Harvard University Department of Physics

R.I.P. Professor Nicolaas Bloembergen

Nicolaas (Nico) Bloembergen, Gerhard Gade University Professor, Emeritus, and 1981 Nobel Laureate, passed away at his home in Tucson, Arizona, on September 5, 2017.

Mimicking Birdsongs

Mahadevan Lab has developed a simple device to recreate complex birdsongs with the help of soft materials and mathematical modeling. The study finds that the inherent complexity in birdsongs might actually be the result of a simple controllable instability in the structure of the specialized organ used to create song, known as a syrinx...

Mach-Zehnder Interferometry Using Spin- and Valley-Polarized Quantum Hall Edge States in Graphene

Confined to a two-dimensional plane, electrons in a strong magnetic field travel along the edge in one-dimensional quantum Hall channels that are protected against backscattering. These channels can be used as solid-state analogs of monochromatic beams of light...

UPCOMING EVENTS

Today

Monday Colloquium: Achim Rosch (U Cologne), “Whirls in magnets: from skyrmions to magnetic monopoles”
Start: 04:15pm - End: 05:30pm
- 04:15pm
- In magnets lacking inversion symmetry, topologically quantized magnetic whirls, so-called skyrmions, form due to spin-orbit interactions. Skyrmions are tiny, stable, couple extremely efficiently to electric currents and can be manipulated by small forces. They are therefore promising candidates for, e.g., future magnetic memories. The coupling of skyrmions to electrons arises from Berry phases, which can efficiently be described by an artificial electromagnetic field. We investigate how the topology of skyrmion phase can be changed by singular magnetic defects which can be identified as emergent magnetic monopoles.
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Tomorrow

CMP Special Seminar: Achim Rosch (U. of Cologne)
Start: 01:15pm - End: 02:15pm
- 01:15pm
- Title: Pumping approximately integrable systems
  
  Abstract: Weak perturbations can drive an interacting many-particle system far from its initial equilibrium state if one is able to pump into degrees of freedom approximately protected by conservation laws. This concept has for example been used to realize Bose–Einstein condensates of photons, magnons and excitons. Integrable quantum systems, like the one-dimensional Heisenberg model, are characterized by an infinite set of conservation laws. Here, we develop a theory of weakly driven integrable systems and show that pumping can induce large spin or heat currents even in the presence of integrability breaking perturbations, since it activates local and quasi-local approximate conserved quantities. The resulting steady state is qualitatively captured by a truncated generalized Gibbs ensemble with Lagrange parameters that depend on the structure but not on the overall amplitude of perturbations nor the initial state. We suggest to use spin-chain materials driven by terahertz radiation to realize integrability-based spin and heat pumps.
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Wednesday, September 27th

Xingbo Yang; Needleman group - Harvard University
Start: 05:30pm - End: 07:00pm
- 05:30pm
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Monday, October 2nd

Monday Colloquium: Natalia Toro (Perimeter Institute), Title TBA
Start: 04:15pm - End: 05:30pm
- 04:15pm
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Thursday, October 5th

CMP Seminar: Zala Lenarcic (University of Cologne)
Start: 12:00pm - End: 01:00pm
- 12:00pm
- Title: Pumping approximately integrable systems
  
  Abstract: When a degree of freedom is approximately protected by a conservation law even weak perturbations can cause strong response in that quantity and can drive the system far from its equilibrium steady state. A simple classical example is a well-insulated greenhouse, which can be heated up significantly even by weak sunlight, due to the approximate conservation of the energy within the greenhouse. A quantum platform with many conserved quantities is provided, for example, by integrable models. I will present the theory of weakly open and driven integrable systems on the one-dimensional Heisenberg XXZ model, with integrability-breaking perturbations and coupling to the environment relevant for realistic condensed matter or quantum optics setups. XXZ model is particularly interesting since heat and spin current are its (partially) conserved quantities. A remarkable conclusion is the activation of huge heat and spin currents in realistic materials could be achieved by a weak THz laser radiation, paving the way for a new type of heat and spin pumps. I will argue that the concept of the generalized Gibbs ensemble can be approximately but efficiently used to describe the non-equilibrium steady state in above situations.
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