $c$-axis, and is observed to decrease when applying a magnetic field as expected.

cond-mat.supr-con↗

arXiv1996

Constraint Quantization of Slave-Particle Theories

We start from the Barnes-Coleman slave-particle description, where the Hubbard operators $X$ are decomposed into a product of fermionic ($f_α$) and bosonic ($b$) operators. The quantum mechanical constraint $b^{\dagger} b + \sum_α f_α^{\dagger} f_α = 1$ is treated within the framework of Dirac's method for the quantization of classical constrained systems. This leads to modified algebraic properties of the fundamental operators: $b b^{\dagger} b = b$, $f_α f_β^{\dagger} f_γ = δ_{αβ} f_γ$ and $ f_α b^{\dagger}= 0 $. Thereby the algebra of the $X$-operators is preserved exactly on the operator level. Matrix representations of the above algebra are constructed and a resolvent-like perturbation theory for the single-impurity Anderson model is developed.

cond-mat↗

arXiv1996

Collective Spin Modes in Superconducting Double Layers

We investigate a double layer system with tight-binding hopping, intra-layer and inter-layer interactions, as well as a Josephson like coupling. We find that an antiferromagnetic spin polarization induces additional spin-triplet pairing (with $S_z =0$) to the singlet order parameter. This causes an undamped collective mode in the superconducting state below the particle-hole threshold, which is interpreted as a Goldstone excitation.

cond-mat↗