- Email: [email protected]
Recent progress in vortex studies by tunneling spectroscopy
Physica C 437–438 (2006) 145–148 www.elsevier.com/locate/physc
Recent progress in vortex studies by tunneling spectroscopy A. Kohen a,*, T. Cren a, Y. Noat a, T. Proslier a, F. Giubileo b, F. Bobba b, A.M. Cucolo b, N. Zhigadlo c, S.M. Kazakov c, J. Karpinski c, W. Sacks a, D. Roditchev a a
Institut des NanoSciences de Paris, UMR7588 au CNRS, Universite Paris 6 and Paris 7, 140 rue de Lourmel, 75015 Paris, France b Physics Department and INFM-SUPERMAT Laboratory, University of Salerno, via S. Allende, 84081 Baronissi (SA), Italy c Solid State Physics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland Available online 7 February 2006
Abstract Among the methods used to study the vortex state in superconductors, scanning tunneling spectroscopy (STS), is unique in its ability to measure in real space the variations in the local quasiparticle density of states. Thus, as opposed to magnetic imaging, STS gives direct access to the coherence length rather than to the penetration length. Here we discuss two novel methods which enhance the capabilities of STS as a tool for the study of the vortex state. In the first one, called Lazy Fisherman [A. Kohen et al., Appl. Phys. Lett. 86 (2005) 212503], the scanning tunneling microscope’s tip is kept fixed at a selected location while the vortices are being moved by varying the applied magnetic field. By continuously acquiring the local tunneling conductance spectra, dI/dV(V), we detect the changes in the local density of states under the tip due to the vortex motion. With no need for scanning, the method permits one to extend the study of vortices to samples in which scanning is difficult or even impossible due to surface non-uniformity and allows one to detect faster vortex dynamics. To illustrate the approach we study single crystal samples of MgB2. In the second STS method, we replace the commonly used normal metal STM tip by a superconducting (SC) tip which we produce either by mechanically breaking a Nb wire under vacuum in the STM chamber [A. Kohen et al., Physica C 49 (2005) 18] or by gluing a piece of a crystal of MgB2 [F. Giubileo et al., Phys. Rev. Lett. 87 (2001) 177008]. The use of a SC tip enhances the energy resolution of STS in comparison to that obtained with a normal metal tip. The method is illustrated by using Nb and MgB2 tips to perform a simultaneous topographic and spectroscopic imaging on 2H–NbSe2. 2006 Published by Elsevier B.V. PACS: 68.37.Ef; 74.25.Op; 74.50.+r; 74.78.Nd Keywords: Scanning tunneling microscopy; Superconducting tip; Critical field
1. Introduction Since its introduction by Hess et al. [4] as a tool to investigate the vortex lattice, STS has been used successfully in various materials such as NbSe2 [4], BSCCO [5], YBCO [6] and MgB2 [7]. The method is based on measuring the tunneling spectra between a normal metal tip and a superconducting (SC) sample as a function of position over an
*
Corresponding author. E-mail address: [email protected] (A. Kohen).
0921-4534/$ - see front matter 2006 Published by Elsevier B.V. doi:10.1016/j.physc.2005.12.025
area containing several vortices. In such a case, the observed changes in the local density of states (DOS) reflect the spatial variation of the SC order parameter due to the existence of the vortex lattice. The maximum contrast in the image is observed between the points exactly at a vortex center, exhibiting a normal state DOS, and those in between the vortices showing the maximal gap value. This contrast diminishes as one increases either the temperature or the applied magnetic field, as both actions reduce the gap. The contrast is further diminished by the finite resolution of the STMs spectroscopic mode, limited by the voltage source’s bias jitter and the effect of thermal smearing.
