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Publication Highlights
### Driven-Dissipative Supersolid in a Ring Cavity

F. Mivehvar et al., Phys. Rev. Lett. 120, 123601 (2018)

Supersolids, a mysterious phase of matter consisting of a crystal which can flow without friction, have been elusive to experimental confirmation till last year, where the first realisations using ultracold atomic systems have been achieved. These atomic implementations however take place in driven-dissipative systems, a situation which lies outside the?thermal equilibrium scenario so far considered in theory.

In this work, we study for the first time the effect of the openness of the system on the main features of a supersolid, and find that its hallmarks can be robust against drive and dissipation whenever the latter preserve spatial translation invariance.

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Supersolids, a mysterious phase of matter consisting of a crystal which can flow without friction, have been elusive to experimental confirmation till last year, where the first realisations using ultracold atomic systems have been achieved. These atomic implementations however take place in driven-dissipative systems, a situation which lies outside the?thermal equilibrium scenario so far considered in theory.

In this work, we study for the first time the effect of the openness of the system on the main features of a supersolid, and find that its hallmarks can be robust against drive and dissipation whenever the latter preserve spatial translation invariance.

Institute's News
### New Research Group: Mesoscopic Physics of Life

We are glad to announce the arrival of Dr. Christoph A. Weber, who heads the research group ‘Mesoscopic Physics of Life' since March 1, 2018. The group is interested in intra-cellular organisation and aims in particular to understand the role of phase transitions inside cells, including the impact of phase separation and protein aggregation during development and in the context of disease. Further interests are to unravel physical principles underlying the early formation of proto-cells at the origin of life.

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Awards and Honors
### Markus Heyl receives the 2018 Bernhard Heß prize

The distinction is awarded to outstanding young scientists by the University of Regensburg. Markus Heyl receives the prize for his contributions to the field of nonequilibrium quantum many-body systems and, in particular, for the development of the theory of dynamical quantum phase transitions. The prize is donated with 2.000 Euro, and the awardees are invited to give a guest lecture at the University of Regensburg.

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Publication Highlights
### Universality of clone dynamics during tissue development

S. Rulands et al., Nature Physics (2018)

The development of an organism relies on the tightly orchestrated behavior of many cells. How do these cells self-organize in order to build complex structures like the heart or the brain? To achieve this the fate of these cells must be precisely regulated and understanding the mechanisms of cell fate regulation is key for treating diseases that occur upon dysregulation, such as cancer or diabetes. The fate behaviour of stem and progenitor cells is reflected in the time evolution of their progeny, termed clones, which serve as a key experimental observable. But what can we actually learn from such clones about the processes that regulate their fate during development? Drawing on the results of genetic tracing studies, we show that, despite the complexity of organ development, clonal dynamics may converge to a critical state characterized by universal scaling behaviour of clone sizes. We show how this identification of universal scaling dependences may allow lineage-specific information to be distilled from experiments. Our study shows the emergence of core concepts of statistical physics in an unexpected context, identifying cellular systems as a laboratory to study non-equilibrium statistical physics.

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The development of an organism relies on the tightly orchestrated behavior of many cells. How do these cells self-organize in order to build complex structures like the heart or the brain? To achieve this the fate of these cells must be precisely regulated and understanding the mechanisms of cell fate regulation is key for treating diseases that occur upon dysregulation, such as cancer or diabetes. The fate behaviour of stem and progenitor cells is reflected in the time evolution of their progeny, termed clones, which serve as a key experimental observable. But what can we actually learn from such clones about the processes that regulate their fate during development? Drawing on the results of genetic tracing studies, we show that, despite the complexity of organ development, clonal dynamics may converge to a critical state characterized by universal scaling behaviour of clone sizes. We show how this identification of universal scaling dependences may allow lineage-specific information to be distilled from experiments. Our study shows the emergence of core concepts of statistical physics in an unexpected context, identifying cellular systems as a laboratory to study non-equilibrium statistical physics.

