Always getting null values in controller for ajax post in mvc4

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Ajax

JSON

MVC4

Hi i am trying an ajax post from my view to controller for getting the list of some data.
But when I tried to access (break point checked in controller) the properties of the object (searchTerm) it always showed null value. I tried without stringifying and stringifying.
My view is look like this.

@model IEnumerable<mvc41.models.student>

<link href="../../Content/jquery-ui-1.10.3.custom.min.css" rel="stylesheet" type="text/css" />
<script src="../../Scripts/jquery-1.9.1.js" type="text/javascript"></script>
<script src="../../Scripts/jquery-ui-1.10.3.custom.min.js" type="text/javascript"></script>
<script type="text/javascript">
    $(function () {
        $("#txtSearch").autocomplete({
            source: '@Url.Action("GetStudents")',
            minLength: 1
        });

        $("#btnSearch").click(function () {
            var searchTerm = $("#txtSearch").val();
             $.ajax({
                url: 'Home/ListStudent',
                type: 'POST',
                contentType: 'application/json',
                dataType: 'json',
                data: JSON.stringify(searchTerm),
                success: function (data) {
                    alert(data.success);
                },
                error: function () {
                    alert("error");
                }
            });
            alert(searchTerm);
        });
    });
</script>
@if (TempData["alertMessage"]!= null) {
  <script type="text/javascript">
          alert("@TempData["alertMessage"]");
      </script>
}

<div style="font-family:Arial">
@using (@Html.BeginForm())
{
    Name: 
    @Html.TextBox("searchTerm", null, new { id = "txtSearch" })
    <input type="submit" value="Search"  id = "btnSearch"/>
}

and in Controller i have json method.

[HttpPost]
       public JsonResult ListStudent(string searchTerm)
       {
           StudentDataContext db = new StudentDataContext();
           List<student> students;
           if (string.IsNullOrEmpty(searchTerm))
           {
               students = db.Students.ToList();
           }
           else
           {
               students = db.Students
                   .Where(s => s.Name.StartsWith(searchTerm)).ToList();
           }
           return Json(students,JsonRequestBehavior.AllowGet);
       }

if anyone know where i am doing wrong please let me know.
Thanks in advance.