Magnetic Force Microscopy study of layered superconductors in vectorial magnetic fields
- Autores: Alexandre Correa Orellana
- Directores de la Tesis: Hermann Suderow (dir. tes.), Carmen Munuera López (dir. tes.)
- Lectura: En la Universidad Autónoma de Madrid ( España ) en 2018
- Idioma: inglés
- Número de páginas: 176
- Tribunal Calificador de la Tesis: Federico José Mompeán García (presid.), Isabel Guillamón Gómez (secret.), Yonathan Anahory (voc.), Miguel Moreno Ugeda (voc.), Alexandre Bouzdine (voc.)
- Programa de doctorado: Programa de Doctorado en Física de la Materia Condensada, Nanociencia y Biofísica por la Universidad Autónoma de Madrid; la Universidad de Murcia y la Universidad de Oviedo
- Materias:
- Enlaces
- Tesis en acceso abierto en: Biblos-e Archivo
- Resumen
This thesis is focused on the set-up and use of a cryogenic magnetic force microscope (MFM) in a three axis vector magnet. We have studied superconducting layered and quasi-two dimensional compounds. In particular, we address the superconducting properties of graphene deposited on an isotropic s-wave superconductor $\beta-$Bi$_2$Pd, of a layered cuprate superconductor (BiSr$_2$CaCu$_2$O$_8$), of a layered iron based material (Ca(Fe$_{0.965}$Co$_{0.035}$)$_2$As$_2$) and of the s-wave superconductor $\beta-$Bi$_2$Pd.
MFM measures the magnetic properties of a surface by tracing the force when a magnetic tip is scanned over a magnetic sample. The interaction is mutual, the tip feels the magnetic properties of the sample and viceversa. By adjusting the scanning height, we can go from a non-invasive situation to manipulation, very much the same as in atomic manipulation using a STM. Here, in a MFM, the objects that are usually studied are much larger than atoms. Magnetic interactions usually extend over larger distances and therefore often the spatial resolution is of the order of the nm or above. This tool is ideal to study the magnetic profile generated by superconductors in the mixed state. Abrikosov vortices have a magnetic shape that is determined by the penetration depth, which is most often well above the nm range.
Here we are interested in the properties specific to two-dimensional and quasi two-dimensional superconducting systems. The associated confinement of superconductivity brings about new aspects. An important one is that, in the limit of extremely thin samples, the penetration depth often diverges. This makes the MFM useless to identify vortices or study magnetic textures, because the magnetic contrast decreases accordingly. Thus, instead of using single layers, we have focused on layered superconductors and hybrid structures combining a bulk superconductor with a 2D system as graphene. Another important aspect is that vortices are no longer lines of magnetic flux but disks. This implies that their mobility and pinning properties change considerably. Also, highly anisotropic properties can produce structural transitions, which are often of first order and can lead to coexistence of superconducting and normal domains. The MFM is there an ideal tool, with which we can make combined magnetic and structural studies, the latter by measuring the non-magnetic interaction between tip and sample, and making Atomic Force Microscopy (AFM). Finally, interactions might induce novel p-wave or unconventional superconducting states. This has been a recent focus, with the discovery of Majorana end states in proximity induced small superconducting structures. The spectroscopic features of such structures are well addressed in literature and it is generally acknowledged that studying the magnetic textures is the next important step. By inducing superconductivity in graphene, we have searched for unconventional behavior.
%The local properties of thin lakes of these superconductor and graphene were also characterized using a combination of our LT-MFM and a room temperature atomic force microscope (RT-AFM).