146
A. Kohen et al. / Physica C 437–438 (2006) 145–148
While the first can be reduced by improving the electronics, the second is intrinsic and therefore cannot be removed. The main disadvantages of the method is the need for highly flat and uniform samples to avoid the loss of image contrast due to noise generated by abrupt topographic changes, or even worse, crashing of the tip, and the relatively large time needed to obtain a single image. This time being between a few minutes and several hours (depending on whether one suffices in a single voltage map dI=dV jV ¼ V 0 or measures the full dI/dV(V) curve). As the vortex lattice needs to be stationary during the entire scan time, the method is not useful for the study of fast vortex dynamics. Here we describe two improvements which address exactly these limitations. First we address the problem of non uniform surfaces. We present a pseudo image of the vortex profile in MgB2 obtained without scanning the tip. We have obtained this image using the Lazy Fisherman mode of operation [1] in which the tip is held at a constant position while the vortex lattice is slowly moved by varying the value of the applied magnetic field. This proves that the STM can be used even in cases where scanning is hard or impossible. Furthermore, with no need to scan, the method allows detecting faster dynamics, limited by the time needed to obtain a single spectrum (ms). We secondly address the problem of the intrinsic thermal smearing. We do this by replacing the conventionally used normal metal tip by a SC one. In this case the peaked DOS in the tip is convoluted with that of the sample, thus highly reducing the thermal smearing and increasing the energy resolution. While the idea to use SC tips is not new [8–10] it was not a priori obvious that it can be used to image the vortex lattice. So far, publications using SC tips have been limited to the spectra obtained at a single position; in fact the ability to make a full spectroscopic map has not been shown. Further, the applied magnetic field might affect the gap in the tip and at worse provoke the normal state. Moreover, even if the tip remains SC, the field is inhomogeneous due to the existence of the vortex lattice and this might complicate the interpretation of the results. Lastly, a repulsive force should appear between the SC tip and the vortices and this might cause a vortex motion or distort their shape. Here we present images of the NbSe2 vortex lattice obtained in the STS mode with a SC tip thus showing that SC tips can be used to produce a full spectroscopic map of the vortex lattice. We obtain these images using both mechanically cut Nb tips [2] and MgB2 tips [3].
suring the I–V characteristics, one samples the different spectra within the vortex unit cell. In other words, consecutively measured spectra vary between gapped ones, representing the SC DOS in between the vortices, and completely flat ones, representing the DOS in the vortex core. The disadvantage of the method is that one cannot measure the distance between the tip and the vortex core and thus cannot directly plot the zero bias conductance (ZBC) as a function of this distance. This procedure is the one used in conventional STM studies of the vortex lattice in order to extract the vortex profile and the SC coherence length. We have shown that by studying the distribution of the observed zero bias conductance over a narrow field range we can reconstruct the ZBC profile using the fact that the probability to measure a ZBC value between r1 and r2 is proportional to r(r1)2 r(r2)2, r being the distance from the vortex center. The above method is used to present a pseudo image of spectral variations along a single vortex extracted from the data obtained on a c-axis oriented surface of a MgB2 single crystal using a normal metal tip. As in our method the magnetic field is constantly changing, the variations in the spectra reflect two different phenomena. First, they depend on the value of the applied field through the change in the inter vortex distance and the maximum order parameter value and second, on the change in the relative position of the tip within the vortex unit cell due to the vortex motion. In order to isolate the changes in the spectra, due solely to the relative tip’s position within the vortex lattice unit cell, we examine the spectra obtained over a short time interval. The time interval is chosen to be short enough to ensure that the inter-vortex distance does not significantly change. We have chosen a time interval of 6 s, which corresponds to a field change of 0.018 T. This assures, on the one hand, that indeed Dd/d < 5%, where d is the inter
2. Pseudo vortex image using the lazy fisherman method In a previous publication [1] we have introduced the lazy fisherman mode of operation which allows using the STM for vortex study in samples in which scanning is hard or impossible. In this mode of operation, the tip is held stationary in selected position of the sample while the vortices are being moved by slowly varying the value of the applied magnetic field. We have shown that by continuously mea-
Fig. 1. Pseudo image of a vortex in an MgB2 single crystal obtained at a field of 0.16 T and a temperature of 6.5 K.The image was obtained by the Lazy Fisherman method and the spectra are presented reordered by their ZBC value.
