Evidence for Magnetic Field Induced Changes of the Phase of Tunneling States: Spontaneous Echoes in (KBr)$_{1-x}$(KCN)$_x$ in Magnetic Fields

TL;DR

This study demonstrates that magnetic fields can alter the phase of tunneling states in a crystalline material, affecting its dielectric properties and echo amplitudes, revealing new quantum effects in solid-state systems.

Contribution

It provides the first direct evidence that magnetic fields influence the phase of coherent tunneling states in a crystal, expanding understanding beyond glasses.

Findings

Magnetic fields significantly affect echo amplitudes in KBr:CN crystals.

Evidence shows magnetic fields change the phase of tunneling systems.

The effect is comparable or stronger than in amorphous glasses.

Abstract

Recently, it has been discovered that in contrast to expectations the low-temperature dielectric properties of some multi-component glasses depend strongly on magnetic fields. In particular, the low-frequency dielectric susceptibility and the amplitude of coherent polarization echoes show striking non-monotonic magnetic field dependencies. The low-temperature dielectric response of these materials is governed by atomic tunneling systems. We now have investigated the coherent properties of tunneling states in a crystalline host in magnetic fields up to 230$ $mT. Two-pulse echo experiments have been performed on a KBr crystal containing about 7.5% CN$^-$. Like in glasses, but perhaps even more surprising in the case of a crystalline system, we observe a very strong magnetic field dependence of the echo amplitude. Moreover, for the first time we have direct evidence that magnetic fields change the phase of coherent tunneling systems in a well-defined way. We present the data and discuss the possible origin of this intriguing effect.