Publication Highlights
### Dynamical quantum phase transitions: a review

M. Heyl, Rep. Prog. Phys. (2018)

Quantum theory provides an extensive framework for the description of the equilibrium properties of quantum matter. Yet experiments in quantum simulators have now opened up a route towards generating quantum states beyond this equilibrium paradigm. While these states promise to show properties not constrained by equilibrium principles such as the equal a priori probability of the microcanonical ensemble, identifying general properties of nonequilibrium quantum dynamics remains a major challenge especially in view of the lack of conventional concepts such as free energies. The theory of dynamical quantum phase transitions attempts to identify such general principles by lifting the concept of phase transitions to coherent quantum real-time evolution. This review provides a pedagogical introduction to this field. Starting from the general setting of nonequilibrium dynamics in closed quantum many-body systems, we give the definition of dynamical quantum phase transitions as phase transitions in time with physical quantities becoming nonanalytic at critical times. We summarize the achieved theoretical advances as well as the first experimental observations, and furthermore provide an outlook onto major open questions as well as future directions of research.

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Quantum theory provides an extensive framework for the description of the equilibrium properties of quantum matter. Yet experiments in quantum simulators have now opened up a route towards generating quantum states beyond this equilibrium paradigm. While these states promise to show properties not constrained by equilibrium principles such as the equal a priori probability of the microcanonical ensemble, identifying general properties of nonequilibrium quantum dynamics remains a major challenge especially in view of the lack of conventional concepts such as free energies. The theory of dynamical quantum phase transitions attempts to identify such general principles by lifting the concept of phase transitions to coherent quantum real-time evolution. This review provides a pedagogical introduction to this field. Starting from the general setting of nonequilibrium dynamics in closed quantum many-body systems, we give the definition of dynamical quantum phase transitions as phase transitions in time with physical quantities becoming nonanalytic at critical times. We summarize the achieved theoretical advances as well as the first experimental observations, and furthermore provide an outlook onto major open questions as well as future directions of research.

Publication Highlights
### Topological Classification of Crystalline Insulators through Band Structure Combinatorics

Jorrit Kruthoff et al., Phys. Rev. X 7, 041069

Topological insulators are exotic materials that are electrical insulators in their interior but can conduct electricity on their surface, and their discovery has fundamentally changed our understanding of how phases of matter may be organized. Most phases of matter are categorized by the symmetries that they break. Crystals break translational symmetry, magnets break rotational symmetry, and so on. Topological insulators, however, show that some phases can be distinct even though their symmetries are equal. Researchers have come up with a classification scheme--called the tenfold way--which allows for the categorization of topological phases depending on some general properties, such as whether or not they have time-reversal or particle-hole symmetry. We complement this categorization by providing a method for listing all possible topologically distinct phases of matter that do not have external symmetries but do have the types of internal (or lattice) symmetries that appear in the atomic arrangements of real solid materials. We explicitly list all possible phases in two-dimensional materials and provide an intuitive and easily applicable method for identifying the phases possible within a given lattice type in any dimension. Our method matches the known results based on the mathematically involved predictions of K-theory, which is known to be a rigorous way of identifying all possible topological phases. It thus provides insight into this mathematical arena based on a physical understanding of topological band structures. We also show how our method can be used to study the transitions between topological phases and predict whether they will result in topologically protected Weyl semimetals. This new classification can now be used to guide the search for new types of topological materials and related edge modes or exotic intermediate phases.

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Topological insulators are exotic materials that are electrical insulators in their interior but can conduct electricity on their surface, and their discovery has fundamentally changed our understanding of how phases of matter may be organized. Most phases of matter are categorized by the symmetries that they break. Crystals break translational symmetry, magnets break rotational symmetry, and so on. Topological insulators, however, show that some phases can be distinct even though their symmetries are equal. Researchers have come up with a classification scheme--called the tenfold way--which allows for the categorization of topological phases depending on some general properties, such as whether or not they have time-reversal or particle-hole symmetry. We complement this categorization by providing a method for listing all possible topologically distinct phases of matter that do not have external symmetries but do have the types of internal (or lattice) symmetries that appear in the atomic arrangements of real solid materials. We explicitly list all possible phases in two-dimensional materials and provide an intuitive and easily applicable method for identifying the phases possible within a given lattice type in any dimension. Our method matches the known results based on the mathematically involved predictions of K-theory, which is known to be a rigorous way of identifying all possible topological phases. It thus provides insight into this mathematical arena based on a physical understanding of topological band structures. We also show how our method can be used to study the transitions between topological phases and predict whether they will result in topologically protected Weyl semimetals. This new classification can now be used to guide the search for new types of topological materials and related edge modes or exotic intermediate phases.