A. Kohen et al. / Physica C 437–438 (2006) 145–148
vortex distance and on other hand, allows us to measure a large number of spectra (1 0 0), giving an adequate sampling within the vortex unit cell. Then, we have reordered the 100 spectra by their zero bias value. Fig. 1 shows the data obtained in the field range H = 0.15–0.168 T after the reordering procedure. Evidently, the spectra showing the lowest zero bias conductance correspond to the point of symmetry equidistant from the vortices while those showing a constant value represent the spectra obtained closest to the center of the vortex core. Thus we can produce pffiffiffi a pseudo image of the vortex profile for 0 < r < d= 3. In fact in Ref. [1] we have shown how to use the histogram of these ZBC values to find the function ZBC = ZBC(r) for all r.
147
Fig. 3. Conductance map obtained from a full TS map of 128 · 128 spectra measured on an area of 350 nm2 showing the vortex lattice in NbSe2 imaged by a superconducting MgB2 tip at T = 4.5 K and H = 0.23 T. The conductance map is shown at a bias voltage of 3.5 mV.
3. Vortex imaging with superconducting tips In two of our previous publications we have described two methods of preparing superconducting STM tips. Both techniques, gluing a small MgB2 crystal [3] on a PtIr wire or mechanically breaking a Nb wire in the STMs UHV chamber [2], have yielded tips capable of atomic resolution and showing gapped dI/dV spectra proving that they are in the SC state. Here we use, for the first time, both kinds of tips to image the vortex lattice in NbSe2. We obtain these images by preforming a full spectroscopic map of the NbSe2 over an area of 0.35 lm2 measuring at each point both the z value of the topographic image and the full I(V) curve (10 mV < V < 10 mV). In Fig. 2 we show two images obtained with a Nb tip at a field of 0.06 T (applied perpendicular to the sample’s surface). Fig. 2a presents dI/dV(V = V0, x, y) for V0 = 1.5 meV and Fig. 2b for V0 = 3 meV both images show the same triangular vortex lattice with an inter vortex distance of 210 nm p which fits well with the theoretical value ffiffiffiffi given by 50 nm= H , but with an inverted contrast. While with a normal tip one usually uses V0 = 0 (i.e. the ZBC) to get the best contrast, in the case of a SC tip the ZBC is almost zero both in between the vortices and in the center of the vortices thus no contrast appears for the zero bias image. On the other hand, a contrast should appear for
V0 > D1, where D1 is the SC OP in the tip. In this voltage range the conductance in the vortex core being governed by the DOS of the tip starts to increase while that in between the vortices stays almost zero up to D1 + D2. At higher voltages the conductance in between the vortices surpasses that of the core, due to the peak at the gap edge, and the contrast in the images is inverted. At finite temperatures these values shift slightly due to the effect of thermal smearing. Fig. 3 shows a triangular vortex lattice obtained for a field of 0.23 T in an NbSe2 crystal, however, this time, with an MgB2 tip. The image is presented for V0 = 3 meV, the contrast in this image is not as good as in the one obtained using the Nb tip which is a result of the larger ratio between the sample’s and the tip’s gap values, 2.5 for MgB2 in comparison to 1.5 in the case of Nb. This makes the differences between the in core (NIS) and in between vortices (SIS) spectra less significant. However, using these tips to measure a Josephson current [11] the larger gap in MgB2 is favorable. A larger gap results in a larger Josephson coupling energy which should allow measuring a Josephson current at higher temperatures and at higher resistance values. As so far Josephson currents measured by STM where limited to a single position [9,12], the latter should be of importance as it allows a larger tip sample distance. This reduces the chances of crashing the tip and could assist in obtaining a full map of the Josephson current variations as a function of position. As we clearly image a vortex lattice, flux penetration to the tip apex is of minor importance. In some MgB2 tips using a normal metal sample, with the Lazy Fisherman mode, we did observe non monotonic variations in the spectra, suggesting flux penetration. 4. Summary
Fig. 2. Conductance maps obtained from a full TS map of 256 · 256 spectra measured on an area of 380 nm2 showing the vortex lattice in NbSe2 imaged by a superconducting Nb tip at T = 4.5 K and H = 0.06 T. (a) The conductance map at a bias voltage of 1.5 mV. (b)The conductance map at a bias voltage of 3 mV.
