# Group of Prof. Dr. Jörg Schmalian

The research activities of the group of Prof. Jörg Schmalian are in theoretical condensed matter physics. The main interest is the investigation of strongly correlated quantum mechanical many body systems, particularly their new collective behavior emerging due to competing interactions, disorder, non-equilibrium conditions or finite geometries. We are interested in understanding new materials with novel electric, optic, magnetic or thermal properties. Using quantum statistical mechanics and many-body theory, we work on phenomena such as superconductivity, quantum phase transitions, hydrodynamic transport, nano-electronics, magnetism, disordered systems and non-equilibrium dynamics.

We investigate the interplay of strong disorder and quantum fluctuations and disorder induced quantum criticality, as well as the effects of disorder on clean quantum critical points. For example, geometric criticality near a percolation transition leads to a novel universality class for the quantum phase transition in diluted Josephson-junction arrays, antiferromagnets, and interacting bosons. Most recently we demonstrated that this leads to critical properties not captured by the usual Ginzburg-Landau-Wilson description of phase transitions, such as complex critical exponents, log-periodic oscillations and dynamically broken scale invariance.

further references:

T. Vojta and J. Schmalian, Phys. Rev. Lett. 95, 237206 (2005)

A. J. Millis, D. K. Morr, J. Schmalian Phys. Rev. Lett. 87 167202 (2001)

In strongly correlated materials the microscopic mechanism of Cooper pairing, the detailed nature of the pairing state, and the dependence of the transition temperature on the characteristic coupling constants are fundamentally distinct from the known results for electron-phonon induced superconductivity. We investigate phenomena such as superconductivity close to quantum critical points, superconductivity in resonating valence bond systems or superconductivity due to strong quantum valence skipping fluctuations.

further references:

T. Kondo, R. Khasanov, T. Takeuchi, J. Schmalian, A. Kaminski, Nature 457, 296 (2009)

M. Dzero, J. Schmalian Phys. Rev. Lett. 94, 157003 (2005)

Ar. Abanov, A. Chubukov, and J. Schmalian , Europhys. Lett. 55, 369 (2001)

J. Schmalian, Phys. Rev. Lett. 81, 4232 (1998)

The emergent relativistic symmetry of electrons in graphene near its neutrality point can efficiently be described as a quantum critical system, where the scale inveriance of the Dirac spectrum can be exploited to investigate many body effects due to electron-electron Coulomb repulsion or electron phonon interaction. In particular, scaling laws, valid near the neutrality point, dictate the nontrivial magnetic and charge response of interacting graphene with enhanced diamagnetic response and modified electronic compressibility, and universal answers for the hydrodynamic transport such as low shear viscosity. Further examples are the investigation of nonlinear transport, implications for nano-electronics and photoexcited carriers in graphene.

further references:

M. Müller, J. Schmalian, and L. Fritz, Phys. Rev. Lett. 103, 025301 (2009)

L. Fritz., J. Schmalian, M. Mueller, S. Sachdev, Physical Review B 78, 085416 (2008)

The cooperative rearrangement of groups of many molecules has long been thought to underlie the dramatic slowing of liquid dynamics on cooling towards the glassy state. Similar behavior emerges in electronic many body systems with collective degrees of freedom competing for mutually incompatible ordered states. We investigate the formation of such glassy states and effects such as dynamical heterogeneity, barrier fluctuations, or defect nucleation and slow defect motion.

further references:

J. D. Stevenson, J. Schmalian, P. G. Wolynes, Nature-Physics 4 268 (2006).

J. Schmalian, P. G. Wolynes, Phys. Rev. Lett. 85, 836 (2000)