Abstract:
The discoveries of new materials with unexpected properties have alw
ays driven the advancement of technology. To give an example\, liquid crys
tals were found in 1888\, have been investigated since then and are now us
ed in all LCD displays. Today\, one group of materials where new phases oc
cur are so-called quantum materials. In these\, the appearance of unconven
tional states of matter is related with the fact that electrons interact s
trongly with each other. My group’s motivation for research is to unders
tand this intriguing relation. In particular\, we are interested in unconv
entional superconductivity and unconventional metallic states such as topo
logical systems\, heavy-Fermi and non-Fermi liquids. In my talk I will mai
nly focus on two groups of materials\, namely topological semimetals and h
eavy fermion systems. In topological systems\, the low energy excitations
behave like relativistic particles and hence allow to investigate predicti
ons from high energy physics. Our aim here is to find evidence for Weyl fe
rmionic behavior in bulk experimental probes like low-temperature resistiv
ity. In heavy fermion systems\, superconductivity evolves out of unconvent
ional ground states. In these materials we are interested in non-Fermi liq
uid behavior near quantum critical points and unconventional superconducti
ng states.
My group’s expertise lies in high sensitivity - low tem
perature measurements and especially in the detection of quantum oscillati
ons in metals. These oscillations are a direct consequence of the quantiza
tion of the electron orbits in a magnetic field. We measure thermodynamic
and transport probes such as magnetic susceptibility\, torque or resistivi
ty. These techniques yield macroscopic information on the appearing phases
. Additionally\, quantum oscillations can be detected in those measurement
s if requirements of extremely pure materials\, low temperatures\, high ma
gnetic fields and low noise levels are fulfilled. My group is one of the f
ew in the world specialized in this quasiparticle spectroscopy that gives
microscopic information on the energy eigenstates\, scattering processes a
nd the interactions between charge carriers in the metal.
In this ta
lk I will present how we have been using these experimental techniques to
answer the scientific questions mentioned above. Investigations of the top
ological Weyl semimetals TaAs\, TaP\, NbAs and NbP\, have allowed to test
if the chiral anomaly\, a theoretical prediction from high-energy physics\
, induces a longitudinal magnetoresistance. This response should depend on
the energy eigenstates of the electrons in these materials. Therefore\, w
e have established the latter via quantum oscillations and hence were able
to show that in TaAs and NbAs\, electrons behave like Weyl fermions [1\,2
]. Additionally\, our group has found evidence that an apparent negative l
ongitudinal magnetoresistance can easily arise from an inhomogeneous curre
nt distribution caused by the extreme field-induced resistivity anisotropy
typical for any compensated semimetal [1\,3]. We succeeded revealing that
when the current is flowing homogeneously\, the magnetoresistance does no
t show clear signs of the chiral anomaly [4].
Unconventional superco
nductivity (SC) remains one of the most intriguing phenomena in condensed
matter physics. We have recently discovered CeRh2As2\, an outstanding unco
nventional superconductor [5]. This compound\, which probably presents qua
drupole order at around 0.4 K is locally non-centrosymmetric at the Ce-pos
ition while keeping an overall centrosymmetry. The superconducting state e
volves below 0.26 K. Most peculiarly\, it has two superconducting states a
s a function of magnetic field B||c and a critical field curve presenting
a sharp kink at 4 T and increasing up to 14 T (see the figure). In my talk
I will give details about this discovery and how it can be understood whe
n considering that the local symmetry can give rise to spin-orbit interact
ion.
[1] F. Arnold et al.\, Nat. Commun. (2016)
[2] F
. Arnold et al.\, Phys. Rev. Lett. (2016)
[3] R. D. dos Reis et al.\
, New J. Phys. (2016)
[4] M. Naumann et al.\, Phys. Rev. Mat. (2020)
[5] S. Khim et al.\, submitted (2020)
Where: Zoom Meeting:< /p>
https://tu-dresden.zoom.us/j/85193786651?pwd=Q0V0KzR2emsyRnhvdzdLR
DdTSnBaZz09 (https://tu-dresden.zoom.us/j/85193786651?pwd=Q0V0KzR2emsyRnhv
dzdLRDdTSnBaZz09)
Meeting ID: 851 9378 6651
Passcode: reG3NboG
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