We have shown two methods, Lazy Fisherman, allowing to use STS in samples where scanning is hard and the use of a SC tips to enhance the spectroscopic resolution. The latter could be used to study the vortex lattice at elevated
148
A. Kohen et al. / Physica C 437–438 (2006) 145–148
temperatures or magnetic fields and open the door for the use of the STM to measure Josephson current maps, i.e., a Josephson STM. References [1] A. Kohen et al., Appl. Phys. Lett. 86 (2005) 212503. [2] A. Kohen et al., Physica C 49 (2005) 18.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
F. Giubileo et al., Phys. Rev. Lett. 87 (2001) 177008. H.F. Hess et al., Phys. Rev. Lett. 62 (1989) 214. S.H. Pan et al., Phys. Rev. Lett. 85 (2000) 1536. I.M. Maggio-Aprile et al., Phys. Rev. Lett. 75 (1995) 2754. M.R. Eskildsen et al., Phys. Rev. Lett. 89 (2002) 187003. S.H. Pan et al., APL 73 (1998) 2992. O. Naaman et al., Rev. Sci. Instrum. 72 (2001) 1688. H. Suderow et al., Physica C 369 (2002) 106. B.D. Josephson, Phys. Lett. 1 (1962) 251. J.G. Rodrigo et al., Phys. Rev. B 50 (1994) 12788.
Recent progress in vortex studies by tunneling spectroscopy A. Kohen a,*, T. Cren a, Y. Noat a, T. Proslier a, F. Giubileo b, F. Bobba b, A.M. Cucolo b, N. Zhigadlo c, S.M. Kazakov c, J. Karpinski c, W. Sacks a, D. Roditchev a a
Institut des NanoSciences de Paris, UMR7588 au CNRS, Universite Paris 6 and Paris 7, 140 rue de Lourmel, 75015 Paris, France b Physics Department and INFM-SUPERMAT Laboratory, University of Salerno, via S. Allende, 84081 Baronissi (SA), Italy c Solid State Physics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland Available online 7 February 2006
Abstract Among the methods used to study the vortex state in superconductors, scanning tunneling spectroscopy (STS), is unique in its ability to measure in real space the variations in the local quasiparticle density of states. Thus, as opposed to magnetic imaging, STS gives direct access to the coherence length rather than to the penetration length. Here we discuss two novel methods which enhance the capabilities of STS as a tool for the study of the vortex state. In the first one, called Lazy Fisherman [A. Kohen et al., Appl. Phys. Lett. 86 (2005) 212503], the scanning tunneling microscope’s tip is kept fixed at a selected location while the vortices are being moved by varying the applied magnetic field. By continuously acquiring the local tunneling conductance spectra, dI/dV(V), we detect the changes in the local density of states under the tip due to the vortex motion. With no need for scanning, the method permits one to extend the study of vortices to samples in which scanning is difficult or even impossible due to surface non-uniformity and allows one to detect faster vortex dynamics. To illustrate the approach we study single crystal samples of MgB2. In the second STS method, we replace the commonly used normal metal STM tip by a superconducting (SC) tip which we produce either by mechanically breaking a Nb wire under vacuum in the STM chamber [A. Kohen et al., Physica C 49 (2005) 18] or by gluing a piece of a crystal of MgB2 [F. Giubileo et al., Phys. Rev. Lett. 87 (2001) 177008]. The use of a SC tip enhances the energy resolution of STS in comparison to that obtained with a normal metal tip. The method is illustrated by using Nb and MgB2 tips to perform a simultaneous topographic and spectroscopic imaging on 2H–NbSe2. 2006 Published by Elsevier B.V. PACS: 68.37.Ef; 74.25.Op; 74.50.+r; 74.78.Nd Keywords: Scanning tunneling microscopy; Superconducting tip; Critical field
1. Introduction Since its introduction by Hess et al. [4] as a tool to investigate the vortex lattice, STS has been used successfully in various materials such as NbSe2 [4], BSCCO [5], YBCO [6] and MgB2 [7]. The method is based on measuring the tunneling spectra between a normal metal tip and a superconducting (SC) sample as a function of position over an
*
Corresponding author. E-mail address: [email protected] (A. Kohen).