Awards and Honors
### "Physik-Preis Dresden“ zum zweiten Mal verliehen

Am Dienstagabend, dem 5. November 2017, wurde der „Physik-Preis Dresden“ der TU Dresden und des Max-Planck-Instituts für Physik komplexer Systeme (MPI-PKS) zum zweiten Mal verliehen. Preisträger ist der französische Physiker Prof. Dr. Jacques Prost, emeritierter CNRS Forschungsdirektor am Institut Curie in Paris.
Prof. Jacques Prost erhielt den Preis für seine herausragenden wissenschaftlichen Verdienste in der Anwendung der Physik des Nichtgleichgewichts auf vielfältige Fragestellungen in der Biologie. Er ist der internationale Pionier in der mesoskaligen Physik biologischer Systeme. Mit seinen originellen und kreativen Ideen hat Jacques Prost neue Konzepte zur Erforschung biologischer Systeme etabliert und die Rolle physikalischer Prinzipen in der Biologie hervorgehoben. Als ein regelmäßiger Gast in Dresden hat er die Entwicklung der Dresdner biophysikalischen Gemeinschaft wesentlich vorangetrieben und zu zahlreichen Forschungsaktivitäten an der Schnittstelle zwischen Physik und Biologie angeregt.
Der Gastgeber des Abends, Prof. Roland Ketzmerick, Sprecher der Fakultät Physik der TU Dresden, war überaus erfreut, dass mit Jacques Prost eine so bedeutende internationale Forscherpersönlichkeit geehrt wurde. „Ich wünsche der Dresdner Biophysik auch weiterhin hervorragende Kooperationen mit ihm“, so Ketzmerick.
Zurzeit arbeitet Prost intensiv an dem Projekt "Physik aktiver Gele und des Zellskeletts" mit Prof. Stephan Grill vom BIOTEC (TU Dresden) und Prof. Frank Jülicher vom MPI-PKS zusammen. Die Auszeichnung mit dem „Physik-Preis Dresden“ sei für ihn eine große Ehre, und er bedankte sich mit einigen deutschen Worten bei Preisstifter Peter Fulde, der Preis-Jury sowie seinen Eltern, die ihn die Bedeutung der Freundschaft innerhalb Europas von Anfang an gelehrt haben. „Ich habe in Dresden echte Freunde gefunden“, betonte Prost.
Der „Physik-Preis Dresden“ wurde 2015 von dem Dresdner Physiker Prof. Peter Fulde, dem Gründungsdirektor des MPI-PKS gestiftet. Die Preisträger werden von einer gemeinsamen Jury der TU Dresden und des MPI-PKS bestimmt, deren diesjähriger Vorsitzender Prof. Dr. Matthias Vojta von der Fakultät Physik ist. Neben dem zentralen Kriterium der wissenschaftlichen Exzellenz ist für die Entscheidung vor allem wichtig, dass die Arbeiten der Preisträger für die Zusammenarbeit zwischen beiden Dresden-concept-Partnern MPI-PKS und TU Dresden von besonderer Bedeutung sind und deren Verbindung langfristig weiter gestärkt wurde.

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Publication Highlights
### Live cell X-ray imaging of autophagic vacuoles formation and chromatin dynamics in fission yeast

Strelnikova et al., Scientific Reports 7, 13775 (2017)

The major challenge of X-ray imaging of living cellular specimens with a higher resolution than the optical resolution is the very low lethal radiation dose. Owing to the radiation damage and a low electron density contrast, sequential X-ray imaging of live cells was not possible so far. In our manuscript, we demonstrate the first X-ray movies of living yeast cells showing the dynamics of the autophagic vacuole formation and chromosome motion. It is a new way of seeing physiological processes of living organisms at nanoscale resolution. Moreover, the found lower lethal dose for dividing cells in comparison to non-dividing cells, might be an alternative approach to selective killing of malicious cells.