In the third chapter of the thesis, we have focused on the exfoliation and deposition of layered superconductors and on the study of graphene/superconductor interfaces. 2D superconductivity in thin films and crystal flakes has attracted the attention of many researchers in the last decade \cite{Profeta2012,DiBernardo2017,Narayan2015,Manzeli2017,Dong2017,Varela2017,Brun2017,Uchihashi2017,Bollinger2016}. For example, superconducting crystals like BSCCO or TaS$_{2}$ have been successfully exfoliated down to a single layer and deposited in a substrate in the past \cite{Wang2012,Huang2015,Moratalla2016}. In addition, a lot of work has been done trying to induce superconductivity in graphene in contact with a superconductor due to the proximity effect \cite{Profeta2012,DiBernardo2017,Komatsu2012,Aristizabal2009,Hayashi2010,Chuanthesis,Kim2012}. In this thesis, we have measured the magnetic profile of a Bi-2212 flake below the superconducting transition, developed an experimental procedure to localize graphene flakes deposited on top of a $\beta$-Bi$_2$Pd single crystal and demonstrated the possibility to depositing thin flakes of the $\beta$-Bi$_2$Pd superconductor on a substrate.
The vortex distribution in a superconductor at very low fields is still an open debate in the scientific community. For example, bitter decoration experiments performed in the single gap, low-$\kappa$ superconductor, Nb, shows areas where flux expulsion coexists with regions showing a vortex lattice. Moreover, Scanning Hall Microscopy experiments have shown vortex chains and clusters in ZrB$_{12}$ (0.8<$\kappa$<1.12) at very low fields \cite{Ge2014}. Both experiments were explained with the existence of an attractive term in the vortex-vortex interaction in superconductors with $\kappa$ < 1.5 . This regime is known as the \textit{Intermediate Mixed State}. On the other hand, the existence of vortex free areas between cluster and stripes of vortices at very low fields was also reported in the multigap superconductor MgB$_2$ \cite{Moshchalkov2009,Gutierrez2012,Nishio2010}. In this case, the authors propose that this behavior corresponds to a new state that they called \textit{type 1.5 superconductivity}, due to the existence of two different values of the Ginzburg-Landau parameter, $\kappa$, for the two gaps of the compound. In addition, a recent theoretical work has also proposed that pinning may have an important role in the formation of the vortex patterns in MgB$_{2}$ \cite{Zhao2017}. Comparatively, $\beta-$Bi$_2$Pd has a small, yet sizable, value of $\kappa\approx 6$. It has very weak pinning and is a single gap isotropic superconductor \cite{Herrera2015,Herrera2016,Kacmarcik2016}. This allows us to characterize the vortex distribution at very low fields in a material with only one gap and a moderate value of $\kappa$ for the first time. We have found vortex clusters and stripes as in the case of low-$\kappa$ or multigap superconductors. But, in this case, they are associated with local changes in the value of the penetration depth of the superconductor. We have also measured the vortex lattice at low temperatures of a $\beta$-Bi$_2$Pd single crystal with a graphene sheet deposited on top and found that the penetration depth increases, particularly at steps and wrinkles of the graphene surface. These results are presented in chapter 4.
Ca(Fe$_{0.965}$Co$_0.035$)$_2$As$_2$ is an iron-based compound with extremely high sensitivity to pressure and strain. Due to the presence of Ca ions, small pressures result in dramatic changes in the ground state of the system. We have characterized the formation of alternating superconducting antiferromagnetic domains at low temperatures and related them with the separation of the material in two structural phases. The results are collected in chapter 5.
In the last chapter of the thesis, we focus on the local manipulation of superconducting vortices in the high-temperature cuprate superconductor BiSr$_2$CaCu$_2$O$_8$. It has a two-dimensional layered structure, with superconductivity taking place in the copper oxide planes. When a magnetic field is applied tilted with respect to the c crystallographic axis, the vortex lattice decomposes into two systems of vortices, perpendicular to each other. There are Josephson, coreless vortices parallel to the layers and Abrikosov vortices located in the copper oxide planes, called pancake vortices. In our work, we use the MFM tip to manipulate pancake vortices at low temperatures and have determined the force needed to move combined pancake and Josephson vortex lattices.