0921-4534/$ - see front matter 2006 Published by Elsevier B.V. doi:10.1016/j.physc.2005.12.025
area containing several vortices. In such a case, the observed changes in the local density of states (DOS) reflect the spatial variation of the SC order parameter due to the existence of the vortex lattice. The maximum contrast in the image is observed between the points exactly at a vortex center, exhibiting a normal state DOS, and those in between the vortices showing the maximal gap value. This contrast diminishes as one increases either the temperature or the applied magnetic field, as both actions reduce the gap. The contrast is further diminished by the finite resolution of the STMs spectroscopic mode, limited by the voltage source’s bias jitter and the effect of thermal smearing.
146
A. Kohen et al. / Physica C 437–438 (2006) 145–148
While the first can be reduced by improving the electronics, the second is intrinsic and therefore cannot be removed. The main disadvantages of the method is the need for highly flat and uniform samples to avoid the loss of image contrast due to noise generated by abrupt topographic changes, or even worse, crashing of the tip, and the relatively large time needed to obtain a single image. This time being between a few minutes and several hours (depending on whether one suffices in a single voltage map dI=dV jV ¼ V 0 or measures the full dI/dV(V) curve). As the vortex lattice needs to be stationary during the entire scan time, the method is not useful for the study of fast vortex dynamics. Here we describe two improvements which address exactly these limitations. First we address the problem of non uniform surfaces. We present a pseudo image of the vortex profile in MgB2 obtained without scanning the tip. We have obtained this image using the Lazy Fisherman mode of operation [1] in which the tip is held at a constant position while the vortex lattice is slowly moved by varying the value of the applied magnetic field. This proves that the STM can be used even in cases where scanning is hard or impossible. Furthermore, with no need to scan, the method allows detecting faster dynamics, limited by the time needed to obtain a single spectrum (ms). We secondly address the problem of the intrinsic thermal smearing. We do this by replacing the conventionally used normal metal tip by a SC one. In this case the peaked DOS in the tip is convoluted with that of the sample, thus highly reducing the thermal smearing and increasing the energy resolution. While the idea to use SC tips is not new [8–10] it was not a priori obvious that it can be used to image the vortex lattice. So far, publications using SC tips have been limited to the spectra obtained at a single position; in fact the ability to make a full spectroscopic map has not been shown. Further, the applied magnetic field might affect the gap in the tip and at worse provoke the normal state. Moreover, even if the tip remains SC, the field is inhomogeneous due to the existence of the vortex lattice and this might complicate the interpretation of the results. Lastly, a repulsive force should appear between the SC tip and the vortices and this might cause a vortex motion or distort their shape. Here we present images of the NbSe2 vortex lattice obtained in the STS mode with a SC tip thus showing that SC tips can be used to produce a full spectroscopic map of the vortex lattice. We obtain these images using both mechanically cut Nb tips [2] and MgB2 tips [3].
