LOWT-MFM-OF-TIS · Low Temperature Magnetic Force Microscopy Study of Topological Insulators

Low Temperature Magnetic Force Microscopy Study of Topological Insulators

First period: 01/09/10-31/08/12 Our original goal was to detect the topological magneto-electric (TME) effect that has been predicted in topological insulators. Topological insulators are a new class of materials the hallmark of which is a surface state with unique properties. This surface state is topologically protected against many kinds of disorder. Much excitement surrounds the surface states in topological insulators because they may support Majorana modes which are central to some theoretical realizations of future quantum computers. The TME effect itself for is a direct fingerprint of the special properties of the surface states. Being a ‘smoking gun’ effect – discovering the TME effect will validate the ideas behind a large body of theoretical work in an important emerging field. Our main tool in this work is magnetic force microscopy. The magnetic force microscope (MFM) consists of a sharp magnetic tip that we bring to the surface of a sample and raster parallel to the surface. The magnetic tip allows us to apply both electric and magnetic fields as well as to measure the magnetic and electric properties and response from the sample. For this work we are used the magnetic tip to locally apply either an electric field or a magnetic field and to detect the corresponding magnetic response. Our original work plan consisted of the following: 1. Develop devices that will allow us to create the conditions required to give rise to the TME effect in available materials. 2. Search of the TME effect using MFM at low temperatures (down to 4.2K) and ultra-low temperatures (down to 50mK). 3. Study the effect as a function of various parameters such as temperature and applied magnetic field. Our interim achievements were as follows: 1. Our new 4.2K MFM is up and running well. Our 50mK MFM is in the development stage. 2. We developed a process for depositing thin ferromagnetic insulator films, which were part of the original devices we proposed. 3. We had to develope a new concept for the experiment. One of the main problems that we are faced was that even the best samples in existence have a significant concentration of dopants. As a result even the best of samples to which we had access were not bulk insulators. In order to overcome this difficulty we developed a new measurement protocol which allowed us to both control density and to look for a magneto-electric response from the sample. 4. This new protocol requires devices that we did not initially plan to implement. We developed the first generation of these devices. 5. We have performed several exploratory runs using our new 4.2K MFM. Our results were negative with the first generation devices. Because of timeline constraints we decided to differ the development of the next generation devices to a later date and turned our focus to an alternative project, as described below. Second Period (01/09/12-31/08/14) Recent work by many groups presents newly discovered exotic phenomena in BaFe2(As1-xPx)2, a pnictide superconductor, including indications for a quantum critical point near optimal doping (e.g. Analytis et al., Nat. Phys. 10, 194 (2014); Shibauchi et al., Annu. Rev. Condens. Matter Phys. 5, 113 (2014)). Perhaps the most startling property discovered in BaFe2(As1-xPx)2 is a peak in λ (the superconducting penetration depth) near optimal doping (Hashimoto et al., Science 336, 1554 (2012)). This is significant both because the origin of superconductivity in the pnictides, one of the only two families of high temperature superconductors known, is still under debate and because λ, which is notoriously hard to measure, is one of the best characterizations of the superconducting state. As far as we know until our work there are no additional reports of a peak in λ. Thus we set out to determine whether the peak exists if we use a completely different technique than the techniques with which the original result was obtained. Reproducing a surprising and important result by a totally different technique is significant in itself but it is even more significant in light of impact the original observation had, including a flurry of theoretical work struggling to explain the surprising peak (e.g. Chowdhury et al., Phys. Rev. Lett. 111, 157004 (2013)). Our approach is to use magnetic force microscopy (MFM) to measure λ as well as T_C (the superconducting transition temperature). Unlike in the original sample-averaged measurements, MFM is a local method so our results are local as well. In addition to verifying the existence of the peak, we show for the first time that the relationship between the local T_C and the local λ is very similar on both sides of optimal doping. In addition, we report for the first time in this material that λ shoots up as T_C goes down near the underdoped edge of the superconducting dome. This is a strong indication of mixing between a spin density wave state and superconductivity. In addition we use magnetic imaging of superconducting vortices, which are attracted to defects in the material, to show that there are twin-boundaries inside the superconducting dome. This provides another validation that indeed there is a mixed state in underdoped BaFe2(As1-xPx)2.

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