Teemu Ojanen

Helical Fermi arcs and surface states in time-reversal invariant Weyl semimetals

Weyl semimetals are gapless three-dimensional topological materials where two bands touch at an even number of points in the Brillouin zone. In this work we study a zinc-blende lattice model realizing a time-reversal invariant Weyl semimetal. The bulk dynamics is described by 12 helical Weyl nodes. Surface states form a peculiar quasi-two-dimensional helical metal fundamentally different from the Dirac form typical for topological insulators. The allowed direction of velocity and spin of low-energy surface excitations are locked to the cubic symmetry axes. The studied system illustrates the general properties of surface states in systems with common crystal symmetries.

Thermal rectification in nonlinear quantum circuits

We present a theoretical study of radiative heat transport in nonlinear solid-state quantum circuits. We give a detailed account of heat rectification effects, i.e. the asymmetry of heat current with respect to a reversal of the thermal gradient, in a system consisting of two reservoirs at finite temperatures coupled through a nonlinear resonator. We suggest an experimentally feasible superconducting circuit employing the Josephson nonlinearity to realize a controllable low temperature heat rectifier with a maximal asymmetry of the order of 10%. Strikingly, we discover that rectification can change sign as a function of temperature. Heat transport in nanoscale structures has become an active and rapidly growing research area. Progress in experimental methods has enabled the study of fundamental issues, and lately the field has seen major breakthroughs, such as the measurement of quantized heat transport [1], and manipulation of thermal currents using external control fields [2, 3]. In solid-state systems electron–electron and electron–phonon scattering are the most important channels for small systems to exchange energy with the environment. However, recently it was understood that at low temperatures one needs to take into account the radiative channel which becomes the dominant relaxation method in mesoscopic samples below the phonon–photon crossover [2, 4, 5]. In this paper we study rectification effects in thermal transport mediated by electromagnetic fluctuations in solid-state nanostructures. In a two-terminal geometry a finite rectification means that heat current is not simply reversed when the thermal gradient changes sign, but also the absolute magnitude of the current changes. We define the rectification R as

Topological states in engineered atomic lattices

Individual vacancies in a chlorine monolayer on copper can be manipulated with scanning tunnelling microscopy to engineer artificial lattices that have topologically nontrivial electronic states.

Mesoscopic photon heat transistor.

We show that the heat transport between two bodies, mediated by electromagnetic fluctuations, can be controlled with an intermediate quantum circuit--leading to the device concept of a mesoscopic photon heat transistor (MPHT). Our theoretical analysis is based on a novel Meir-Wingreen-Landauer-type of conductance formula, which gives the photonic heat current through an arbitrary circuit element coupled to two dissipative reservoirs at finite temperatures. As an illustration we present an exact solution for the case when the intermediate circuit can be described as an electromagnetic resonator. We discuss in detail how the MPHT can be implemented experimentally in terms of a flux-controlled SQUID circuit.

Single-electron heat diode: Asymmetric heat transport between electronic reservoirs through Coulomb islands

We introduce a functional nanoscale device, a single-electron heat diode, consisting of two quantum dots or metallic islands coupled to electronic reservoirs by tunnel contacts. Electron transport through the system is forbidden but the capacitive coupling between the two dots allows electronic fluctuations to transmit heat between the reservoirs. When the reservoir temperatures are biased in the forward direction, heat flow is enabled by a four-step sequential tunneling cycle, while in the reverse-biased configuration this process is suppressed due to Coulomb blockade effects. In an optimal setup the leakage heat current in the reverse direction is only a few percent of the forward current.

Majorana states in helical Shiba chains and ladders

Motivated by recent proposals to realize Majorana bound states in chains and arrays of magnetic atoms deposited on top of a superconductor, we study the topological properties of various chain structures, ladders, and two-dimensional arrangements exhibiting magnetic helices. We show that magnetic domain walls where the chirality of a magnetic helix is inverted support two protected Majorana states giving rise to a tunneling conductance peak twice the height of a single Majorana state. The topological properties of coupled chains exhibit nontrivial behavior as a function of the number of chains beyond the even-odd dichotomy expected from the simple

Ground-state cooling of mechanical motion in the unresolved sideband regime by use of optomechanically induced transparency

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Theory of single-electron heat engines coupled to electromagnetic environments

nature of coupled Majorana states. In addition, it is possible that a ladder of two or more coupled chains exhibit Majorana edge states even when decoupled chains are trivial. We formulate a general criterion for the number of Majorana edge states in multichain ladders and discuss some experimental consequences of our findings.

Visualizing the chiral anomaly in Dirac and Weyl semimetals with photoemission spectroscopy

We present a scheme for cooling mechanical motion to the ground state in an optomechanical system. Unlike standard sideband cooling, this scheme applies to the so-called unresolved sideband regime, where the resonance frequency of the mechanical mode is much smaller than the cavity linewidth. Ground state cooling becomes possible when assuming the presence of an additional, auxiliary mechanical mode and exploiting the effect of optomechanically induced transparency. We first consider a system where one optical cavity interacts with two mechanical modes, and show that ground state cooling of the unresolved mechanical mode is possible when the auxiliary mode is in the resolved sideband regime. We then present a modified setup involving two cavity modes, where both mechanical modes are allowed to be in the unresolved sideband regime.

Thermal conductance in a spin-boson model: Cotunneling and low-temperature properties

We introduce a new class of mesoscopic heat engines consisting of a tunnel junction coupled to a linear thermal bath. Work is produced by transporting electrons up against a voltage bias like in ordinary thermoelectrics but heat is transferred by microwave photons, allowing the heat bath to be widely separated from the electron system. A simple and generic formalism capable of treating a variety of different types of junctions and environments is presented. We identify the systems and conditions required for maximal efficiency and maximal power. High efficiencies are possible with quantum dot arrays but high power can also be achieved with metallic systems.