suring the I–V characteristics, one samples the different spectra within the vortex unit cell. In other words, consecutively measured spectra vary between gapped ones, representing the SC DOS in between the vortices, and completely flat ones, representing the DOS in the vortex core. The disadvantage of the method is that one cannot measure the distance between the tip and the vortex core and thus cannot directly plot the zero bias conductance (ZBC) as a function of this distance. This procedure is the one used in conventional STM studies of the vortex lattice in order to extract the vortex profile and the SC coherence length. We have shown that by studying the distribution of the observed zero bias conductance over a narrow field range we can reconstruct the ZBC profile using the fact that the probability to measure a ZBC value between r1 and r2 is proportional to r(r1)2 r(r2)2, r being the distance from the vortex center. The above method is used to present a pseudo image of spectral variations along a single vortex extracted from the data obtained on a c-axis oriented surface of a MgB2 single crystal using a normal metal tip. As in our method the magnetic field is constantly changing, the variations in the spectra reflect two different phenomena. First, they depend on the value of the applied field through the change in the inter vortex distance and the maximum order parameter value and second, on the change in the relative position of the tip within the vortex unit cell due to the vortex motion. In order to isolate the changes in the spectra, due solely to the relative tip’s position within the vortex lattice unit cell, we examine the spectra obtained over a short time interval. The time interval is chosen to be short enough to ensure that the inter-vortex distance does not significantly change. We have chosen a time interval of 6 s, which corresponds to a field change of 0.018 T. This assures, on the one hand, that indeed Dd/d < 5%, where d is the inter
2. Pseudo vortex image using the lazy fisherman method In a previous publication [1] we have introduced the lazy fisherman mode of operation which allows using the STM for vortex study in samples in which scanning is hard or impossible. In this mode of operation, the tip is held stationary in selected position of the sample while the vortices are being moved by slowly varying the value of the applied magnetic field. We have shown that by continuously mea-
Fig. 1. Pseudo image of a vortex in an MgB2 single crystal obtained at a field of 0.16 T and a temperature of 6.5 K.The image was obtained by the Lazy Fisherman method and the spectra are presented reordered by their ZBC value.
A. Kohen et al. / Physica C 437–438 (2006) 145–148
vortex distance and on other hand, allows us to measure a large number of spectra (1 0 0), giving an adequate sampling within the vortex unit cell. Then, we have reordered the 100 spectra by their zero bias value. Fig. 1 shows the data obtained in the field range H = 0.15–0.168 T after the reordering procedure. Evidently, the spectra showing the lowest zero bias conductance correspond to the point of symmetry equidistant from the vortices while those showing a constant value represent the spectra obtained closest to the center of the vortex core. Thus we can produce pffiffiffi a pseudo image of the vortex profile for 0 < r < d= 3. In fact in Ref. [1] we have shown how to use the histogram of these ZBC values to find the function ZBC = ZBC(r) for all r.
147
Fig. 3. Conductance map obtained from a full TS map of 128 · 128 spectra measured on an area of 350 nm2 showing the vortex lattice in NbSe2 imaged by a superconducting MgB2 tip at T = 4.5 K and H = 0.23 T. The conductance map is shown at a bias voltage of 3.5 mV.
