David Nozadze

Disorder promotes ferromagnetism: rounding of the quantum phase transition in Sr(1-x)Ca(x)RuO3.

The subtle interplay of randomness and quantum fluctuations at low temperatures gives rise to a plethora of unconventional phenomena in systems ranging from quantum magnets and correlated electron materials to ultracold atomic gases. Particularly strong disorder effects have been predicted to occur at zero-temperature quantum phase transitions. Here, we demonstrate that the composition-driven ferromagnetic-to-paramagnetic quantum phase transition in Sr(1-x)Ca(x)RuO3 is completely destroyed by the disorder introduced via the different ionic radii of the randomly distributed Sr and Ca ions. Using a magneto-optical technique, we map the magnetic phase diagram in the composition-temperature space. We find that the ferromagnetic phase is significantly extended by the disorder and develops a pronounced tail over a broad range of the composition x. These findings are explained by a microscopic model of smeared quantum phase transitions in itinerant magnets. Moreover, our theoretical study implies that correlated disorder is even more powerful in promoting ferromagnetism than random disorder.

Compressibility as a probe of quantum phase transitions in topological superconductors

The non-Abelian statistics of Majorana fermions, their role in topological quantum computation, and the possibility of realizing them in condensed matter systems, has attracted considerable attention. While there have been recent reports of zero energy modes in single particle tunneling density of states, their identity as Majorana modes has so far not been unequivocally established. We make predictions for the local compressibility

Composition-tuned smeared phase transitions

\kappa_{\rm{loc}}

Quantum Griffiths Singularities in Ferromagnetic Metals

, tuned by changing the chemical potential

Disorder correlations at smeared phase transitions

\mu

Modification of smeared phase transitions by spatial disorder correlations

in a semiconducting nanowire with strong spin-orbit coupling and in a Zeeman field in proximity to a superconductor, that has been proposed as a candidate system for observing Majorana modes. We show that in the center of the wire, the topological phase transition is signaled by a divergence of

Transport properties in antiferromagnetic quantum Griffiths phases

\kappa_{\rm{loc}}

Numerical method for disordered quantum phase transitions in the large‐N limit

as a function of

Topological states in normal and superconducting p-wave chains

\mu

Compressibility as a probe of topological quantum phase transitions in 1D systems

which is an important diagnostic of the topological phase transition. We also find that a single strong impurity potential can lead to a local {\it negative} compressibility at the topological phase transition. The origin of such anomalous behavior can be traced to the formation of Andreev bound states close to topological phase transitions. Measurable by a scanning electron transistor, the compressibility includes contributions from both single particle states and collective modes and is therefore a complimentary probe from scanning tunneling spectroscopy.