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The major challenge of X-ray imaging of living cellular specimens with a higher resolution than the optical resolution is the very low lethal radiation dose. Owing to the radiation damage and a low electron density contrast, sequential X-ray imaging of live cells was not possible so far. In our manuscript, we demonstrate the first X-ray movies of living yeast cells showing the dynamics of the autophagic vacuole formation and chromosome motion. It is a new way of seeing physiological processes of living organisms at nanoscale resolution. Moreover, the found lower lethal dose for dividing cells in comparison to non-dividing cells, might be an alternative approach to selective killing of malicious cells.

Publication Highlights
### Interaction Dependent Heating and Atom Loss in a Periodically Driven Optical Lattice

Martin Reitter et al., Phys. Rev. Lett. 119, 200402

Time periodic driving, for example in the form of coherent radiation, is a standard tool for the manipulation of small quantum systems like single atoms. With the advent of highly controllable and well isolated quantum gases of neutral atoms, periodic driving has recently become a powerful tool also for the coherent control of many-body systems. A milestone is the possibility to couple the motion of neutral atoms to artificial magnetic fields, which allows to study quantum Hall physics with atomic quantum gases. However, periodically driven many-body systems suffer from unwanted intrinsic heating, the time scale of which should be large compared to the duration of the experiment. In a joint work of Scientists from Munich, Cambridge and Dresden, the interaction dependence of such intrinsic heating has now been measured for the first time and compared to theory. Interestingly, for sufficiently large driving frequencies, heating was found to be reduced via a new form of evaporative cooling.

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Time periodic driving, for example in the form of coherent radiation, is a standard tool for the manipulation of small quantum systems like single atoms. With the advent of highly controllable and well isolated quantum gases of neutral atoms, periodic driving has recently become a powerful tool also for the coherent control of many-body systems. A milestone is the possibility to couple the motion of neutral atoms to artificial magnetic fields, which allows to study quantum Hall physics with atomic quantum gases. However, periodically driven many-body systems suffer from unwanted intrinsic heating, the time scale of which should be large compared to the duration of the experiment. In a joint work of Scientists from Munich, Cambridge and Dresden, the interaction dependence of such intrinsic heating has now been measured for the first time and compared to theory. Interestingly, for sufficiently large driving frequencies, heating was found to be reduced via a new form of evaporative cooling.

Publication Highlights
### High-temperature nonequilibrium Bose condensation induced by a hot needle

Alexander Schnell et al., Phys. Rev. Lett. 119, 140602

When a physical system is brought in weak thermal contact with an environment of temperature T, it will appproach an equilibrium state. The properties of this state depend on the temperature of the environment only. For example, a quantum gas of bosonic particles will form a Bose-Einstein condensate, where a macroscopic fraction the particles occupies the same state, below a condensation temperature T_c. In contrast, when a system is coupled to two heat baths of different temperatures T_1 and T_2, the situation changes in a fundamental way. In this case the system will assume a non-equilibrium steady state, which depends on the very details of both baths and not simply on their temperatures. It is an interesting question in how far this fact can be used to manipulate the far-from equilibrium state of the system in a controlled fashion by bath engineering. In this paper, we describe an astonishing effect: a Bose gas that is embedded in an environment of temperature T>>T_c can form a Bose condensate when coupled to a "hot needle", a second local bath of even larger temperature T_h>>T. Moreover, depending on the parameters and unlike in thermal equilibrium, the condensate can also be formed in an excited state of the system.

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When a physical system is brought in weak thermal contact with an environment of temperature T, it will appproach an equilibrium state. The properties of this state depend on the temperature of the environment only. For example, a quantum gas of bosonic particles will form a Bose-Einstein condensate, where a macroscopic fraction the particles occupies the same state, below a condensation temperature T_c. In contrast, when a system is coupled to two heat baths of different temperatures T_1 and T_2, the situation changes in a fundamental way. In this case the system will assume a non-equilibrium steady state, which depends on the very details of both baths and not simply on their temperatures. It is an interesting question in how far this fact can be used to manipulate the far-from equilibrium state of the system in a controlled fashion by bath engineering. In this paper, we describe an astonishing effect: a Bose gas that is embedded in an environment of temperature T>>T_c can form a Bose condensate when coupled to a "hot needle", a second local bath of even larger temperature T_h>>T. Moreover, depending on the parameters and unlike in thermal equilibrium, the condensate can also be formed in an excited state of the system.