3. Vortex imaging with superconducting tips In two of our previous publications we have described two methods of preparing superconducting STM tips. Both techniques, gluing a small MgB2 crystal [3] on a PtIr wire or mechanically breaking a Nb wire in the STMs UHV chamber [2], have yielded tips capable of atomic resolution and showing gapped dI/dV spectra proving that they are in the SC state. Here we use, for the first time, both kinds of tips to image the vortex lattice in NbSe2. We obtain these images by preforming a full spectroscopic map of the NbSe2 over an area of 0.35 lm2 measuring at each point both the z value of the topographic image and the full I(V) curve (10 mV < V < 10 mV). In Fig. 2 we show two images obtained with a Nb tip at a field of 0.06 T (applied perpendicular to the sample’s surface). Fig. 2a presents dI/dV(V = V0, x, y) for V0 = 1.5 meV and Fig. 2b for V0 = 3 meV both images show the same triangular vortex lattice with an inter vortex distance of 210 nm p which fits well with the theoretical value ffiffiffiffi given by 50 nm= H , but with an inverted contrast. While with a normal tip one usually uses V0 = 0 (i.e. the ZBC) to get the best contrast, in the case of a SC tip the ZBC is almost zero both in between the vortices and in the center of the vortices thus no contrast appears for the zero bias image. On the other hand, a contrast should appear for
V0 > D1, where D1 is the SC OP in the tip. In this voltage range the conductance in the vortex core being governed by the DOS of the tip starts to increase while that in between the vortices stays almost zero up to D1 + D2. At higher voltages the conductance in between the vortices surpasses that of the core, due to the peak at the gap edge, and the contrast in the images is inverted. At finite temperatures these values shift slightly due to the effect of thermal smearing. Fig. 3 shows a triangular vortex lattice obtained for a field of 0.23 T in an NbSe2 crystal, however, this time, with an MgB2 tip. The image is presented for V0 = 3 meV, the contrast in this image is not as good as in the one obtained using the Nb tip which is a result of the larger ratio between the sample’s and the tip’s gap values, 2.5 for MgB2 in comparison to 1.5 in the case of Nb. This makes the differences between the in core (NIS) and in between vortices (SIS) spectra less significant. However, using these tips to measure a Josephson current [11] the larger gap in MgB2 is favorable. A larger gap results in a larger Josephson coupling energy which should allow measuring a Josephson current at higher temperatures and at higher resistance values. As so far Josephson currents measured by STM where limited to a single position [9,12], the latter should be of importance as it allows a larger tip sample distance. This reduces the chances of crashing the tip and could assist in obtaining a full map of the Josephson current variations as a function of position. As we clearly image a vortex lattice, flux penetration to the tip apex is of minor importance. In some MgB2 tips using a normal metal sample, with the Lazy Fisherman mode, we did observe non monotonic variations in the spectra, suggesting flux penetration. 4. Summary
Fig. 2. Conductance maps obtained from a full TS map of 256 · 256 spectra measured on an area of 380 nm2 showing the vortex lattice in NbSe2 imaged by a superconducting Nb tip at T = 4.5 K and H = 0.06 T. (a) The conductance map at a bias voltage of 1.5 mV. (b)The conductance map at a bias voltage of 3 mV.
We have shown two methods, Lazy Fisherman, allowing to use STS in samples where scanning is hard and the use of a SC tips to enhance the spectroscopic resolution. The latter could be used to study the vortex lattice at elevated
148
A. Kohen et al. / Physica C 437–438 (2006) 145–148
temperatures or magnetic fields and open the door for the use of the STM to measure Josephson current maps, i.e., a Josephson STM. References [1] A. Kohen et al., Appl. Phys. Lett. 86 (2005) 212503. [2] A. Kohen et al., Physica C 49 (2005) 18.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
F. Giubileo et al., Phys. Rev. Lett. 87 (2001) 177008. H.F. Hess et al., Phys. Rev. Lett. 62 (1989) 214. S.H. Pan et al., Phys. Rev. Lett. 85 (2000) 1536. I.M. Maggio-Aprile et al., Phys. Rev. Lett. 75 (1995) 2754. M.R. Eskildsen et al., Phys. Rev. Lett. 89 (2002) 187003. S.H. Pan et al., APL 73 (1998) 2992. O. Naaman et al., Rev. Sci. Instrum. 72 (2001) 1688. H. Suderow et al., Physica C 369 (2002) 106. B.D. Josephson, Phys. Lett. 1 (1962) 251. J.G. Rodrigo et al., Phys. Rev. B 50 (1994) 12788.