Quartic metal: Spontaneous breaking of time-reversal symmetry due to
four-fermion correlations in Ba$_{1-x}$K$_x$Fe$_2$As$_2$
- URL: http://arxiv.org/abs/2103.17190v2
- Date: Mon, 5 Jul 2021 16:31:06 GMT
- Title: Quartic metal: Spontaneous breaking of time-reversal symmetry due to
four-fermion correlations in Ba$_{1-x}$K$_x$Fe$_2$As$_2$
- Authors: Vadim Grinenko, Daniel Weston, Federico Caglieris, Christoph Wuttke,
Christian Hess, Tino Gottschall, Ilaria Maccari, Denis Gorbunov, Sergei
Zherlitsyn, Jochen Wosnitza, Andreas Rydh, Kunihiro Kihou, Chul-Ho Lee, Rajib
Sarkar, Shanu Dengre, Julien Garaud, Aliaksei Charnukha, Ruben H\"uhne,
Kornelius Nielsch, Bernd B\"uchner, Hans-Henning Klauss, and Egor Babaev
- Abstract summary: Superconducting states are a spontaneously broken symmetry corresponding to long-range coherence of fermion pairs.
We show that the formation of fermionic bound states leads to spontaneous breaking of time-reversal symmetry above the superconducting transition temperature.
- Score: 3.076082310544129
- License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
- Abstract: Discoveries of ordered quantum states of matter are of great fundamental
interest, and often lead to unique applications. The most well known example --
superconductivity -- is caused by the formation and condensation of pairs of
electrons. A key property of superconductors is diamagnetism: magnetic fields
are screened by dissipationless currents. Fundamentally, what distinguishes
superconducting states from normal states is a spontaneously broken symmetry
corresponding to long-range coherence of fermion pairs. Here we report a set of
experimental observations in hole doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ which are
not consistent with conventional superconducting behavior. Our specific-heat
measurements indicate the formation of fermionic bound states when the
temperature is lowered from the normal state. However, for $x \sim 0.8$,
instead of the standard for superconductors, zero resistance and diamagnetic
screening, for a range of temperatures, we observe the opposite effect: the
generation of self-induced magnetic fields measured by spontaneous Nernst
effect and muon spin rotation experiments. The finite resistance and the lack
of any detectable diamagnetic screening in this state exclude the spontaneously
broken symmetry associated with superconducting two-fermion correlations.
Instead, combined evidence from transport and thermodynamic measurements
indicates that the formation of fermionic bound states leads to spontaneous
breaking of time-reversal symmetry above the superconducting transition
temperature. These results demonstrate the existence of a
broken-time-reversal-symmetry bosonic metal state. In the framework of a
multiband theory, such a state is characterized by quartic correlations: the
long-range order exists only for {\it pairs} of fermion pairs.
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