Study of valence band electronic states of near-surface atoms of niobium used for superconducting cavity

Highlights

• The valence band electronic states of technical-Nb material investigated.

• The electronic states of valence band of Nb sample evaluated.

• The valence band spectra of outer-surface of Nb atoms show nonmetallic character.

• The study helps in understanding the electronic properties of materials.

Abstract

The results of investigations conducted on the valence band structure of the near-surface atoms of niobium (Nb) sample, taken from a sheet that is used for fabricating superconducting radio frequency (SRF) cavity, are presented. The valence band electronic states were excited using low energy ($\le 50$ eV) ultraviolet radiation. It was observed that a) the valence band spectrum primarily consists of oxygen 2p-derived states, b) a typical feature expected near the Fermi edge in the valence band of Nb is conspicuously absent. Analysis of the wide band profile provides a number of electronic states peaked at ∼6.3, 7.6, 9.2, and 10.9 eV. After sputtering the surface, a small peak appeared at ∼1.4 eV near the Fermi energy edge. The Nb 4d states showed a resonance involving intra-atomic transition and also hybridization with the states of O 2p. The present study will be useful in identification of the electronic states of Nb that are accountable for superconductivity related issues in an SRF cavity.

Introduction

Achieving high electric field gradient, in the accelerating cavities at low operating cost, has been a major challenge for the accelerator scientists world-wide for a long time. To achieve this goal, having a reliable and efficient superconducting cavity is highly desirable. For last five decades, elemental niobium (Nb) is being used for the fabrication of superconducting radio frequency (SRF) cavities in particle accelerators [1,2]. The Nb SRF cavities are broadly classified into ‘high field-gradient’ and ‘high quality-factor’ categories. The high electric field accelerating gradient operation of an SRF cavity is usually limited either by the increased radio frequency (RF) power loss, or by the quenching of the cavity. These limitations are primarily governed by the cleanliness of the cavity surface and the surface contaminants, rather than the fundamental limitations of the cavity material. Achieving a high accelerating field gradient, concurrently with a high quality-factor ($Q\ge {10}^{11}$) [3], still remains a distant dream.

It is well known that some light element impurities get introduced into the cavity surface during the fabrication/processing of the cavity [4]. It is also well established that the near-surface chemical and physical properties of a niobium SRF cavity, up to a few tens of nm from the top of the surface, are considerably different from those of the bulk material [[3], [4], [5], [6], [7], [8], [9], [10], [11], [12]]. In fact, the response of the cavity to the RF field is quite sensitive to quality of this near-surface region of Nb metal. Therefore, a deeper understanding of the electronic properties of near surface atoms is needed to identify the mechanisms responsible for the performance degradation of these cavities. So far it is not known how the contaminants modify the electronic structure of Nb metal. Being a transition metal, it is expected that the partially filled valence orbitals ($4d^4$) of Nb will easily overlap with orbitals of other nearest neighbour coordinating elements present in the sample. Therefore, it is important to investigate electronic structure of surface of an Nb-SRF cavity.

X-ray photo-electron spectroscopy (XPS) is a surface investigative technique extensively used to identify the chemical structure near the surface of materials. The high energy of X-rays used in XPS ionizes the core-shell electrons from an element. In last four decades, several XPS studies [[13], [14], [15], [16], [17], [18]] have been carried out to understand the chemical structure of the near-surface region of Nb. All these studies were focused mainly on finding out which elements, in what chemical state, are present in the near surface of the sample. For this purpose, the binding energy of Nb 3d (core-levels) was earmarked to evaluate the formation of chemical bonds of the impurities atoms with Nb. Although the Nb 4d valence electrons play quite dynamic role in superconductivity, to the best of our knowledge, information concerning the 4d valence band electronic states of SRF niobium is not available.

Ultraviolet photo-electron spectroscopy (UPS), wherein only the valence orbitals are ionized, is an extremely useful tool for studying the occupied part of the valence band [19]. The localization/delocalization of the valence electrons in an Nb atom is highly sensitive to the presence of the impurity atoms or their complexes in the near-surface region. Therefore, experimental investigation of the electronic structure of Nb is very much needed to understand the fundamental processes responsible for limitations and variability in the performance of an Nb-SRF cavity.

In the present study, the valence band states of the surface atoms have been investigated using UPS. Further, an attempt has been made to analyse the observed features in the valence band spectrum and to identify the orbital origin of the peaks.